Airport weather and METAR decoder

A weather report cannot answer that, and any tool that says yes or no is inventing. A METAR describes the weather; the decision belongs to the airline and to air traffic control, and it depends on things the report does not contain: where the aircraft currently is, whether the crew is within duty hours, whether the alternate is open. What the report does tell you is how close conditions are to approach limits — and that is stated here in words: the condition category, the cloud base and the gap between temperature and dew point.

They are condition categories computed from two numbers: visibility and the height of the lowest broken or overcast layer. VFR means visibility over 5 miles and cloud above 3,000 feet, so visual flight is possible. MVFR is 3–5 miles or 1,000–3,000 feet. IFR is 1–3 miles or 500–1,000 feet, instrument approaches only. LIFR is below a mile or under 500 feet, right at the limit. For a passenger the letter matters less than the consequence: at IFR, landing intervals stretch and delays become likely.

The dew point is the temperature at which air becomes saturated and moisture starts to condense. When the actual temperature falls to it, fog forms. A one-degree gap means a night of cooling is enough to hide the runway. That is why the two numbers sit side by side in the report, and why they predict morning delays better than a general weather forecast does.

Major airports issue a METAR every half hour, plus an off-schedule SPECI when the weather changes sharply. Smaller aerodromes report less often, and some transmit nothing at all. We always show the age of the observation and flag anything older than ninety minutes: the weather may have changed completely, and presenting it as current would be untrue.

Not every airport has a weather station feeding the international exchange. That is especially true of small regional aerodromes and of airports in countries that do not publish openly. It is not a tool failure: the station simply is not there, and we say so rather than showing you the weather from a neighboring field.

Visibility of 10 kilometers or more — the maximum the METAR format encodes. Lower values are written in meters as four digits: 0800 is eight hundred meters. American reports use statute miles instead, so there you will see 10SM or 1/2SM. The tool shows whichever the original used rather than silently converting.

Ordinary visibility is measured across the horizon generally; RVR is measured along one specific runway by instruments beside its lights. In fog the two diverge several-fold, and the decision to approach is made on RVR. That is why it appears only when it becomes critical — its very presence is a signal.

A METAR is an observation: what is happening right now. A TAF is an aerodrome forecast covering six to thirty hours ahead, stating when and how the weather will change. It contains groups such as BECMG (gradual change), TEMPO (temporary) and PROB30 (thirty per cent probability) — all decoded here in the same way as the report itself.

Yes, but they are two different forecasts and the difference matters. The aviation forecast (TAF) is issued by the aerodrome's weather service for six to thirty hours ahead — it is the time band above, and it carries cloud base and visibility, the very numbers that delay flights. Beyond that horizon no aerodrome forecast exists: the services do not issue one. So the week below is an ordinary model forecast for the airport's coordinates: temperature, wind, chance of precipitation. It is good for planning a trip, but no approach is decided on it, and we label it as a separate source rather than passing it off as aviation weather.

Aircraft limits are set by the component across the runway, not by the strength of the wind. The same line "23015G25KT" is almost straight down runway 06/24 and bothers nobody, while on runway 18/36 it is a 15-knot crosswind gusting 25 — the limit for many types. That is why the "Runways and wind" tab resolves the wind onto every landing direction. Runway headings there are true, matching METAR: runway numbers are magnetic, and subtracting one from the other directly would be wrong by 11 degrees in Moscow and up to twenty-five in Norway and Iceland.