Reading METAR for drone pilots

A METAR is a coded weather observation from an airport, usually issued every 30 or 60 minutes. For drone pilots the useful parts are the wind and gusts (measured about 10 m above the ground), visibility, cloud, and temperature with dew point. This guide decodes each group, with worked examples, and explains how Altiwind uses METARs to check its forecast.

Anatomy of a METAR

Here is a made-up example in the standard international format:

EGLL 221050Z 24012G25KT 9999 SCT030 BKN045 14/08 Q1012

Wind

The wind group is the one drone pilots read first.

• Direction is where the wind blows from, in degrees true. 240 is from the south-west.
• Speed is usually in knots (KT). A few countries report in metres per second (MPS).
• Gust follows a G. In 24012G25KT the mean is 12 kt (6.2 m/s, 22 km/h, 14 mph) and the gust 25 kt (12.9 m/s, 46 km/h, 29 mph).
• 00000KT means calm. VRB03KT means light wind from a variable direction.
• 210V270, after the wind group, means the direction is swinging between 210° and 270°.

Gusts are only shown when they are 10 kt or more above the mean, so no "G" does not mean smooth air. The mean wind covers the last 10 minutes in the international format; US stations use a 2-minute mean. Our guide to gusts vs sustained wind explains the difference.

Quick conversions: 1 kt = 0.514 m/s = 1.852 km/h = 1.15 mph. The wind speed converter does the rest.

Visibility and weather

• Four digits in metres: 9999 is 10 km or more, 3200 is 3,200 m, 0000 is under 50 m.
• US reports use statute miles: 10SM is 10 miles (16 km), 1/2SM is half a mile.
• Weather codes: RA rain, DZ drizzle, SN snow, SH showers, TS thunderstorm, BR mist, FG fog, HZ haze. - means light and + means heavy, so +SHRA is heavy rain showers. FZ means freezing, as in FZFG (freezing fog).

Altiwind's visibility check uses 5 km and 1.5 km as its lines: 5 km or more is within your limits, 1.5 km to 5 km is marginal, below 1.5 km is outside your limits.

Cloud

• Amount in eighths of the sky (oktas): FEW 1–2, SCT 3–4, BKN 5–7, OVC 8.
• Height in hundreds of feet above the airport: BKN004 is 400 ft, about 120 m. That is cloud at drone height.
• VV001 means the sky is hidden, with vertical visibility of 100 ft, usually in fog.
• NSC means no significant cloud and NCD no cloud detected by an automatic station.

Temperature and dew point

14/08 is 14 °C with a dew point of 8 °C. An M means minus: M02/M09 is −2 °C and a dew point of −9 °C. When the two numbers are close and getting closer, fog or low cloud is likely; see fog, rain and humidity.

CAVOK

CAVOK means "ceiling and visibility OK": visibility of 10 km or more, no cloud below 5,000 ft (or the highest minimum sector altitude, if higher), no cumulonimbus or towering cumulus, and no significant weather.

CAVOK says nothing about wind. A beautiful CAVOK day can still be far too windy for a small drone.

Worked examples

Foggy morning: LFPG 220630Z 00000KT 0200 FG VV001 04/04 Q1024

Calm wind, visibility 200 m in fog, sky hidden, and temperature equal to dew point at 4 °C. The wind is no problem; the visibility is well outside your limits.

Cold and clear, US style: KDEN 221853Z 33008KT 10SM FEW120 M02/M09 A3012 RMK AO2

Wind from 330° at 8 kt (4.1 m/s, 15 km/h, 9 mph), visibility 10 miles, a few clouds at 12,000 ft, −2 °C with a dew point of −9 °C. A3012 is pressure in inches of mercury (30.12) and AO2 marks an automated station. Light wind and good visibility, but below 5 °C, so Altiwind shows its cold-battery warning.

CAVOK but windy: LPPT 221330Z 32018G28KT CAVOK 22/12 Q1018

Perfect sky, but the wind is 18 kt (9.3 m/s, 33 km/h, 21 mph) gusting 28 kt (14.4 m/s, 52 km/h, 32 mph). Even at 10 m, those gusts are above the rated maximum of a typical 12 m/s mid-size drone.

What a METAR can and cannot tell you

• It is measured at 10 m. The WMO standard puts wind sensors 10 m above open, level ground. The wind at 120 m is usually stronger.
• It is measured at the airport. Airports are flat and open. Your spot may be more sheltered, or more exposed on a hill or a coast.
• It is a past observation. The latest report can be up to an hour old, although airports also issue special reports (SPECI) when the weather changes significantly.

How Altiwind uses METARs

Altiwind reads METARs from the nearest airport within 25 km, via aviationweather.gov. For its verified badge, it compares the last 6–24 hours of reported wind and gusts with 2–3 forecast models, picks the model with the smallest error for that area, and shows the bias, for example: "Checked vs EGLL 40 min ago: running 3 km/h high". See verified forecast.

Sources

• Met Office, How to decode a METAR
• NOAA Aviation Weather Center, METAR data
• NOAA/NWS, ASOS User's Guide (US wind averaging and gust rules)
• WMO, Guide to Instruments and Methods of Observation (WMO-No. 8), Volume I, Chapter 5 (10 m wind exposure)