Gusts, lulls and squalls
1The lesson in plain words
The mean wind is an average. Gusts are short bursts above it; the gust factor tells you how much stronger they are.
Mean and gust
Wind is never steady. Here we use the World Meteorological Organization (WMO) convention: mean wind is averaged over 10 minutes. A Beaufort force describes wind strength. A gust is a brief peak measured over about 3 seconds. A lull is a brief drop. Check your forecast service’s averaging period. Choose sail area for the gusts you expect.
Gust factor
The gust factor is the gust divided by the mean wind. With a mean of 20 knots and gusts of 28 knots, the gust factor is 28 ÷ 20 = 1.4. Over the open sea in steady air it is often about 1.2–1.4. Off high land, near cliffs and under showers it can reach 1.5 or more.
Why gusts happen
Gusts come from turbulence, or irregular, swirling air movement. Rough land and buildings disturb the flow. In unstable air, rising and sinking currents bring fast air from higher up down to the surface. That is why cold, showery air is so gusty, while warm, stable air over a cold sea is steadier.
Squalls
A squall is a sudden, longer increase. The WMO definition: a rise of at least 16 knots (8 m/s) to 22 knots (11 m/s) or more, lasting at least a minute. Squalls come with shower clouds, lines of towering thunderstorm clouds and cold fronts, often with a sudden wind shift and heavy rain. Squalls and thunderstorms have their own unit later in this module.
Sailing for the gusts
To reef is to reduce sail area. Choose the sail area for the gusts you expect. Watch upwind for dark patches on the water and for shower clouds: they mark gusts arriving. In gusty offshore winds, stay well off the downwind side of high land.
2Key data
- Mean wind
- WMO convention: 10-minute average
- Gust
- Peak over about 3 seconds
- Gust factor
- Gust ÷ mean; e.g. 28 ÷ 20 = 1.4
- Typical gust factor
- Open sea ~1.2–1.4; off high land or in showers 1.5+
- Squall (WMO)
- Rise of ≥16 kn to ≥22 kn, lasting at least 1 minute
3Key points
- 1
Here, mean wind means a 10-minute average; gusts are brief peaks.
- 2
Gust factor = gust ÷ mean wind.
- 3
Unstable, showery air and high land make gusts stronger.
- 4
A squall: sudden rise of at least 16 kn to at least 22 kn, lasting at least 1 minute.
- 5
Reef for the gusts and watch the water upwind.
4Frequent mistakes
Wrong: A forecast of force 4 means no wind over 16 knots.
Right: Force 4 is a mean of 11–16 knots. Gusts of 20 knots or more are possible, especially in showers.
Wrong: A gust and a squall are the same thing.
Right: A gust lasts seconds. A WMO squall rises suddenly by at least 16 knots to at least 22 knots and lasts at least a minute.
5Practice case & exam keys
The forecast gives a mean north-westerly of 20 knots, gusting 30 knots, with cold air and showers behind a front. You will sail past high cliffs on the downwind side of an island, where the wind blows off the land. What is the gust factor, and how do you prepare?
Show the answer
The gust factor is 30 ÷ 20 = 1.5. Off the cliffs, gusts can be stronger and come down steeply. Reef to handle 30 knots or more, keep well off the cliffs, and watch for shower clouds and dark water upwind.
Exam keys
- WMO convention: Mean wind = 10-minute average; gust = about 3-second peak.
- Gust factor = gust ÷ mean; higher in unstable air, near high land and under showers.
- Squall (WMO): sudden rise of at least 16 kn to at least 22 kn, lasting at least 1 minute.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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