# aviationwheater.siten.co — full text > The complete text of every guide in this language, so an answer engine can read the catalogue in one request. Nothing here is absent from the visible pages. ## Crosswind and Runway Condition: Turning Wind Into a Number https://aviationwheater.siten.co/guides/crosswind-and-runway-conditions Updated 2026-08-09 · Weather and flight planning - A reported wind means nothing until it is resolved against the runway heading; the clock rule is close enough for the decision. - Always use the gust figure against a limit, and the worst bearing when a variable direction group is reported. - A maximum demonstrated crosswind is what was tested, not necessarily a hard limit — the operator’s figure is the hard one. - Runway contamination reduces grip, which raises stopping distance and lowers the usable crosswind at the same time. - Tailwind limits are much tighter than crosswind limits and are the check most often forgotten. A reported wind is not a limit. It is two numbers that have to be resolved against a runway heading before they mean anything, and the part that matters is the component across the runway rather than the total. The arithmetic is simple enough to do in your head, and doing it in your head is the point: by the time you are on the approach, the wind has changed twice and the number you calculated on the ground is history. ### The calculation The crosswind component is the reported wind speed multiplied by the sine of the angle between the wind and the runway. Nobody computes a sine on an approach, so the working approximation is the clock rule, and it is close enough for a decision. | 10° | Roughly 1/6 | 3 kt | | 20° | Roughly 1/3 | 7 kt | | 30° | 1/2 | 10 kt | | 45° | Roughly 0.7 | 14 kt | | 60° | Roughly 0.9 | 17 kt | | 90° | All of it | 20 kt | Runway numbers are magnetic and METAR wind is true. In most of the world the variation is a few degrees and does not change the answer; at high latitudes it can be large enough to matter. ### Which wind figure to use Always the gust, not the mean. A wind of 24012G28KT is a 28-knot wind for the purposes of a limit check, because the gust is what arrives at the moment of the flare. The mean is for planning fuel and the approach; the gust is for the limit. - Use the gust value against the crosswind limit, every time. - If a variable direction group is present — 240V300 — check the worst bearing in the arc, not the mean. - The tower wind is measured near the middle of the runway; conditions at the threshold and in the flare can differ. - Wind over obstacles, hangars or trees near the threshold produces mechanical turbulence that no report shows. - On a gusty day the headwind component varies too, which changes the approach speed additive. The variable group is the one people ignore. A crosswind that is comfortable at 240° and outside limits at 300° is reported as being at both. ### What a demonstrated crosswind is, and is not The figure in the flight manual is usually a maximum demonstrated crosswind — the strongest crosswind in which the aircraft was shown to be controllable during certification. It is not necessarily a structural or regulatory limit, and it is frequently misread in both directions: as an absolute prohibition, or as a soft suggestion. - Demonstrated means it was tested there, not that more is impossible. - Operators routinely impose their own limits, which are lower and are hard limits. - Limits are usually reduced for wet or contaminated runways, sometimes substantially. - Some aircraft have separate limits for takeoff and landing, and for autoland. - Crew experience and recency are the informal limit, and the one most often exceeded. Reduced-limit tables for contaminated runways are the part to read before the day you need them. That is where a comfortable 25-knot crosswind becomes a 15-knot one. ### Runway condition, and why it moves the limit Braking action and directional control both come from the tyre’s grip. Water, slush, snow and ice reduce it, so the ability to keep the aircraft straight against a crosswind falls at the same time as the stopping distance rises. That is why the two subjects are the same subject. | 6 — dry | Full | Published limit applies | | 5 — good | Near full | Usually the published limit | | 4 — good to medium | Reduced | Operator reduction typically begins | | 3 — medium | Noticeably reduced | Substantial reduction | | 2 — medium to poor | Poor | Large reduction, aquaplaning risk | | 1 — poor | Very poor | Severe reduction; ice | | 0 — less than poor | Almost none | Operations normally suspended | Aquaplaning deserves its own mention: above a speed that depends on tyre pressure, a layer of standing water can lift the tyre off the surface entirely, at which point braking and steering both go to almost nothing. ### Reading it off the report - Take the wind group, and use the gust figure. - Compare the direction against the runway in use; if a variable group is present, use the worst bearing. - Apply the clock rule for the crosswind component and the complementary figure for head or tailwind. - Check the runway condition report — a RCC group, a CLRD entry or the reported braking action. - Apply the operator’s reduced limit for that condition, not the dry-runway figure. - Check the tailwind component separately; limits there are much lower, usually around 10 to 15 knots. Step six is skipped surprisingly often. A tailwind limit is far tighter than a crosswind limit and it is easy to focus on the cross component alone. ### What a passenger sees - An aircraft on final pointing noticeably away from the runway — that is the crab, and it is the normal technique. - A late alignment just before touchdown, sometimes with a wing low. - A firm landing on a wet or contaminated runway; that is deliberate, to break through the water film and get the wheels turning. - A go-around when a gust arrives at the wrong moment — a normal manoeuvre and not an emergency. - A runway change, which is why the aircraft suddenly taxis further than expected. The crab looks alarming and is the correct answer to a crosswind. The aircraft is tracking the centreline while pointing into the wind, which is exactly what it should be doing. Q: How do you calculate a crosswind component? A: Multiply the wind speed by the sine of the angle between the wind and the runway. In practice, use the clock rule: at 30° take half the wind speed, at 45° take about 70 per cent, at 60° about 90 per cent, and at 90° all of it. Always use the gust figure rather than the mean, because the gust is what arrives in the flare, and if a variable direction group is reported, use the worst bearing in the arc rather than the average. Q: What is the maximum crosswind for landing? A: There is no universal figure. The flight manual usually publishes a maximum demonstrated crosswind — the strongest tested during certification — which is often not a structural or regulatory limit, and operators then impose their own lower and firmer limits. Those limits are typically reduced further for wet or contaminated runways, sometimes substantially. Crew experience and recency form the practical limit, and it is the one exceeded most often. Q: Why does a wet runway reduce the crosswind limit? A: Because both stopping and steering come from tyre grip, and water reduces it. With less grip the aircraft is harder to hold straight against a crosswind at the same time as the landing distance required increases. Above a speed that depends on tyre pressure, standing water can lift the tyre off the surface entirely — aquaplaning — at which point braking and directional control are almost gone. That is why runway condition codes and crosswind limits are read together. ## The Preflight Weather Briefing: A Checklist That Works https://aviationwheater.siten.co/guides/preflight-weather-briefing-checklist Updated 2026-08-09 · Weather and flight planning - Brief outward-in: synoptic picture, route, destination, alternate, departure, warnings, NOTAMs. - Read the destination before the departure — reading the departure first is how people talk themselves into starting. - Compare the current METAR against what the TAF forecast for this hour; a forecast already failing will be amended. - The freezing level and the cloud tops are derived numbers that decide the route, and neither appears in a METAR. - Write the briefing down: twenty records make your own misjudgement pattern visible in a way reading never will. The mistake in a weak briefing is almost never a missing product. It is the order. People open the destination METAR first, form an impression, and then read everything else as confirmation of the impression they already have. A briefing works outward-in: the synoptic picture first, then the route, then the aerodromes, then the warnings that cut across all of it. Doing it in that order means the aerodrome reports arrive as detail on a picture you already understand, rather than as a picture you build from one report. ### The order, and why it is that order - The synoptic picture. Where are the fronts, the lows and the jet? What is moving, and how fast? This sets expectations for everything else. - The route. Significant weather charts and upper winds at your levels. Icing and turbulence areas, convection, and the fuel implication of the wind. - The destination. TAF first, for the window around arrival plus realistic holding. METAR second, to see how the forecast is verifying. - The alternate. Its own TAF, read against the higher alternate planning minima, and far enough away to be under different weather. - The departure aerodrome. METAR and TAF for the departure window, plus de-icing and runway condition in winter. - The warnings. SIGMET, AIRMET, volcanic ash and tropical cyclone advisories across the whole route. - NOTAMs. Not weather, but they decide whether the approach you are planning against actually exists today. Step three before step five is deliberate. The question that decides whether the flight is viable is at the destination, and reading the departure first is how people talk themselves into starting. ### How the TAF is verifying One check separates a competent briefing from a thorough one: compare the current METAR against what the TAF said for this hour. If the forecast is verifying well, you can lean on the rest of it. If conditions are already worse than forecast at this point, the later part of the TAF is suspect and an amendment is probably coming. - Forecast conditions matching observation — the TAF is tracking; trust the later groups. - Observed worse than forecast — the deterioration is early; expect an amendment and plan pessimistically. - Observed better than forecast — pleasant, but do not plan on it continuing; forecasters are deliberately conservative. - A TAF issued several hours ago with rapidly changing conditions — check for an AMD before anything else. This check takes fifteen seconds and it is the single highest-value item in the whole sequence. ### The freezing level and the cloud tops Two derived numbers do most of the work on the route and neither appears directly in a METAR. The freezing level tells you where the icing band starts and whether your climb or descent passes through it inside cloud. The cloud tops tell you whether you can be on top, and therefore whether the ride and the icing exposure change with a level request. | Where is the freezing level? | Upper wind and temperature data, significant weather chart | | Am I in cloud through the icing band? | Cloud layers from the TAF and the area forecast | | What are the tops? | Area forecast, satellite imagery, pilot reports | | Will the wind cost or save fuel? | Upper wind charts at planned levels | | Where is the jet, and the shear beside it? | Upper wind chart; CAT areas on the significant weather chart | ### The checks people skip - The alternate’s own TAF. Nominating an alternate without reading it is common and pointless. - The trend group. A METAR at the limit with a BECMG improving is a different flight from one with NOSIG. - Amendments. A TAF read an hour ago may have been replaced twice, and amendments cluster when things are deteriorating. - Sunset and sunrise times. Weather that is workable in daylight can be a different proposition at night, and night VFR rules differ by state. - Runway condition and braking action in winter, which changes the landing distance required more than the ceiling does. - The return. For a same-day return, the destination’s later forecast matters as much as the arrival one. None of these is obscure. They get skipped because the briefing felt finished after the destination METAR looked acceptable. ### Writing it down A briefing that exists only in your head cannot be compared with reality later, and comparison is how judgement is built. A short written record — the numbers you decided against, the alternate you chose and why, the item you were unsure about — turns each flight into evidence about how you read weather. - Note the destination and alternate ceilings and visibilities you planned against. - Note the worst TEMPO or PROB value you accepted, and why it was acceptable. - Note the fuel decision and what drove it. - Note the one thing you were least sure about. - After the flight, write what actually happened in a single line. Do this for twenty flights and the pattern in your own misjudgements becomes obvious, which is not something any amount of reading will give you. ### What a briefing cannot do It cannot make the decision. Everything above produces a picture; the go or no-go rests with the pilot in command, against the operator’s minimums, the aircraft’s equipment and limitations, and an honest assessment of recency and fatigue. A thorough briefing that ends in a decision to stay on the ground is a successful briefing. And it has to be an official one. Explanatory sites — including this one — are for understanding what the products mean, never for obtaining them. Q: What should a preflight weather briefing include? A: Seven things, in order: the synoptic picture, the route weather and upper winds, the destination TAF and METAR, the alternate’s own TAF, the departure aerodrome, the warnings — SIGMET, AIRMET, volcanic ash and cyclone advisories — and finally NOTAMs. The order matters as much as the contents: working outward-in means the aerodrome reports arrive as detail on a picture you already understand rather than as an impression you then confirm. Q: How long before a flight should I check the weather? A: In layers. A first look the day before establishes what kind of day it will be and whether the trip is realistic. A full briefing goes in the last few hours, when TAFs covering your window are current. A final check immediately before departure catches amendments, which are issued precisely when conditions are deteriorating and are the most common thing to be caught out by. For a longer flight, the destination forecast should be rechecked en route where that is possible. Q: What is the most commonly skipped item in a weather briefing? A: The alternate’s own TAF. An alternate is nominated, the fuel is carried, and nobody reads the forecast for the place — which defeats the purpose, since an alternate under the same weather system as the destination is not an alternate. Close behind it are the trend group on a marginal METAR, and checking for an amended TAF before departure. All three take under a minute and all three change decisions. ## Weather Minimums: What VFR and IFR Actually Require https://aviationwheater.siten.co/guides/weather-minimums-vfr-ifr Updated 2026-08-08 · Weather and flight planning - VFR minimums protect see-and-avoid through visibility and cloud separation; IFR minimums protect the approach through decision height and RVR. - VFR, MVFR, IFR and LIFR are summary categories from the US National Weather Service, not legal limits for a particular flight. - VFR requirements rise with altitude and are stricter in controlled airspace — the shape is consistent even though the figures are national. - Approach minima are a pair: a decision height and a visibility, and lower categories need equipment, aircraft, crew and procedures all at once. - An alternate must be far enough away to have different weather, and its own forecast must meet the higher alternate planning minima. Visual flight rules require you to see. Instrument flight rules require you to be equipped, qualified and cleared not to. That is the whole distinction, and every minimum that follows from it is an attempt to write down how much seeing is enough. The numbers vary by country, by airspace class and by altitude, which frustrates people looking for one answer. What does not vary is the structure, and once the structure is clear the local table is easy to read. ### What the flight rules actually mean Under VFR the pilot is responsible for seeing and avoiding other aircraft and terrain, so the rules specify how much visibility and how much separation from cloud is required to make that possible. Under IFR the aircraft is separated by air traffic control and navigated by instruments, so the limiting numbers move to the approach: how low the cloud base and how poor the visibility can be while the runway environment can still be acquired in time to land. - VFR minimums protect see-and-avoid, so they are about flight visibility and distance from cloud. - IFR minimums protect the approach, so they are about ceiling and runway visual range at the decision point. - Special VFR is a limited clearance to operate below normal VFR minimums in a control zone. - Marginal VFR is not a legal category; it is an informal warning that conditions are close to the limit. The most common serious accident in general aviation involves a VFR flight continuing into instrument conditions. The minimums exist to make that boundary visible before it is crossed. ### The flight categories used in reports | VFR | Above 3,000 ft | More than 5 statute miles | | MVFR — marginal | 1,000 to 3,000 ft | 3 to 5 statute miles | | IFR | 500 to below 1,000 ft | 1 to less than 3 statute miles | | LIFR — low IFR | Below 500 ft | Less than 1 statute mile | These four categories come from the United States National Weather Service and are used worldwide as informal shorthand. They summarise conditions; they are not the legal minimum for any specific flight, which depends on the rules, the airspace, the aircraft and the crew. ### VFR minimums vary, and here is the pattern Exact figures are set nationally, but the shape is consistent almost everywhere. Requirements increase with altitude, because closing speeds are higher. They are more demanding in controlled airspace than outside it. And they relax at low level in uncontrolled airspace, where traffic is slower and less dense. | Controlled airspace, at or above 3,000 ft | 8 km visibility, 1,500 m horizontally and 300 m vertically from cloud | | Controlled airspace, below 3,000 ft | 5 km visibility, same cloud separation | | Uncontrolled, low level, slow aircraft | Reduced visibility permitted, clear of cloud and in sight of the surface | | Control zone, special VFR | By clearance only, with a ground visibility floor | | Night VFR | More restrictive or not permitted at all, depending on the state | Always read your own authority’s table. This is the shape of the rule, not a substitute for the regulation that applies to you. ### IFR approach minimums An instrument approach has a published minimum, and it is not one number but a pair: a height at which the approach must be discontinued if the required visual reference is not in sight, and a visibility or runway visual range below which the approach may not be started or continued. Precision approaches with vertical guidance go lower than non-precision ones, and better ground equipment plus crew qualification goes lower still. | Non-precision (VOR, NDB) | Around 400–600 ft above the aerodrome | 1,500 m or more | | RNP with vertical guidance | Around 250–300 ft | 1,000–1,200 m | | ILS CAT I | 200 ft | 550 m RVR | | ILS CAT II | 100 ft | 300 m RVR | | ILS CAT IIIb | Below 50 ft or no decision height | 75–125 m RVR | CAT II and III need certified ground equipment, certified aircraft, a qualified crew and low-visibility procedures in force at the aerodrome. All four have to be present, which is why a CAT III capable aircraft still diverts sometimes. ### Alternates, and the rule that catches people out Planning does not stop at the destination. If the forecast at the destination is below a defined threshold for a window around the estimated arrival, an alternate must be nominated, and fuel carried to reach it after an approach. Some rules require two alternates when the weather is genuinely marginal, and an alternate whose own forecast is poor does not qualify. - Read the destination TAF for the window around your arrival, not just the arrival minute. - Apply the applicable alternate requirement — the thresholds differ by state and operator. - Choose an alternate far enough away that it is not under the same weather system. - Check the alternate’s own TAF against the higher alternate planning minima. - Carry the fuel: to the destination, an approach, to the alternate, an approach there, plus reserves. The mistake that recurs is choosing a close alternate. Close is convenient and useless — the point of an alternate is to be somewhere the weather is different. ### Reading the weather against the minimum - Compare the forecast ceiling against the decision height for the approach you expect to fly. - Compare the forecast visibility or RVR against the approach minimum, using the worst TEMPO or PROB value in the window. - Check whether low-visibility procedures would be in force, which changes the arrival rate as well as the minimum. - Look at the trend group — a NOSIG at the limit is very different from a BECMG improving. - Check the wind against the runway that the approach serves; a legal ceiling on an unusable runway is not a plan. Everything on this page is educational. The applicable minimums for a given flight come from the state’s regulations, the operations manual and the approach chart, and the commander applies them. Q: What is the difference between VFR and IFR minimums? A: VFR minimums are about being able to see: they specify flight visibility and distance from cloud so that see-and-avoid works, and they vary by airspace class and altitude. IFR minimums are about the approach: they specify a decision height and a visibility or runway visual range at which the approach must be discontinued if the runway environment is not in sight. VFR minimums protect the whole flight; IFR minimums bite at the very end of it. Q: What do VFR, MVFR, IFR and LIFR mean on a weather map? A: They are summary flight categories defined by ceiling and visibility: VFR is a ceiling above 3,000 feet with more than 5 statute miles visibility; MVFR is 1,000 to 3,000 feet or 3 to 5 miles; IFR is 500 to below 1,000 feet or 1 to under 3 miles; LIFR is below 500 feet or under 1 mile. They come from the United States National Weather Service and are used worldwide as shorthand. They summarise conditions rather than defining what any particular flight is permitted to do. Q: Can a flight land in fog? A: Yes, within limits and with the right combination of equipment. A CAT I ILS typically needs 550 m runway visual range and a 200 ft decision height. CAT II goes to 300 m and 100 ft, and CAT IIIb can operate at 75 m with little or no decision height. But all of it requires certified ground equipment, a certified aircraft, a qualified crew and low-visibility procedures in force at the aerodrome — remove any one and the aircraft diverts even though it is technically capable. ## Winter Operations: De-icing, Holdover Times and Contaminated Runways https://aviationwheater.siten.co/guides/winter-operations-and-de-icing Updated 2026-08-08 · Regional aviation weather - The clean aircraft principle is absolute: no takeoff with contamination adhering to any critical surface, in any amount. - De-icing removes contamination; anti-icing protects for a calculated holdover time that starts when the application starts. - Holdover times collapse in heavy snow and freezing rain, sometimes to single-digit minutes, forcing re-treatment. - Runway condition codes from 6 down to 0 feed directly into the landing and takeoff distance calculation. - De-icing capacity is the binding constraint on a snowy morning, and a slow queue consumes the capacity it is waiting for. Winter does not make flying dangerous. It makes it slow, and it makes it arithmetic. Every departure gains a de-icing step with a clock attached. Every runway gains a condition code that changes the distance required to stop. Every arrival rate falls because clearance equipment has to occupy the surface. None of that is dramatic, and together it is why a snowy morning turns a working schedule into a queue by lunchtime. ### The clean aircraft principle The rule is absolute and it is the foundation of everything else: an aircraft may not take off with frost, ice, snow or slush adhering to any critical surface. Not a thin layer, not if it looks like it will blow off, not if the delay is expensive. The reason is the same as for in-flight icing — the wing is a shape, and contamination changes it — but on the ground it happens at exactly the moment the margins are thinnest. - Frost on the upper wing surface is enough to matter, even when it is barely visible. - Contamination is checked by inspection, not by assumption. - Cold-soaked fuel frost can form on a wing on a warm day after a long flight. - Snow that appears dry can be hiding a layer of ice beneath it. - The final responsibility for the check sits with the commander. This is one of the few areas in aviation with no judgement scale at all. There is no acceptable amount of contamination. ### De-icing and anti-icing are two different jobs | Type I | De-icing — removes contamination | Heated, low viscosity, runs off quickly, minimal protection | | Type II | Anti-icing — protects | Thickened, stays on the wing, sheds during the takeoff roll | | Type III | Anti-icing for slower aircraft | Sheds at lower rotation speeds | | Type IV | Anti-icing — longest protection | Most viscous, longest holdover time | A one-step treatment removes and protects with a single fluid; a two-step uses Type I to clean and then Type II or IV to protect. Which is used depends on the conditions and the operator’s procedure. ### Holdover time, and why it collapses The holdover time is the period during which anti-icing fluid can be expected to prevent ice forming on the treated surface. It is read from published tables against the fluid, the concentration, the outside air temperature and the type and rate of precipitation — and it starts at the beginning of the anti-icing application, not at the end. | Light snow, −3 °C | Tens of minutes with Type IV | | Moderate snow | Substantially shorter | | Heavy snow | Very short — sometimes single-digit minutes | | Freezing drizzle | Short, and the tables have hard limits | | Freezing rain | Extremely short; some conditions are outside the tables entirely | | Ice pellets | Restricted or not permitted at all | When the holdover expires before takeoff, the aircraft is de-iced again. It is not estimated, extended or judged by eye — which is precisely why a heavy snow morning produces a departure queue rather than a slow trickle. ### Contaminated runways On the ground the runway itself becomes a variable. Contamination — water, slush, snow, compacted snow or ice — reduces braking action and, for depths above a threshold, adds displacement and impingement drag during the takeoff roll. Modern reporting uses a runway condition code from 6 (dry) down to 0 (worse than poor), assessed per third of the runway, and the crew computes performance against it. | 6 | Dry | Dry | | 5 | Good | Frost, dry snow up to 3 mm | | 4 | Good to medium | Compacted snow at −15 °C or below | | 3 | Medium | Wet snow, dry snow over 3 mm, compacted snow above −15 °C | | 2 | Medium to poor | Standing water or slush over 3 mm | | 1 | Poor | Ice | | 0 | Less than poor | Wet ice, water on compacted snow | The step from code 3 to code 1 is not a small change in the numbers. It can move a landing from comfortable to not legally possible at the planned weight. ### Why the schedule falls apart - Every departure gains a de-icing step, and there is a finite number of rigs and pads. - Holdover clocks force re-treatment when the queue moves slowly, which lengthens the queue further. - Runway clearance closes one surface at a time, so movement capacity drops while the demand is unchanged. - Reduced braking action increases spacing between arrivals. - Crew duty limits start to bite on a day of long delays, producing cancellations that are not weather-caused in any direct sense. The feedback loop in step two is the important one. De-icing capacity is the constraint, and a queue that forces re-treatment consumes the very capacity it is waiting for. ### What passengers see, and what it means - A long wait on stand or at a remote pad with a vehicle spraying the wings — normal, and the safest part of the morning. - Orange or green fluid running off the wing during the takeoff roll — that is the anti-icing fluid shedding, exactly as designed. - A return to the pad after a long wait — the holdover expired, and the alternative is not acceptable. - Long taxi times behind a plough or sweeper — clearance takes priority over movements. None of this is a sign that something is wrong. It is a sign that the process is being followed at the point where it is most inconvenient to follow it. Q: Why do planes need de-icing? A: Because a wing works by its shape, and frost, snow or ice on the upper surface disrupts the airflow enough to reduce lift and raise the stall speed at the moment the aircraft has the least margin — during rotation and initial climb. The clean aircraft principle is absolute: nothing takes off with contamination adhering to a critical surface. De-icing fluid removes what is there; anti-icing fluid then protects the surface for a calculated period called the holdover time. Q: What is holdover time? A: The period during which anti-icing fluid can be expected to prevent ice forming on treated surfaces, read from published tables against the fluid type and concentration, the outside air temperature and the type and rate of precipitation. It begins when the anti-icing application starts. If it expires before the aircraft takes off, the treatment must be repeated rather than estimated or extended. In heavy snow or freezing rain the times can fall to a few minutes, which is what turns a snowy morning into a departure queue. Q: Can aircraft land on a snow-covered runway? A: Often yes, within calculated limits. Runway condition is reported as a code from 6 for dry down to 0 for wet ice, assessed for each third of the runway, and the crew computes the landing distance required against that code, the aircraft weight and the wind. Compacted snow at low temperature gives reasonably good braking; wet ice gives almost none. The step from medium to poor is not a small adjustment — it can make a landing at the planned weight impossible, which is when a diversion becomes the answer. ## Monsoon and Tropical Aviation Weather: Flying the Wet Season https://aviationwheater.siten.co/guides/monsoon-and-tropical-aviation-weather Updated 2026-08-08 · Regional aviation weather - The monsoon is a seasonal wind reversal, not a storm — it changes the regime for months and is planned for in advance. - Its costs are standing water, sustained low ceilings and embedded convection that only radar can find. - Equatorial convection is a daily cycle: clearest early, worst in the afternoon, gust-front windshear in the evening. - Tropical cyclones close airports rather than complicate them, and the forecast track supports pre-emptive cancellation. - Wet-season TAFs lean on TEMPO and PROB groups because the convection is certain and its placement is not. The monsoon is not a storm. It is a seasonal reversal of the prevailing wind, driven by the difference in how fast land and ocean heat, and it changes the weather regime over a subcontinent for months at a time. For aviation the consequence is specific and predictable: months of low ceilings, reduced visibility, standing water on runways, and convection embedded inside layers that look uniform from the flight deck. None of that is dramatic. All of it is operationally expensive. ### What the monsoon does to an operation The southwest monsoon over South Asia runs from roughly June to September, and it arrives as a wall of moisture rather than as individual weather events. Rainfall rates are high enough to produce standing water faster than drainage can clear it. Ceilings sit low for days. And embedded convection is the specific hazard: cells hidden inside a wider layer, which cannot be seen visually and have to be found on radar. - Standing water and aquaplaning risk, which changes the landing distance required. - Low ceilings for extended periods rather than in short episodes. - Embedded cumulonimbus inside stratiform cloud — invisible, radar-only. - Reduced visibility in continuous heavy rain, well below what the ceiling alone suggests. - Crosswind from a prevailing direction that may not favour the main runway. The monsoon is one of the few weather regimes where the schedule is planned around it in advance. Airlines add block time and plan for reduced arrival rates for the season, rather than absorbing it day by day. ### Tropical convection is daily, not seasonal Away from the monsoon belt, in the equatorial tropics, the dominant pattern is diurnal. Surface heating builds cumulus through the morning, cells mature in the afternoon, and the whole cycle repeats the next day. Tops routinely exceed anything an airliner can overfly, which means avoidance is lateral and planned into the fuel, not improvised. | Early morning | Clearest period, occasional fog or low stratus | Best departure window | | Late morning | Cumulus building rapidly | Deviations begin | | Afternoon | Mature cells, highest tops | Peak deviations, holding, occasional diversion | | Evening | Cells decaying, outflow and gust fronts | Windshear risk on approach | | Night | Residual layers, occasional overnight systems | Generally workable | This is why so many tropical long-haul departures are scheduled at night or early morning. The pattern is reliable enough to build a timetable around. ### Tropical cyclones Typhoons, hurricanes and cyclones are the same phenomenon under different regional names, and for aviation they close airports rather than complicate them. The forecast track is published days ahead by the responsible centre, which is why airlines cancel pre-emptively — a cancellation two days out is cheaper and safer than a diversion on the day. | Western North Pacific | Typhoon | Effectively year-round, peak July–October | | North Atlantic and eastern Pacific | Hurricane | June–November | | North Indian Ocean | Cyclone | April–June and October–December | | South Indian Ocean and South Pacific | Cyclone | November–April | Even well outside the eye, the outer bands produce severe turbulence, heavy rain and strong crosswinds, so the operational impact is much wider than the storm itself. ### How the reports read differently The code is the same; what fills it is not. TAFs in the tropics lean heavily on TEMPO and PROB groups because the convection is certain to happen and its exact placement is not. Visibility groups swing hard within a single hour. And where the aerodrome network is sparse, the nearest useful observation may be a long way from where you are going. - Expect PROB30 or PROB40 TSRA on almost every wet-season afternoon TAF; it is the honest forecast, not a hedge. - TEMPO groups carrying 1000 or 0800 visibility in heavy rain are routine. - CB in the cloud group is common rather than exceptional. - Windshear reports around the evening gust front deserve particular attention. - Haze — HZ — can persist for weeks in some years and is a separate problem from the rain. ### Planning around it - Fly early where the schedule allows: the diurnal convective cycle is reliable enough to plan around. - Carry realistic extra fuel for deviations and holding, not the legal minimum, through the wet season. - Choose an alternate that is genuinely independent — one far enough away not to be under the same system. - Read the runway condition and expect reduced braking action; recompute landing distance for a wet or contaminated surface. - Watch for the gust front on approach in the evening, which is where windshear appears after the cells decay. - Check the tropical cyclone advisories for the basin days ahead, not on the morning. None of this is exotic. It is ordinary planning done with the season’s numbers rather than the annual average. Q: How does the monsoon affect flights? A: Through sustained conditions rather than individual events. Months of heavy rain produce standing water on runways, which increases the landing distance required and brings aquaplaning risk. Ceilings sit low for days at a time, cutting arrival rates. Visibility drops in continuous rain well below what the cloud base alone would suggest. And convection embedded inside otherwise uniform layers cannot be seen visually, so it has to be found on radar. Airlines add block time for the season rather than absorbing it day by day. Q: Is it dangerous to fly during the monsoon? A: It is more demanding, not intrinsically dangerous. The operation is designed around it: crews are trained for contaminated runway performance, aircraft are certified for heavy rain, radar finds the embedded cells, and airports in monsoon regions have drainage and procedures built for it. What passengers experience is more delay, more holding and more turbulence on approach. The genuine hazard cases — a cell over the airfield, severe windshear on a gust front — are the ones the operation is set up to avoid rather than to fly through. Q: Why do tropical thunderstorms happen every afternoon? A: Because it is a diurnal cycle rather than a weather system. Strong equatorial surface heating and abundant moisture build cumulus through the morning, cells mature in the afternoon when the heating peaks, and they decay in the evening — then the same thing happens the next day. Tops routinely exceed the altitude an airliner can overfly, so avoidance is lateral and planned into the fuel. It is also why many tropical long-haul departures are scheduled at night or early morning. ## Aviation Weather by Region: What Each Part of the World Briefs For https://aviationwheater.siten.co/guides/aviation-weather-by-region Updated 2026-08-07 · Regional aviation weather - Moisture, temperature gradient and terrain explain nearly all the regional variation in aviation weather. - Twelve regions, twelve different briefings — the aircraft does not change, the atmosphere does. - The ITCZ and the monsoon migrate across the year, and the migration is what you plan capacity around. - Report format is international but coverage is not: station density, reporting hours and automation all vary. - Regional climatology gives you the shape of the problem; only the TAF and an official briefing answer tomorrow. Every pilot who moves between continents notices the same thing: the briefing changes shape. In northern Europe the argument is about visibility and contamination. In Southeast Asia it is about convection, every single afternoon. In the Gulf it is about heat and dust. The aircraft has not changed; the atmosphere has. This guide is the map of that — what each region briefs for, when its demanding season falls, and which national service is the authority for it. ### Why weather is a regional subject Three things vary systematically with geography and produce almost all of the difference: how much moisture the air can hold, how strong the temperature gradients are, and what the terrain does to the wind. Warm oceans give convection. Steep gradients give jet streams and the clear-air turbulence beside them. Mountains give wave and rotor. Everything in the table below follows from those three. - Tropical latitudes: daily deep convection, high cloud tops, tropical cyclones in season. - Mid-latitudes: frontal systems, jet streams and the winter–summer swing between contamination and convection. - Continental interiors: the widest temperature range, severe convection and the strongest surface heating. - Maritime margins: fog, low stratus and persistent wind rather than storms. - Mountainous regions: wave, rotor, downdraughts and rapid local changes. ### The regions at a glance | North America | Convection, icing, snow, fog | May–August convection; November–March winter | | Western Europe | Fog, low visibility, crosswind, icing | October–February | | Northern Europe | Snow, icing, contamination, crosswind | November–April | | Mediterranean | Crosswind, autumn convection, dust | June–September wind; September–November storms | | Middle East | Dust, heat, shallow fog, windshear | March–June | | South Asia | Monsoon, low visibility, convection | June–September monsoon; December–January fog | | Southeast Asia | Daily convection, tropical cyclones, haze | All year, worst May–October | | East Asia | Typhoons, clear-air turbulence, dust | July–October | | Oceania | Mountain wave, wind, convection | June–September wind; November–March storms | | South America | Andean wave, convection, volcanic ash | November–March | | Africa | Harmattan dust, ITCZ convection, heat | December–March dust; March–June storms | | North Atlantic and Arctic | Icing, turbulence, snow, ash risk | October–April | Each region has its own page on this site with the representative airports, the hazard definitions and links to the responsible meteorological service. ### The seasonal patterns that move Two of the biggest regional patterns are not fixed at all — they migrate, and the migration is what you plan around. The intertropical convergence zone, the belt where the trade winds meet, moves north and south across the year and drags a band of severe convection with it across Africa, South America and the Indian Ocean. The monsoon does something similar on a larger scale over South and Southeast Asia, reversing the prevailing flow and changing the weather regime wholesale for months. - The ITCZ sits north of the equator in the northern summer and south of it in the northern winter. - The southwest monsoon runs roughly June to September over South Asia, the northeast monsoon roughly October to December. - Tropical cyclone seasons differ by basin: the western Pacific runs almost year-round, the North Atlantic June to November. - The northern jet stream strengthens and moves south in winter, which is why winter transatlantic crossings are faster eastbound and rougher. - Harmattan dust over West Africa follows the northeast trades from about December to March. Seasonal climatology tells you what to expect and what to plan capacity around. It never tells you what tomorrow will do, and confusing the two is the classic mistake in reading a guide like this one. ### Where the reports themselves differ Format is international, but coverage is not. Dense networks in North America, Europe, Japan and Australia produce reports from small aerodromes at half-hourly intervals. Elsewhere the network is sparser, some stations report only during operating hours, and automated stations may not report present weather at all. A missing METAR is not good weather; it is a missing METAR. | Station density | Long gaps between observations on some routes | | Reporting hours | No report outside the aerodrome’s operating hours | | Automation | AUTO reports may omit present weather and cloud type | | Visibility units | Statute miles and inHg in North America, metres and hPa elsewhere | | Remarks section | Rich in North America, often empty elsewhere | | Access | Some states restrict briefing systems to licence holders | ### Using this practically - Start with the regional pattern to know what the season is likely to throw at you. - Read the aerodrome TAFs for the actual forecast — the pattern is context, never a substitute. - Check which service is authoritative for the airspace and use its briefing channel. - Note where the observation network thins, and plan alternates accordingly. - Watch for the migrating features — ITCZ position and monsoon onset move by weeks, not on a calendar. Every regional page on this site links the responsible national service. Read this guide for the shape of the problem, then go to them for the answer. Q: Which region has the most difficult aviation weather? A: There is no single answer, because the difficulty is of different kinds. Northern Europe and Canada impose the highest winter workload through contamination and de-icing. Southeast Asia has the most persistent convective hazard, with deep cells almost every afternoon. The North Atlantic combines icing, long overwater legs and very few alternates. Andean and Himalayan operations add terrain and wave to everything else. Crews describe whichever they fly least often as the hardest, which is itself informative. Q: Does aviation weather really differ that much by region? A: Yes, and it changes the whole shape of a briefing. Moisture, temperature gradient and terrain vary systematically with geography, and those three produce nearly all the difference. A briefing in northern Europe is largely about visibility and runway condition; the same briefing in the Gulf is about dust and density altitude; in Southeast Asia it is about where the afternoon cells will build. The report format is identical everywhere — what is in it is not. Q: When is the worst season for flying weather? A: It depends entirely on where. In the northern mid-latitudes there are two peaks: winter contamination and summer convection. In the tropics, convection is daily rather than seasonal, and the sharper marker is the tropical cyclone season for the basin. In South Asia, the southwest monsoon from June to September dominates, with a second problem in the December–January fog season. Any single global answer is wrong for most of the planet. ## Turbulence: The Four Types, and Which Ones Can Be Forecast https://aviationwheater.siten.co/guides/turbulence-types-and-forecasts Updated 2026-08-07 · Weather hazards in flight - Turbulence is four unrelated phenomena — convective, mechanical, clear-air and wake — with different causes and very different forecastability. - Clear-air turbulence has no cloud, no radar return and no visual cue; pilot reports are the only real-time source. - Mountain wave is smooth, but the rotor beneath it produces severe turbulence and the downdraughts can exceed an aircraft’s climb rate. - Severity levels describe the effect on aircraft and occupants — severe includes the aircraft being momentarily out of control. - The aircraft is not at risk in ordinary turbulence; unbelted occupants are, and that is the whole of the passenger-side mitigation. Turbulence is the weather passengers notice and the one they most misunderstand, because the single word covers four unrelated phenomena. They have different causes, different warning signs and very different forecastability. The practical consequence is that some turbulence is avoidable by planning and some is not, and knowing which kind you are dealing with is the difference between a reroute and a seatbelt sign. ### The four types | Convective | Rising and sinking air in and near storms | In and around cumulus and cumulonimbus | Area yes, cell no | | Mechanical | Wind over terrain or obstacles | Low level, downwind of ridges and buildings | Yes, from wind and terrain | | Clear-air | Wind shear at the edges of jet streams | Cruise levels, no cloud at all | Partly — probability, not position | | Wake | The vortices behind another aircraft | Behind and below a preceding aircraft | Managed by separation, not forecast | Mountain wave is a specific and severe form of mechanical turbulence, and it deserves its own treatment because it reaches cruise levels far from the terrain that caused it. ### Clear-air turbulence, the one with no warning CAT occurs where fast-moving air shears against slower air, principally at the boundaries of jet streams, and it produces the sudden events that make the news. There is no cloud, no radar return and no visual cue whatsoever — the only sensors are other aircraft and the model forecast. It is most common in winter when jet streams are strongest, and the strongest jet stream on the planet, over East Asia in winter, is the most reliable producer of it. - Look for it near the jet core, particularly on the cold side and below the axis. - Winter maximum, when the temperature gradient and therefore the jet is strongest. - Forecast as probability areas on significant weather charts, not as locations. - Pilot reports are the primary real-time source, which is why they are passed along the route. - Rising exposure in recent decades is an active area of research linked to jet stream changes. This is the type that injures unbelted passengers in cruise. It arrives without a cue, which is precisely why the seatbelt sign advice is not theatre. ### Mountain wave and rotor When strong wind blows across a ridge with a stable layer above, the air oscillates downwind in standing waves that can extend to the stratosphere and hundreds of kilometres away. The wave itself is smooth; the danger is the rotor beneath it — a rolling, violent circulation under the wave crests — and the sustained downdraughts, which can exceed the climb rate of many aircraft. | Lenticular cloud | Smooth, lens-shaped, stationary | Marks the wave; smooth but indicates strong flow | | Rotor cloud | Ragged, rolling, below the wave | Severe to extreme turbulence | | Cap cloud | Over the ridge, spilling down the lee | Terrain obscured and strong downdraught | | Clear wave | No cloud in dry air | All of the above with no visual warning | The classic setup is wind above about 25 knots across the ridge, roughly perpendicular, with a stable layer near ridge height. Those conditions are visible in a forecast a day out. ### How turbulence is reported and forecast Turbulence severity is reported in four levels, and the definitions are about the effect on the aircraft and occupants rather than a measurement. Forecasts come as SIGMETs for severe turbulence, graphical products by flight level, and increasingly as model-derived indices — but the accepted currency in flight is still the pilot report. | Light | Slight erratic changes; occupants feel a slight strain against the belt | | Moderate | Changes in altitude and attitude; occupants feel definite strain; loose objects move | | Severe | Large abrupt changes; aircraft momentarily out of control; occupants forced violently against belts | | Extreme | Aircraft violently tossed; possible structural damage | Note that severe is defined partly as the aircraft being momentarily out of control. It is a rare report, and it is why crews pass it on immediately. ### Is it dangerous? For the aircraft, essentially never. Airframes are certified to withstand loads far beyond what turbulence realistically imposes, and structural damage from turbulence is extremely rare. For people inside the cabin, it is the leading cause of in-flight injuries, and the overwhelming majority of those injuries happen to people who were not wearing a seatbelt when the seatbelt sign was on, or during unexpected clear-air turbulence in cruise when the sign was off. - The aircraft is not at risk in ordinary turbulence, including severe turbulence. - Cabin crew, who are standing by definition, are injured most often. - Almost all passenger injuries are avoidable with a fastened belt. - Crews slow to the turbulence penetration speed, which reduces the loads. - Reroutes and level changes are requested to avoid known areas, and pilot reports drive that. The honest summary: turbulence is uncomfortable and occasionally frightening, and the risk it poses is almost entirely about whether you are strapped in. ### Avoiding it in planning - Read the significant weather chart for forecast CAT areas at your cruise level. - Check the jet stream position and pick a level away from the shear zone where possible. - For low-level flight, check wind speed and direction against the terrain — perpendicular flow over a ridge above about 25 knots means wave and rotor. - Read SIGMETs for the whole route, not just the departure and arrival areas. - Note recent pilot reports along the track; they are the only direct measurement available. - In the cabin, keep the belt fastened whenever seated. It is the whole of the passenger-side mitigation. Q: What causes clear-air turbulence? A: Wind shear at the boundaries of jet streams, where fast-moving air rubs against slower air and the flow breaks down into eddies. It happens in cloudless air, which is why there is no visual cue and no radar return — the only real-time sensors are other aircraft. It peaks in winter when jet streams are strongest, and is most common near the jet core, particularly on the cold side and below the axis. Forecasts give probability areas rather than positions. Q: Is turbulence dangerous for the aircraft? A: Almost never. Airframes are certified for loads well beyond what turbulence realistically produces, and structural damage from it is extremely rare. The real risk is to people inside the cabin: turbulence is the leading cause of in-flight injuries, and the overwhelming majority of those injuries are to people who were not wearing a seatbelt. That is the entire reason the advice to stay belted when seated is repeated so insistently. Q: Can turbulence be forecast accurately? A: It depends on the type. Mechanical turbulence and mountain wave are forecast well, because they follow from wind speed and direction over known terrain — a day of strong perpendicular flow across a ridge is predictable. Convective turbulence is forecast at the level of an area, not an individual cell. Clear-air turbulence is forecast as probability regions from model shear indices, which is useful for choosing a cruise level but never tells you exactly where you will meet it. Wake turbulence is not forecast at all; it is managed by separation rules. ## Thunderstorms and Convective Weather: Why Aircraft Go Around Them https://aviationwheater.siten.co/guides/thunderstorms-and-convective-weather Updated 2026-08-07 · Weather hazards in flight - A cumulonimbus concentrates seven hazards at once, which is why avoidance is categorical rather than a judgement call. - The developing stage is the most deceptive: severe updraughts before the radar echo has built. - The conventional minimum is 20 NM from an active cell, more downwind, and no gap narrower than about 20 NM. - Where an individual cell will stand is not forecastable a day ahead; radar and satellite are the tools inside the last hour. - Turbulence near storms causes most in-flight injuries, and nearly all of them to unbelted passengers. A thunderstorm is not bad weather. It is a machine that concentrates six separate hazards into a few cubic kilometres of air, and aircraft do not fly through one for the same reason they do not fly into terrain. The rule that follows is simple and almost never broken: you go around, not over, not under, and not through. Everything else in this guide is detail about how far around, and why the alternatives do not work. ### What is inside a cumulonimbus - Severe or extreme turbulence, capable of exceeding the structural design load in the worst cells. - Updraughts and downdraughts that can exceed anything an aircraft can outclimb. - Hail, which can be thrown out of the top and sides and encountered in clear air beside the cell. - Lightning, which rarely damages an aircraft badly but can and does cause it. - Heavy precipitation that reduces visibility to nothing and can affect engine operation. - Microbursts on the approach and departure path — a downdraught and outflow that has caused multiple accidents. - Airframe icing throughout the mid-levels of the cell. Any one of those would be a reason to keep away. A mature cell contains all seven simultaneously, which is why the response is categorical rather than a judgement call. ### The three stages, and which one is worst | Cumulus (developing) | 15–20 minutes | Updraught throughout | Turbulence, rapid growth, no radar echo yet | | Mature | 15–30 minutes | Updraught and downdraught together | All hazards at maximum, including hail and microburst | | Dissipating | 20–30 minutes | Downdraught throughout | Still severe; outflow and windshear persist | The developing stage is deceptively dangerous because the cell has not yet produced enough precipitation to show clearly on radar, and it can grow through a cruising level in minutes. ### How far around The standard guidance is to remain at least 20 nautical miles from a thunderstorm, and further from a severe cell or one with an anvil, because hail can be thrown well outside the visible cloud. In practice crews avoid on radar with a wide margin, upwind rather than downwind where possible, and never accept a gap between two cells that is closing. - 20 NM is the conventional minimum from an active cell. - Give more room downwind — the anvil throws hail and turbulence out that side. - Do not fly a gap under about 20 NM wide between two cells; gaps close faster than they look. - Never fly under a cell: that is where the microburst and the heaviest precipitation are. - Overflying requires a very large margin above the top, and the tops in the tropics routinely exceed what airliners can reach. Radar shows precipitation, not turbulence. A cell in the developing stage or one with a hail shaft above the beam can look far weaker on screen than it is. ### Squall lines, embedded cells and the tropical case Isolated cells are the easy problem. Squall lines — organised lines of storms sometimes hundreds of kilometres long — remove a route segment entirely, and the only options are around the end or waiting. Embedded thunderstorms hidden inside a wider stratiform layer are dangerous precisely because they cannot be seen, only detected. And in the tropics, where the convection is daily rather than seasonal, cells build higher and the diversion becomes routine planning rather than an exception. | Air-mass, isolated | Summer afternoons, mid-latitudes | Deviations of a few miles | | Squall line | Ahead of a cold front | Route segment closed for hours | | Embedded | Within a warm-frontal layer | Radar-only detection; unexpected severe turbulence | | Supercell | Continental interiors | Extreme hazard, very wide avoidance | | Tropical, daily | Intertropical convergence zone | Continuous deviation planning, high tops | ### What the forecast can and cannot tell you Convection is the hardest thing in the forecaster’s job. Whether a region will develop storms is often known a day ahead with reasonable confidence. Where a specific cell will stand at a specific hour is not knowable at that range — which is exactly why TAFs express it as PROB30 or PROB40 and why the useful tools inside a few hours are radar and satellite rather than a forecast made yesterday. - Convective outlooks and significant weather charts give the area and the general timing. - TAF PROB groups are the aerodrome-level hedge. - SIGMETs are issued for observed or expected severe convection and are the warning that matters in flight. - Weather radar, on board and on the ground, is the tool in the last hour. - Satellite imagery shows the rapidly growing tops that radar has not caught yet. This is the phenomenon where a forecast is least specific and the live picture matters most. Treat a convective TAF as a warning to look, not as a schedule. ### What passengers experience On the ground, thunderstorms produce ground stops and long taxi queues, because departures have to be metered onto the routes that are still open. In the air, they produce deviations that add fuel and time, a seatbelt sign that stays on, and occasionally a diversion when a cell parks over the destination. Turbulence encountered near storms is the main cause of in-flight injuries, and almost all of those injuries are to people who were not wearing a seatbelt. That last point is the only piece of advice in this guide that a passenger can act on, and it is the one that has been demonstrated to matter. Q: Why do planes avoid thunderstorms instead of flying through them? A: Because a mature cumulonimbus contains severe turbulence, violent updraughts and downdraughts, hail, lightning, heavy precipitation, microbursts and icing all at once, and the turbulence alone can approach the structural limits of the airframe. There is no technique for flying through one safely, so the standard is to avoid it laterally by at least 20 nautical miles. Flying under a cell puts the aircraft in the microburst and the heaviest rain; flying over one requires a margin above tops that often exceed the aircraft ceiling. Q: How close can an aircraft fly to a thunderstorm? A: The conventional minimum is 20 nautical miles from an active cell, and more downwind, because hail can be thrown out of the anvil into apparently clear air well beyond the visible cloud. Crews also refuse gaps narrower than about 20 nautical miles between two cells, because those gaps close faster than they appear to on radar. Radar shows precipitation rather than turbulence, so a developing cell can be far more dangerous than its echo suggests. Q: Can weather radar see all thunderstorms? A: No, and the gaps are important. Airborne radar detects precipitation, so a cell in the developing stage that has not yet produced enough rain returns a weak echo while already containing severe updraughts. Attenuation means a heavy cell can hide a second one behind it. Hail above the beam may not appear at all. Satellite imagery of rapidly growing tops, and the pattern of the cells rather than the raw returns, fill part of that gap. ## Aircraft Icing Explained: Where It Forms, and Why It Is So Serious https://aviationwheater.siten.co/guides/aircraft-icing-explained Updated 2026-08-06 · Weather hazards in flight - Ice ruins the aerofoil shape: maximum lift falls, stall speed rises, drag and weight increase, and instruments can be blocked. - Two conditions are needed together — visible moisture and a surface at or below freezing; the productive band is roughly 0 °C to −20 °C. - Clear ice is the most dangerous type because it is nearly invisible and forms behind the protected leading edge. - Freezing rain and freezing drizzle exceed what protection systems are certified for; the correct response is to leave the area. - On the ground the clean aircraft principle is absolute, and holdover times collapse in freezing rain and heavy snow. A wing does not fly because it is strong. It flies because of its shape, and that shape is engineered to within fractions of a millimetre at the leading edge. Ice ruins the shape. That is why icing is treated with a seriousness that surprises people who imagine a bit of frost. Roughness the thickness of coarse sandpaper on a leading edge can cost a substantial fraction of the maximum lift and raise the stall speed noticeably — and the ice does not stop accumulating when it becomes inconvenient. ### What actually happens Cloud droplets can remain liquid well below 0 °C — supercooled — and freeze the instant they strike a surface. An aircraft flying through supercooled cloud is a moving freezing surface. The droplets hit the leading edge, freeze, and build outward and backward from there. Three things follow at once, and each of them is bad on its own. - The aerofoil shape changes, so maximum lift falls and the stall speed rises. - Drag increases, so the aircraft needs more power to hold speed and altitude. - Weight increases, which is the least important of the three but arrives with the others. - Control surfaces, pitot tubes, static ports and antennas can ice, giving false or lost indications. - Propeller or fan blade ice sheds unevenly, producing vibration. The stall speed rise is the killer. An aircraft that stalls at an unexpectedly high speed, with degraded controls, in cloud, has very little margin left. ### The three types | Rime | −15 °C to −40 °C, small droplets | Opaque, white, rough | Builds fast on the leading edge; rough shape, easy to see | | Clear (glaze) | 0 °C to −10 °C, large droplets | Transparent, smooth, heavy | Runs back before freezing, forms behind the de-ice boots | | Mixed | −10 °C to −15 °C | Both, in layers | Combines the weight of clear with the roughness of rime | Clear ice is the one that catches people out, because it is nearly invisible from the flight deck and forms aft of the protected surfaces where nothing can remove it. ### The temperature band, and the moisture rule Two conditions have to be met at once: visible moisture, and a surface temperature at or below freezing. Without visible moisture there is nothing to freeze; without the temperature there is nothing to freeze it. The most productive band for structural icing is roughly 0 °C to −20 °C, with the worst accumulation rates around −2 °C to −10 °C where the droplets are largest. - Below about −40 °C the cloud is already ice crystals and does not adhere. - Between 0 °C and −20 °C is the band to plan around. - Freezing rain and freezing drizzle can produce severe icing at temperatures at or just above 0 °C at the aircraft, because the droplets are supercooled from above. - Engine and carburettor icing can occur well above freezing — carburettor ice is possible in warm, humid air at +20 °C. Freezing rain deserves separate respect. It is the one situation where the correct answer is to leave the area immediately rather than manage it, because accumulation rates exceed what any protection system is certified for. ### What the reports and forecasts tell you Icing is forecast rather than observed, and the products are separate from the METAR. AIRMETs and SIGMETs cover icing in a defined area; graphical icing forecasts show probability and severity by flight level; and the METAR contributes indirectly through the temperature, dew point and precipitation codes. | SIGMET (ICE) | Severe icing over a defined area and period | | AIRMET / graphical icing forecast | Moderate icing, probability and severity by level | | METAR FZRA / FZDZ | Freezing precipitation observed at the surface | | METAR temperature/dew point | Saturation and freezing level clues at the aerodrome | | Significant weather chart | Forecast icing areas at cruise levels | | Pilot reports | The only direct observation of icing that exists | Pilot reports matter disproportionately here. Icing is the phenomenon with the largest gap between what is forecast and what is actually encountered, and the only instrument measuring it is an aircraft. ### Ground icing is a separate problem Frost, snow or ice on an aircraft before departure is governed by the clean aircraft principle: nothing takes off with contamination adhering to a critical surface, without exception. De-icing fluid removes what is there; anti-icing fluid buys a protected period called the holdover time, which depends on the fluid, the temperature and the precipitation rate. - Inspect and identify the contamination type. - De-ice — usually heated Type I fluid, which removes but does not protect. - Anti-ice — thickened Type II or IV fluid, which protects for a calculated period. - Start the holdover clock at the beginning of the anti-icing application. - If the holdover time expires before takeoff, the treatment is repeated, not estimated. Holdover tables collapse in freezing rain and heavy snow — sometimes to a few minutes — which is why those conditions produce departure gridlock rather than a slow trickle. ### Reading the risk before you fly - Is there visible moisture along the route at the levels you will use? - Where is the freezing level, and does the climb or descent pass through the 0 °C to −20 °C band inside cloud? - Are FZRA or FZDZ reported anywhere near the route or the destination? - Is the aircraft certified for flight in known icing, and is the protection system serviceable? - Is there an escape — a level, a heading or an altitude that leaves the cloud quickly? For an aircraft not certified for known icing, the plan is to stay out of the conditions, not to manage them. That is a route and altitude decision made on the ground. Q: At what temperature does aircraft icing occur? A: Structural icing needs visible moisture and a surface at or below 0 °C, and the most productive band is roughly 0 °C to −20 °C, with the highest accumulation rates around −2 °C to −10 °C where supercooled droplets are largest. Below about −40 °C the cloud consists of ice crystals that do not adhere. Freezing rain and freezing drizzle are the exception: they can produce severe icing at temperatures at or just above freezing at the aircraft, because the droplets arrive already supercooled. Q: Why is aircraft icing dangerous? A: Because a wing is a shape, and ice changes it. Roughness on the leading edge reduces the maximum lift the wing can produce and raises the stall speed, while drag rises and weight increases. Clear ice is the most dangerous type because it is nearly invisible from the flight deck and forms behind the protected leading edge, where no de-icing system can reach it. Icing can also block pitot tubes and static ports, which corrupts the airspeed and altitude indications at the worst possible moment. Q: How do pilots know if icing is forecast? A: Through separate products from the METAR: SIGMETs for severe icing, AIRMETs and graphical icing forecasts for moderate icing by flight level and probability, and significant weather charts for cruise levels. The METAR contributes indirectly through the temperature and dew point pair and through FZRA or FZDZ codes at the surface. Pilot reports carry unusual weight for icing, because it is the phenomenon where forecast and encountered conditions differ most and an aircraft is the only real sensor. ## METAR and TAF Abbreviations: The Complete Decoder Table https://aviationwheater.siten.co/guides/metar-taf-abbreviations Updated 2026-08-06 · METAR and TAF decoding - METAR weather codes are built from intensity, descriptor and phenomenon in a fixed order — learn the four lists, not the combinations. - BR and FG are the same phenomenon split at 1,000 metres of visibility. - Only BKN and OVC count as a ceiling; CB and TCU are appended to the layer that carries them. - CAVOK is a defined state with four conditions, not a general statement that the weather is nice. - Everything after RMK is a national addition and outside the international standard. The weather codes in a METAR are not arbitrary letters. They are built from four small vocabularies that combine in a fixed order: an intensity marker, then a descriptor, then one to three phenomena. Learn the four lists and you can read a combination you have never seen before. +TSRA is not an entry in a dictionary. It is heavy, thunderstorm, rain — assembled on the spot, the same way the forecaster assembled it. ### How a weather group is built Read left to right: intensity, then descriptor, then phenomenon. Any of the three can be absent. VC replaces the intensity marker to mean in the vicinity — between about 8 and 16 kilometres from the aerodrome, not at it. | Intensity | -, none, + | Light, moderate, heavy | | Proximity | VC | In the vicinity of the aerodrome, not over it | | Descriptor | MI BC PR DR BL SH TS FZ | Shallow, patches, partial, drifting, blowing, showers, thunderstorm, freezing | | Phenomenon | RA SN DZ GR … | The precipitation or obscuration itself | So -SHRA is light showers of rain, +TSRA is a heavy thunderstorm with rain, and FZDZ is freezing drizzle — which is the one that ruins aircraft. ### Descriptors | MI | Shallow | Fog under about two metres deep — the runway may be clear at eye level | | BC | Patches | Not continuous; conditions can change between one end of the runway and the other | | PR | Partial | Covering part of the aerodrome only | | DR | Low drifting | Below eye level — blowing snow or dust under two metres | | BL | Blowing | Raised above eye level; reduces visibility properly | | SH | Showers | Convective, so intermittent and often sharp | | TS | Thunderstorm | Lightning present; implies severe turbulence and possible hail | | FZ | Freezing | Supercooled — freezes on contact with the airframe or the runway | ### Precipitation and obscuration | DZ | Drizzle | BR | Mist — visibility 1,000 m or more | | RA | Rain | FG | Fog — visibility under 1,000 m | | SN | Snow | FU | Smoke | | SG | Snow grains | HZ | Haze | | PL | Ice pellets | DU | Widespread dust | | GR | Hail, 5 mm or larger | SA | Sand | | GS | Small hail or snow pellets | VA | Volcanic ash | | IC | Ice crystals | PO | Dust or sand whirls | | UP | Unidentified precipitation | SQ | Squall | | NSW | No significant weather | FC | Funnel cloud | BR and FG are the same phenomenon divided by a number: 1,000 metres. That single threshold decides which of the two appears. ### Cloud, and the groups that replace it | FEW | 1–2 oktas of the sky | | SCT | 3–4 oktas — scattered | | BKN | 5–7 oktas — broken, counts as a ceiling | | OVC | 8 oktas — overcast, counts as a ceiling | | NSC | No significant cloud below 5,000 ft or the highest minimum sector altitude | | NCD | No cloud detected — an automated station reporting nothing | | SKC / CLR | Sky clear | | VV/// | Sky obscured, vertical visibility not measurable | | CB | Cumulonimbus, appended to a layer | | TCU | Towering cumulus, appended to a layer | | CAVOK | Ceiling and visibility OK — replaces visibility, weather and cloud groups together | CAVOK is not a compliment; it is a defined state. Visibility 10 km or more, no cloud below 5,000 ft or the minimum sector altitude, no CB or TCU, and no significant weather. ### Trend, runway and report-status groups | NOSIG | METAR trend | No significant change expected in the next two hours | | BECMG | Both | Permanent change, occurring gradually | | TEMPO | Both | Temporary fluctuations, each under an hour | | PROB30 / PROB40 | TAF | 30 or 40 per cent probability | | FM | TAF | From this time — a rapid, complete change | | AUTO | METAR | Fully automated observation, no human observer | | COR | METAR | Corrected report, replaces the earlier one | | AMD | TAF | Amended forecast, replaces the earlier one | | R27/0900 | METAR | Runway visual range: 900 m on runway 27 | | R27/CLRD70 | METAR | Runway 27 cleared, friction coefficient 0.70 | | WS RWY24 | METAR | Windshear reported on the approach or departure path of runway 24 | | RMK | METAR | Remarks — national additions, formats vary by state | Everything after RMK is outside the international standard. North American remarks are rich and worth learning; elsewhere the section is often empty. ### Combinations you should recognise on sight - FZRA — freezing rain. Supercooled droplets that freeze on the airframe on contact, and among the most dangerous conditions in aviation. - +TSGR — heavy thunderstorm with hail. Structural damage territory; not a weather to be near. - VCTS — thunderstorms in the vicinity. Nothing over the field yet, and a reason to expect windshear. - BLSN — blowing snow, visibility down at eye level even when the sky is clear. - MIFG — shallow fog. The runway can be invisible from the flight deck while the tower reports good visibility. - SHRA BKN008 — showers with a broken layer at 800 ft; the classic marginal-approach combination. Our decoder handles all of these and prints the expansion in your language. It parses standard formats and can be wrong about a truncated or non-standard report — always read the raw text alongside it. Q: What does CAVOK mean? A: Ceiling and Visibility OK — a defined state, not a general description. It requires visibility of 10 kilometres or more, no cloud below 5,000 feet or below the highest minimum sector altitude, whichever is greater, no cumulonimbus or towering cumulus at any height, and no significant weather. When those conditions are met, CAVOK replaces the visibility, weather and cloud groups in one word. Q: What does FZRA mean in a METAR? A: Freezing rain: rain falling as liquid through air below freezing, which freezes on contact with the airframe or the runway. It is one of the most hazardous conditions reported, because it produces clear ice quickly over large areas of the wing, and it also produces some of the worst braking action on the ground. FZDZ, freezing drizzle, is the same mechanism with smaller droplets and is nearly as serious. Q: How are METAR weather codes constructed? A: From four small vocabularies read left to right: an intensity marker (- for light, nothing for moderate, + for heavy, or VC for in the vicinity), a descriptor (MI, BC, PR, DR, BL, SH, TS, FZ), then one to three phenomena (RA, SN, GR, FG and so on). Any part can be absent. That is why you can read a combination you have never seen: +TSRA is simply heavy, thunderstorm, rain, and BLSN is blowing snow. ## How to Read a TAF: Change Groups, Windows and Probability https://aviationwheater.siten.co/guides/how-to-read-a-taf Updated 2026-08-06 · METAR and TAF decoding - A TAF is a baseline plus change groups, not a sequence of future METARs — every group after the first edits what came before. - The validity window is the group people skip; outside it the forecast says nothing. - BECMG and FM are permanent, TEMPO and PROB are not, and confusing the two families is what leaves aircraft short of fuel. - PROB30 and PROB40 are the only probability values used, because anything likelier is written as BECMG or TEMPO. - Always re-check for an AMD before departure — amendments cluster exactly when conditions are deteriorating. People who can read a METAR often stall on a TAF, and the reason is a category error: they read it as a sequence of future METARs. It is not. A TAF describes a baseline and then the ways that baseline is expected to change, and every group after the first is an amendment to what came before it. Once you read the change groups as edits rather than as separate observations, the format becomes straightforward — and considerably more informative, because it tells you not just what is expected but how confident the forecaster is. ### What a TAF is and is not A TAF is a forecast of conditions in the vicinity of a single aerodrome, generally within about five nautical miles, for a defined validity period — usually 24 or 30 hours, issued four times a day. It includes only weather expected to be operationally significant. That is why a TAF can look sparse next to a METAR: everything not mentioned is expected to stay as described in the prevailing conditions. - Covers one aerodrome and its immediate vicinity, not a region. - Has an explicit validity window; outside it the forecast says nothing at all. - Mentions only significant weather — the absence of a group is information. - Is amended whenever the forecast changes materially, and the amendment replaces the original entirely. An amended TAF carries AMD. When one appears, discard your reading of the previous issue completely rather than merging them. ### The header and the baseline The first line sets the scene: station, issue time, validity period, then the prevailing conditions at the start. EGLL 121103Z 1212/1318 24012KT 9999 SCT030 means Heathrow, issued at 11:03 on the 12th, valid from 12:00 on the 12th to 18:00 on the 13th, initially wind from 240 at 12 knots, visibility 10 km or more, scattered cloud at 3,000 feet. | Station and issue | EGLL 121103Z | Heathrow, issued 11:03 UTC on the 12th | | Validity | 1212/1318 | From 12:00 on the 12th to 18:00 on the 13th | | Prevailing wind | 24012KT | From 240° at 12 knots | | Prevailing visibility | 9999 | 10 km or more | | Prevailing cloud | SCT030 | Scattered at 3,000 ft | The validity period is the group people skip and then misread the whole forecast. A TAF outside its window is history. ### The change groups, and what each one commits to | BECMG 1418/1420 | A permanent change occurring gradually within that window | Assume the new conditions from the end of the window onward | | TEMPO 1500/1504 | Fluctuations lasting under an hour each, less than half the period | Plan for it happening; do not plan on it persisting | | PROB30 1600/1620 | 30 per cent probability of the stated conditions | Significant enough to carry fuel for, not enough to rely on | | PROB40 TEMPO | 40 per cent chance of temporary conditions | The most cautious group in common use | | FM121500 | From that time, a rapid and complete change | Everything before it is superseded at that minute | BECMG and FM are permanent; TEMPO and PROB are not. Mixing those two families up is the most common misreading of a TAF, and it is the one that leaves an aircraft short of fuel. ### Probability, honestly read PROB30 and PROB40 exist because forecasters are being explicit about uncertainty, and no higher value is used — a 50 per cent or greater expectation is stated as BECMG or TEMPO instead. That makes PROB groups unusually informative: their presence means the forecaster considered the scenario likely enough to write down but was not willing to commit to it. - PROB30 TSRA on an afternoon TAF is the standard summer convective hedge. - PROB40 0400 FG on a clear autumn evening should be read as a serious fog warning. - No PROB group does not mean no risk, only that nothing crossed the threshold for one. - Two competing scenarios sometimes appear as a BECMG plus a PROB — read them as the forecaster showing their working. ### Reading one for a real decision - Check the validity window covers your arrival time, plus the holding you would realistically do. - Read the baseline. This is what you get if nothing else in the TAF happens. - Apply the permanent groups in order — FM and BECMG — to build the expected conditions at your time. - Layer the TEMPO and PROB groups on top as things that may happen at your time, and check whether the worst of them is still workable. - Compare the result against the minimum for the approach you expect, and against your alternate’s own TAF. - Re-check for an AMD before departure. Amendments cluster exactly when conditions are changing. Step six is the one that catches people. A TAF read two hours ago may have been replaced twice since, and amendments are issued precisely when the situation is deteriorating. ### Why TAFs are wrong, and in which direction A TAF is a forecast and inherits every limitation of one. Frontal timing is often out by a couple of hours even inside a day. Radiation fog is notoriously hard to time and easy to under-forecast in strength. Convection can be forecast as a probability for a region but not placed on a specific aerodrome a day ahead. Forecasters know all of this, which is why TAFs in uncertain situations widen rather than sharpen — a vague-looking TAF is usually an honest one. Where a TAF is systematically conservative, that is deliberate. The cost of an under-forecast approach minimum is much higher than the cost of carrying extra fuel. Q: What is the difference between a TAF and a METAR? A: A METAR is an observation of what is happening at an aerodrome now; a TAF is a forecast for that aerodrome over a stated period, usually 24 or 30 hours. A METAR describes the whole picture each time it is issued. A TAF describes a baseline and then the changes expected to it, in change groups, and mentions only weather significant for operations. Reading a TAF as a list of future METARs is the most common mistake and leads directly to misjudging fuel. Q: What do BECMG, TEMPO and PROB30 mean in a TAF? A: BECMG marks a permanent change happening gradually within the stated window — after it, assume the new conditions. TEMPO marks fluctuations that last under an hour at a time and cover less than half the period; plan for them occurring but not persisting. PROB30 and PROB40 give the probability of the stated conditions, and no higher figure is used, because anything more likely is written as BECMG or TEMPO. The permanent group family and the temporary one are planned against completely differently. Q: How far ahead is a TAF reliable? A: It varies by phenomenon rather than by hours. Wind and general cloud trends hold up reasonably across the whole validity period. Frontal timing is often uncertain by several hours even within 24. Radiation fog is hard to time and easy to under-forecast. Individual thunderstorm placement is not knowable a day ahead at all, which is why convection appears as PROB groups and why radar in the last hours matters more than yesterday’s forecast. Always check for an amended TAF before you rely on one. ## How to Read a METAR: Every Field, in Order https://aviationwheater.siten.co/guides/how-to-read-a-metar Updated 2026-08-05 · METAR and TAF decoding - A METAR is an observation in a fixed group order — the order never changes and a missing group means nothing to report. - Wind is in degrees true with gusts after a G; a separate 240V300 group shows the direction swinging, which the average hides. - Only BKN and OVC count as a ceiling, and the ceiling is what approach minimums are written against. - The temperature–dew point spread is the derived number that warns about fog; near-freezing with moisture warns about ice. - The operational scan is pressure, wind, ceiling, visibility, present weather, trend — roughly ten seconds. A METAR looks like a password and reads like a sentence. Once you know the order of the groups, the line stops being code: it is an observation written the same way at every aerodrome on the planet, so that a report from Reykjavík and one from Jakarta can be read by the same person without translation. The rule that makes it learnable is that the order never changes. Every group appears in the same place, and any group that is absent means there was nothing to say. ### The worked example Take a routine report and walk through it. EGLL 121250Z 24012KT 9999 FEW035 SCT120 14/09 Q1013 NOSIG says: London Heathrow, on the 12th of the month at 12:50 UTC, wind from 240 degrees at 12 knots, visibility 10 km or more, a few clouds at 3,500 feet and scattered cloud at 12,000 feet, temperature 14 °C with a dew point of 9 °C, pressure 1013 hectopascals, no significant change expected in the next two hours. Nothing in that line is optional decoration. Each group either narrows what can be flown or confirms that nothing is in the way. ### The groups, in the order they appear | Station | EGLL | ICAO identifier of the aerodrome | | Day and time | 121250Z | 12th of the month, 12:50 UTC — always UTC | | Wind | 24012G22KT | From 240° at 12 knots, gusting 22 | | Visibility | 9999 / 1 1/2SM | 10 km or more; or one and a half statute miles | | Present weather | -RA BR | Light rain and mist | | Cloud | BKN012 SCT025CB | Broken at 1,200 ft, scattered cumulonimbus at 2,500 ft | | Temperature / dew point | 14/09 | 14 °C, dew point 9 °C | | Pressure | Q1013 / A2992 | QNH 1013 hPa; or 29.92 inHg | | Trend | NOSIG / TEMPO 3000 RA | No significant change; or a temporary deterioration | AUTO before the wind group means there was no human observer. COR means the report replaces an earlier one — read the corrected version and discard what you remember. ### Wind, and why the small print matters The wind group carries more decisions than any other. Direction is given in degrees true to the nearest ten, speed in knots, and a gust is appended after a G. VRB replaces the direction when the wind is genuinely variable below six knots. A separate group such as 240V300 means the direction has been swinging between those bearings, which matters far more than the average suggests: a runway that is inside limits at 240° may not be at 300°. - 00000KT — calm. - VRB03KT — variable direction, three knots. - 24012G28KT — from 240° at 12, gusting 28; the gust is the number the crosswind calculation uses. - 240V300 — direction varying across that arc. - P99KT — speed above 99 knots, which is rare and never routine. Wind in a METAR is degrees TRUE. The runway numbers and the tower are magnetic. In most of the world the difference is small; near the poles it is not. ### Cloud, coverage and the ceiling Cloud is reported as coverage plus height in hundreds of feet above aerodrome elevation, lowest layer first. Coverage is measured in oktas — eighths of the sky. Only BKN and OVC count as a ceiling, and the ceiling is what most approach minimums are written against. | FEW | 1–2 oktas | No | | SCT | 3–4 oktas | No | | BKN | 5–7 oktas | Yes | | OVC | 8 oktas | Yes | | NSC / NCD | No significant cloud / none detected | No | | VV003 | Sky obscured, vertical visibility 300 ft | Treated as a ceiling | CB and TCU are appended to a layer — SCT025CB — and they are the only cloud types a METAR reports, because they are the only two that change what you do. ### Temperature, dew point and what they warn about The pair is written temperature/dew point in whole degrees Celsius, with M for minus. The gap between them is the single most useful derived number in the report. A small and shrinking spread means the air is close to saturation and fog or low stratus is likely — a spread under about 3 °C on a clear, calm evening is the classic radiation fog setup. A temperature near or below freezing with visible moisture is the icing warning. - 14/09 — a 5 °C spread, no immediate fog concern. - 09/09 — saturated; if visibility is still good, it is about to stop being good. - M03/M05 — minus three, dew point minus five: cold enough to matter in cloud. - A widening spread through the morning is the fog burning off. Dew point never exceeds temperature. If you read a report where it appears to, you have mis-split the group. ### Reading one in ten seconds With practice nobody reads a METAR left to right. The operational scan is: pressure setting, wind against the runway, ceiling, visibility, anything unusual in present weather, then the trend. Everything else is context. - QNH — set the altimeter correctly before anything else. - Wind — runway in use and the crosswind component. - Lowest BKN or OVC layer — is there a ceiling, and where? - Visibility — is the approach you want still legal? - Present weather — TS, FZRA, +SN and windshear reports change the plan, not just the numbers. - Trend — is this improving or falling apart within the hour? Our decoder on the region pages does exactly this scan and prints it in your language. It is a reading aid, not a substitute for reading the raw line yourself. Q: What does METAR stand for? A: It comes from the French météorologique aviation régulière — routine aviation weather observation. It is an observation, not a forecast: it describes measured conditions at an aerodrome at a stated time, normally issued every half hour or every hour. When conditions change significantly between routine issues, a SPECI is put out in the same format. Q: How do I decode a METAR quickly? A: Read it in the order the groups appear, because the order never changes: station, day and time in UTC, wind, visibility, present weather, cloud from the lowest layer up, temperature and dew point, pressure, then the trend. Anything missing simply means there was nothing to report. In practice most people scan rather than read — pressure setting, wind, ceiling, visibility, present weather, trend — which takes about ten seconds once the groups are familiar. Q: What does 9999 mean in a METAR? A: Visibility of 10 kilometres or more. Visibility is reported in metres in four digits, so 0800 is 800 metres and 4000 is 4 kilometres; 9999 is the top of the scale rather than a literal 9,999 metres. North American reports use statute miles instead, written as 10SM or 1 1/2SM, and CAVOK replaces both the visibility and cloud groups when visibility is 10 km or more with no significant cloud or weather. ## Where Aviation Weather Actually Comes From (and Which Sources Count) https://aviationwheater.siten.co/guides/aviation-weather-sources Updated 2026-08-05 · Aviation weather basics - There is no global aviation weather service: each state designates its own authority, which is why the authoritative source changes with the airspace. - Three layers issue the products — the aerodrome service, the meteorological watch office, and the global WAFCs and VAACs. - Consumer apps show real data but rarely guarantee amendments, usually omit SIGMETs and NOTAMs, and silently drop stations that failed to parse. - Three questions settle any source: who issued it, what period it covers, and whether it has been amended. - This site explains the products and links the responsible service; it never issues, relays or aggregates them. There is no global weather service. There is a treaty, a set of formats, and about a hundred and ninety national meteorological services that agree to produce the same products in the same code so that a report from Jakarta reads the same way as a report from Reykjavík. That structure explains something that confuses people who arrive from consumer weather apps: why the authoritative source changes depending on where you are flying, and why an app that shows you a METAR is not the same thing as a briefing. ### Who issues what Under the ICAO framework, each state designates a meteorological authority responsible for aviation weather in its airspace. That authority runs the observations at its aerodromes, issues the TAFs, and operates the watch office that puts out SIGMETs. Two further layers sit above it: World Area Forecast Centres produce the global upper-wind and significant-weather data used for route planning, and Volcanic Ash Advisory Centres cover ash for defined regions. | Aerodrome | Designated national service | METAR, SPECI, TAF, aerodrome warnings | | Airspace | Meteorological watch office | SIGMET, AIRMET, volcanic ash and tropical cyclone SIGMETs | | Global | World Area Forecast Centres (London, Washington) | Upper wind and temperature, significant weather charts | | Volcanic | Nine Volcanic Ash Advisory Centres | Ash advisories and forecast ash cloud positions | | Tropical | Tropical Cyclone Advisory Centres | Cyclone position and forecast track for aviation | This is why our region pages link a different service for each part of the world. There is no single authoritative site, by design. ### The services you will actually meet - United States — the NOAA Aviation Weather Center publishes METAR, TAF, SIGMET and graphical products free and without registration. - Canada — NAV CANADA operates the aviation weather and flight planning service. - United Kingdom — the Met Office provides aviation services, including the London VAAC for volcanic ash. - Germany — DWD runs a pilot briefing service covering central Europe. - India — the India Meteorological Department issues the reports; AAI publishes the aeronautical information. - Brazil — REDEMET, operated by the air force, is the aviation weather portal. - Japan — the Japan Meteorological Agency, including the Tokyo VAAC. - Australia and New Zealand — the Bureau of Meteorology and MetService respectively. Availability differs sharply. Some services publish everything openly on the web; others require a registered account tied to a licence, and a few make raw data available only through a national briefing system. ### Why a weather app is not a briefing Most consumer and enthusiast apps display genuine data — they pull the same METARs and TAFs from public feeds. What they usually do not do is guarantee currency, completeness or the inclusion of the warnings, NOTAMs and amendments that a briefing contains. A TAF that was amended eight minutes ago may not have propagated. A SIGMET may not be shown at all. And an aggregated map often silently drops stations whose reports failed to parse. - No guarantee of currency — feeds lag, and amendments are the ones that matter. - Warnings often missing — SIGMET and AIRMET coverage in third-party apps is inconsistent. - No NOTAMs, so a closed runway or an unserviceable approach aid does not appear. - Silent gaps — a station that failed to parse simply does not show, which looks identical to good weather. - No record — a briefing you obtained through the official channel is logged; an app screen is not. None of that makes the apps useless. It makes them a supplement to a briefing rather than a replacement for one, which is exactly how experienced users treat them. ### Reading a source critically Three questions settle whether what you are looking at can be relied on, and they take about ten seconds. - Who issued it? A named national service or WAFC, or an aggregator that does not say? - When was it issued, and what period does it cover? A TAF outside its validity window is history, not forecast. - Has it been amended? An amended TAF supersedes the original entirely, and amendments cluster exactly when conditions are changing. If a display cannot answer all three, treat it as an indication and go to the source before it matters. ### What this site is, in these terms We are none of the above. We do not observe, forecast, relay or aggregate; we explain what the products contain and how to read them, and we link the responsible service for each region so that you can go to it directly. The decoder on our region pages parses text you paste in, in your browser, and never contacts anything. That is a deliberate boundary. Explaining a report is educational; distributing one carries obligations we are not authorised to take on. Q: Who provides official aviation weather? A: The meteorological authority designated by each state for its own airspace — NOAA’s Aviation Weather Center in the United States, NAV CANADA in Canada, the Met Office in the United Kingdom, DWD in Germany, the IMD in India, REDEMET in Brazil, JMA in Japan, the Bureau of Meteorology in Australia. Above them sit the two World Area Forecast Centres for global upper-air data and nine Volcanic Ash Advisory Centres. There is no single global aviation weather service; the system works because everyone issues the same formats. Q: Can I use a weather app to plan a flight? A: As a supplement, not as the briefing. Most apps show genuine METARs and TAFs, but they rarely guarantee that an amendment has propagated, often omit SIGMETs and AIRMETs entirely, never include NOTAMs, and can silently drop a station whose report failed to parse — which on screen looks exactly like good weather. Flight planning has to rest on an official briefing obtained through the recognised channel for that airspace; the app is for the quick look. Q: Is aviation weather data free? A: Usually yes for the core text products. METARs and TAFs are exchanged internationally under WMO agreements and most national services publish them openly; the United States in particular puts everything on the public web without registration. Graphical products, high-resolution model output and some briefing systems can require an account or a subscription, and a few states restrict access to licence holders. The raw observation and forecast you need to read a situation is almost always free. ## How Weather Affects Flights: Delays, Diversions and Cancellations https://aviationwheater.siten.co/guides/how-weather-affects-flights Updated 2026-08-04 · Aviation weather basics - Most weather reduces capacity rather than stopping flights, and a queue that grows for three hours does not clear when the weather does. - A flight is exposed to weather in at least four places, three of which are invisible from your terminal. - Diversions are triggered mainly by the destination falling below approach minimums while the aircraft is committed. - Outright cancellations come from disruption that is forecast days out: cyclones, major snow events and volcanic ash. - Earlier departures and direct routings are the only real passenger-side levers, plus acting on an airline waiver early. Weather is blamed for more delays than any other single cause, and almost nobody outside the industry knows why a particular delay happened. The board says weather. The sky outside the window is blue. The explanation is that flights are a network, the limiting weather is usually somewhere else, and most weather does not stop aircraft — it slows the rate at which they can be handled. That distinction explains nearly everything a passenger finds confusing. ### Rate, not stoppage Very little weather closes an airport. What weather does is reduce capacity, and capacity is the thing that produces delay. On a clear day a busy runway might accept an arrival every ninety seconds. In low visibility, separation on approach increases and the spacing on the ground grows, so the same runway takes perhaps two-thirds as many. Nothing has stopped; the queue has simply started growing faster than it drains, and a queue that grows for three hours does not clear when the fog does. - Low visibility procedures increase separation and cut the arrival rate. - Snow clearance closes one runway at a time, halving movements for the duration. - De-icing adds fixed minutes per departure and there is a finite number of rigs. - Strong or gusting crosswind slows approaches and forces occasional go-arounds. - Convection removes route segments, so departures are metered onto the remaining ones. This is why delays peak hours after the weather does. The atmosphere recovers quickly; a schedule does not. ### Why your clear sky is irrelevant A flight touches at least four weather locations: departure aerodrome, en-route airspace, destination and the nominated alternate. It also touches the aircraft’s own history — where it was this morning and whether it arrived on time. Any one of those can be the constraint. | Departure aerodrome | De-icing, low visibility taxi, runway change | Yes | | En route | Reroute around convection, extra fuel, longer flight time | No | | Destination | Holding, slot restriction, arrival metering | No | | Alternate | Extra fuel required, sometimes a payload reduction | No | | Aircraft’s previous sector | Late arrival cascades into your departure | No | Roughly the last three rows are invisible from the terminal, and they cause the majority of the delays passengers describe as inexplicable. ### What actually causes a diversion Diversions are rarer than they feel and the triggers are specific. The most common is that conditions at the destination fall below the minimum for the approach in use while the aircraft is already committed, and holding fuel runs out before the weather lifts. The second most common is a runway becoming unusable — contamination, a disabled aircraft, a wind shift beyond limits. Severe convection sitting over the aerodrome produces the dramatic ones. - Conditions drop below the applicable approach minimum and do not recover within the holding fuel available. - Windshear or a crosswind beyond the aircraft or operator limit on every usable runway. - Runway contamination that pushes the landing distance required beyond what is available. - A thunderstorm cell parked over the aerodrome, where the correct answer is to leave and come back. - An aerodrome closure ahead of arrival — snow clearance, an incident, an unrelated emergency. A diversion is a normal, planned outcome, not a failure. Every flight departs with a nominated alternate and the fuel to reach it, precisely because this is expected to happen sometimes. ### The weather that cancels rather than delays Outright cancellation usually means the disruption is expected to last longer than the schedule can absorb, and airlines increasingly cancel in advance rather than let a day collapse in real time. Tropical cyclones, major winter storms and volcanic ash are the three that reliably produce pre-emptive cancellation, because all three are forecast days out and all three make the airport unusable rather than slow. - Tropical cyclone or typhoon landfall — airports close for a day or more, and the network is reshaped either side. - A major snow event — clearance capacity is the binding constraint, not the aircraft. - Volcanic ash in the flight levels — engines cannot ingest it, and the affected airspace simply closes. - Extreme heat at high-altitude airports — some aircraft cannot depart at any useful weight. - Widespread freezing rain — de-icing cannot keep pace and holdover times collapse. ### What you can do with this as a passenger Not much about the weather, but a fair amount about the day. The earliest departures suffer least from cascading delay because the aircraft is already in place overnight. A direct flight removes an entire connection’s worth of exposure. And when disruption is forecast, airlines almost always publish a waiver policy before it starts, which is the cheapest moment to move. - Book earlier flights when a disrupted day is forecast; the cascade builds through the afternoon. - Prefer direct routings during storm or snow season — every connection is another chance to miss one. - Check whether the airline has issued a travel waiver before the day; rebooking is free and easy at that stage. - Read the destination and alternate weather, not just your own airport. None of this is a prediction about a specific flight. It is only where the odds sit, and the odds are what a schedule is built on. Q: Why is my flight delayed when the weather here is fine? A: Because your airport is only one of at least four weather locations that matter: departure, en route, destination and the alternate — plus wherever your aircraft was earlier in the day. Thunderstorms across the route, fog at the destination or a runway closed for snow clearance ahead of your arrival will all delay a flight departing into a clear blue sky. The single most common cause is simpler still: the aircraft was delayed by weather on a previous sector, and that delay follows it through the day. Q: What weather causes the most delays? A: Thunderstorms and low visibility, in most networks. Convection has the biggest en-route impact because cells are avoided laterally and a squall line can shut a whole corridor for hours. Low ceilings and low visibility have the biggest airport impact because they cut the arrival rate, which builds a queue that keeps growing long after the fog has cleared. In northern-hemisphere networks, winter contamination and summer convection produce the two annual delay peaks. Q: Why did my flight divert when the weather looked flyable? A: Almost always because the destination fell below the minimum for the approach in use while the aircraft was already airborne, and the fuel available for holding ran out before conditions recovered. Other triggers are a runway becoming unusable through contamination or a wind shift, and a thunderstorm sitting over the aerodrome. A diversion is a planned outcome rather than a failure — every flight departs with a nominated alternate and the fuel to reach it for exactly this reason. ## What Is Aviation Weather? The Five Numbers That Decide a Flight https://aviationwheater.siten.co/guides/what-is-aviation-weather Updated 2026-08-04 · Aviation weather basics - Aviation weather reports the same atmosphere in units that go straight into a decision: feet, oktas, metres, knots, hectopascals. - Five numbers carry most of the weight — cloud base, visibility, wind, temperature and dew point, and pressure. - The METAR is an observation and the TAF is a forecast; charts and SIGMETs cover the route between aerodromes. - Weather rarely stops flights outright — it reduces rates, adds de-icing minutes and closes route segments, which is what a delay board is showing. - Guides explain the reports; only an official briefing and the pilot in command can decide whether a flight goes. A public forecast says cloudy with a chance of rain. That sentence is useless to a flight, and not because it is wrong — because it answers a question nobody in an operations room is asking. Aviation weather reports the same atmosphere in the terms that actually decide something: how high the cloud starts, how much of the sky it covers, how far you can see along the runway, which way the wind is blowing and how hard, and what the pressure is set to. Five numbers, and between them they determine whether an approach is legal, how much runway is needed, and whether ice will form on the wing. ### Why aviation needs its own weather Everything an aircraft does is a margin calculation, and weather moves the margins. A wing produces less lift in hot, thin air, so a summer afternoon can mean leaving passengers or fuel behind. A wet runway lengthens the distance needed to stop. A cloud base at 400 feet makes an approach that is perfectly routine at 1,400 feet illegal for most operators. None of that is visible in a forecast written for the public, which is why aviation has its own products, its own units and its own vocabulary. - Cloud base is reported in hundreds of feet above the aerodrome, not in vague terms like ‘low cloud’. - Coverage is measured in oktas — eighths of the sky — because the amount decides whether a layer counts as a ceiling. - Visibility is reported in metres almost everywhere, and in statute miles in North America. - Wind is direction to the nearest ten degrees plus speed in knots, because crosswind is a component you calculate. - Pressure is given as QNH so every altimeter in the area reads the same height above sea level. The unit choices look arbitrary until you notice they all exist to be put straight into a calculation. Nothing in an aviation report is descriptive for its own sake. ### The five numbers, and what each one decides | Cloud base | Layer coverage plus height in hundreds of feet | Whether an approach is legal and which one can be flown | | Visibility | Metres, or statute miles in North America | Approach minimums, taxi procedures, whether VFR is possible at all | | Wind | Direction and speed, plus gusts | Runway in use, crosswind component, performance on takeoff | | Temperature and dew point | Degrees Celsius, with the spread | Icing risk, fog formation, engine and wing performance | | Pressure | QNH in hPa, or altimeter setting in inHg | Altimeter setting, and therefore actual height above terrain | The temperature and dew point pair is the one non-pilots overlook. When the two converge, fog is close; when the temperature is near freezing in visible moisture, ice is close. ### The two products everything else hangs off Aviation weather is delivered through a small number of standard products, and two of them carry most of the daily weight. The METAR is an observation: what is measured at the aerodrome right now, issued every half hour or hour, with a SPECI in between when something changes sharply. The TAF is a forecast for the area around that aerodrome, usually covering 24 or 30 hours, written only in terms of what is expected to be operationally significant. - METAR — observed conditions at an aerodrome, the raw material of every briefing. - TAF — aerodrome forecast, written in change groups rather than a continuous description. - SIGMET and AIRMET — warnings of hazardous phenomena en route, issued by the responsible watch office. - Significant weather charts — the graphical picture of fronts, jets and hazard areas at cruise levels. - Volcanic ash advisories — issued by the VAAC, and the only source that matters for ash. A briefing is not one product. It is the sequence: the charts for the big picture, the TAFs for the aerodromes, the METARs for what is actually happening, and the warnings for anything moving. ### How weather becomes a delay Passengers experience weather as a delay board, and the chain behind it is short and mechanical. Low visibility does not stop aircraft landing; it reduces the rate at which they can land, because separation on approach has to increase. A runway with standing water reduces the accelerate–stop margin, so departures slow down. De-icing adds a fixed number of minutes to every departure and there are only so many rigs. Thunderstorms do not close airports so much as close route segments, and a line of cells can shut a corridor for hours. | Low ceiling or visibility | Approach rate falls | Holding, then rolling delay across the day | | Thunderstorms en route | Route segment unusable | Long taxi waits and reroutes | | Snow and ice | De-icing time added per departure | Departure slots collapse | | Strong crosswind | Runway changes or arrival rate falls | Go-arounds, occasional diversion | | High temperature | Performance limited | Weight restrictions, offloaded cargo | ### Where it stops being a website subject Everything above can be learned from published documentation, and this site exists to explain it. What cannot be learned from a website is whether a specific flight should go. That decision rests with the pilot in command, on an official briefing from the meteorological service designated for the airspace, against the operator’s own minimums and the aircraft’s limitations. Read guides to understand what the reports mean. Read the official briefing to fly. Any site that blurs the two is doing something it is not allowed to do. Q: What is aviation weather in simple terms? A: It is meteorological information reported specifically for flight, in units that can be put directly into a decision. Instead of ‘cloudy with rain later’, an aviation report gives the height of each cloud layer in hundreds of feet, how much of the sky it covers, the visibility in metres, the wind direction and speed, the temperature and dew point, and the pressure setting. Those numbers determine whether an approach is legal, how much runway is required and whether ice is likely, which is why aviation maintains its own products rather than using public forecasts. Q: What weather information do pilots actually use? A: Four layers of it. Significant weather charts give the big picture — fronts, jet streams, hazard areas at cruise level. TAFs give the forecast for each aerodrome along the route, including the alternate. METARs give what is actually happening now at those aerodromes. SIGMETs and AIRMETs warn about hazardous phenomena in flight, such as severe turbulence, icing or volcanic ash. A briefing works through all four, in that order, because each answers a different question. Q: Why do flights get delayed even when the weather looks fine? A: Because the limiting weather is usually somewhere else. A clear sky at the departure airport tells you nothing about a line of thunderstorms sitting across the route, fog at the destination, or a runway closed for snow clearance two hours ahead of your arrival. Network effects do the rest: an aircraft delayed by weather at 07:00 in one country is late for its next four sectors, so a calm afternoon at your airport can still produce a two-hour delay.