Sustained wind is the average over several minutes: 10 minutes under World Meteorological Organization (WMO) practice, 2 minutes at US airport stations. A gust is the strongest burst of a few seconds within that time. Your drone feels the wind second by second, so gusts usually decide whether a flight stays within its limits. Altiwind checks both, with estimated gusts at your flying height.
The definitions
| Term | What it is | Where you see it |
|---|---|---|
| Sustained (mean) wind | Average over 10 minutes, measured 10 m above open ground | WMO standard, most METARs outside the US |
| Sustained wind, US | Average over 2 minutes | US automated airport stations (ASOS) |
| Gust | Highest 3-second average in the period | WMO standard |
| Gust, US | Highest 5-second average | ASOS |
| Peak gust in forecasts | Highest gust in the previous hour | Hourly forecasts, including Open-Meteo's 10 m gusts |
The WMO settled on about 3 seconds as a gust length that suits most users of gust data. Even shorter spikes exist, but averaging over 3 seconds smooths them out.
How gusts appear in METARs
Airport reports only show a gust when the wind is properly gusty:
- ICAO-style METARs (used in the UK and most of the world): the gust is shown only when it is 10 kt or more above the mean wind.
24012G25KTmeans a mean of 12 kt gusting 25 kt. - US automated stations: a gust is reported when the peak beats the lull by 10 kt or more over the last 10 minutes, and the lowest gust ASOS reports is 14 kt.
So a METAR without a "G" does not mean a smooth wind. It means the gusts stayed under the reporting threshold, which can still be several knots above the mean. Our METAR guide decodes the rest of the report.
The gust factor
The gust factor is the gust divided by the mean wind. In 24012G25KT it is 25 ÷ 12, about 2.1.
What makes it bigger:
- Rough surroundings. Towns, forest and broken hills stir up turbulence. Open water and flat grassland give smoother air.
- Daytime heating. The US National Weather Service explains that as the ground warms, faster air from higher up mixes down. Gusty surface winds usually start late morning, peak in the afternoon and die down by early evening.
- Showers and fronts. Sudden, strong gusts often arrive with a passing shower or a front.
Why gusts decide the flight
A drone's rated maximum is a limit on what it can push against. A gust above that number can shove the drone off its position faster than it can correct, at exactly the moment it may be close to a tree or a building.
A worked example, at 10 m:
- Mean wind 6 m/s (22 km/h, 13 mph), gusts 11 m/s (40 km/h, 25 mph).
- For a drone rated 10.7 m/s, the mean is 56% of the rating, inside the 60% line. The gusts are 103% of the rating, outside the 100% line.
Judged on the average alone, it would be within limits. Judged on the gusts, it is outside your drone's limits. That is why Altiwind scores both and lets the worst check decide the colour: sustained wind against 60% and 85% of your drone's rating, estimated gusts against 80% and 100%.
How Altiwind estimates gusts aloft
Weather models give gusts at 10 m, but not at 60 or 120 m. Altiwind estimates them in three steps:
- Take the gust excess at 10 m: the gust minus the mean wind.
- Add it to the wind at your chosen height, 30, 60, 90 or 120 m.
- Never go below 1.2× the wind at that height, or below the 10 m gust.
Why add rather than multiply by the gust factor? Near the ground, friction slows the mean wind much more than it shrinks the gusts riding on it, so the 10 m gust factor is inflated by the slow surface mean. The size of a gust above the mean changes far less with height, so carrying that excess up gives a closer estimate than multiplying the stronger wind aloft by a surface ratio.
Example: at 10 m the mean is 5 m/s and the gust is 8 m/s, an excess of 3 m/s. The model gives 7.1 m/s at 120 m, so the estimated gust there is 7.1 + 3 = 10.1 m/s (36 km/h, 23 mph).
For a drone rated 10.7 m/s, the sustained 7.1 m/s is 66% of the rating and the estimated gust is 94%: both marginal. A drone rated 12 m/s would be within its limits on sustained wind (59%) and marginal on gusts (84%).
The floor covers very smooth air. A steady 8 m/s gusting 8.5 m/s has an excess of only 0.5 m/s; with 11 m/s at 120 m that would give 11.5 m/s, so Altiwind uses 1.2 × 11 = 13.2 m/s.
These are always estimated gusts. On stable nights the ground can be calm while the air a few tens of metres up is moving fast, so surface gusts tell you less about conditions aloft. See how wind changes with height, and the gust forecast feature for how gusts show in the app.
Reading gusts in practice
- Compare the gust, not just the mean, with your drone's rating.
- A big gap between the mean and the gust means turbulent air, even if both numbers look modest.
- Expect gustier air near buildings, tree lines and ridges than the forecast suggests.
- Midday and afternoon are usually the gustiest hours on sunny days.
Sources
- WMO, Guide to Instruments and Methods of Observation (WMO-No. 8), Volume I, Chapter 5 (10-minute means, 3-second gusts, 10 m exposure)
- Met Office, How to decode a METAR (gusts shown when 10 kt or more above the mean)
- NOAA/NWS, ASOS User's Guide (2-minute means, 5-second gusts, reporting rules)
- NOAA/NWS, The Front, February 2014 (daily cycle of surface gustiness)
- Open-Meteo, Weather forecast API documentation (10 m gusts as the maximum of the preceding hour)
FAQ
What is a 3-second gust?
It is the highest wind speed averaged over any 3 seconds in the reporting period. The WMO uses it as the standard gust because it captures the bursts that matter without counting every tiny spike.
Why doesn't the METAR show a gust when it feels gusty?
Gusts only appear when they are well above the mean: 10 kt or more in ICAO-style METARs. Smaller gusts go unreported, and the airport may also be more open than your spot.
Should I go by the gust or the average wind?
Both. Altiwind checks sustained wind against 60% and 85% of your drone's rating and estimated gusts against 80% and 100%. The worse of the two decides the colour.
What is a normal gust factor?
It varies with terrain, time of day and weather. Smooth air over open water or flat land gives low factors; towns, forest and sunny afternoons give higher ones, and factors around 2 are common over land. Altiwind doesn't multiply the wind aloft by this factor; it adds the gust excess (gust minus mean), with gusts never less than 1.2× the wind at your height.