Aviation weather, explained line by line

Aviation weather is the same atmosphere reported in a different way, because flights care about different things than picnics do. A public forecast tells you it will be cloudy with a chance of rain. An aviation report tells you the cloud base in hundreds of feet, how much of the sky each layer covers, the visibility in metres or statute miles, the wind direction to the nearest ten degrees, the temperature and dew point, and the pressure setting to a hectopascal — because those five numbers decide whether an approach is legal, how much runway is needed, and whether ice will form on the wing. The two main products are the METAR, an observation of what is happening now, and the TAF, a forecast for a specific aerodrome over a fixed window.

Left to right, in a fixed order: station identifier, day and time in UTC, wind as direction and speed, visibility, present weather, cloud layers from the lowest up, temperature and dew point, pressure, and finally a trend group. So EGLL 121250Z 24012KT 9999 FEW035 SCT120 14/09 Q1013 NOSIG reads as: Heathrow, on the 12th at 12:50 UTC, wind from 240° at 12 knots, visibility 10 km or more, a few clouds at 3,500 ft and scattered at 12,000 ft, 14 °C with a dew point of 9 °C, pressure 1013 hPa, no significant change expected. The decoder on our regions pages will do this for any report you paste, in your own language.

A METAR is an observation and a TAF is a forecast. The METAR describes conditions measured at the aerodrome, normally issued every half hour or hour, with a SPECI in between when something changes sharply. The TAF forecasts conditions in a defined area around the aerodrome for a period that is usually 24 or 30 hours, and it only mentions weather expected to be significant for operations. That is why a TAF looks sparser: it is written in change groups — BECMG, TEMPO, PROB30 — that describe what will alter and when, rather than painting the whole picture again.

It depends on the season and the region, but four causes dominate. Thunderstorms close whole route segments and cause the largest en-route disruption, because cells are avoided laterally and a line of them can shut a corridor for hours. Low visibility and low ceilings reduce the arrival rate at busy airports, which builds delay across the network even when individual flights still land. Snow and ice add de-icing time to every departure and cut the number of movements the runway can take. Strong crosswinds slow arrivals and occasionally force a diversion. In most northern-hemisphere networks, winter contamination and summer convection account for the two annual delay peaks.

No, and it is important to be exact about that. We explain what aviation weather products contain and how to read them; we do not issue, relay or forecast anything. The authoritative source is the meteorological service designated by the state — the Aviation Weather Center in the United States, the Met Office and DWD in Europe, the IMD in India, REDEMET in Brazil, and so on — and each of our region pages links the relevant one. A flight must be planned on an official briefing, and the pilot in command is the only person who can decide whether it goes.

Reliability drops sharply with time, and the useful horizon depends on the phenomenon. Wind and temperature at cruise level are reasonably good three to five days out. Frontal timing — when the rain or the wind shift arrives — is often uncertain by several hours even inside 24 hours, which is why a TAF is amended so frequently. Convection is the hardest: forecasters can say a region is likely to develop storms, but where an individual cell will stand at 16:00 is not knowable a day ahead, which is why the tools that matter for thunderstorms are radar and satellite in the last hours rather than a forecast made yesterday.