Wind
What Is a Squall? How Sailors Spot and Handle Them
Published · Last checked October 2026
A squall is a sudden, sharp increase in wind, typically by 16 knots or more to at least 22 knots, lasting a minute or more, often with heavy rain and a shift in direction. At sea, squalls usually come from cumulonimbus clouds or along a cold front. Sailors spot them by a dark, hard-edged cloud base and a line of disturbed water, and prepare by reefing early.
What causes a squall
Most squalls come from towering cumulonimbus clouds. Rain and cold air falling inside the cloud spread out at the surface as a gust front, giving a burst of wind well ahead of the rain. Squalls also form in lines along active cold fronts, called line squalls, and in showery airstreams behind depressions.
A squall typically lasts minutes rather than hours, but the wind can double in a few seconds and shift direction by 45 degrees or more. That combination catches out boats carrying full sail.
How to spot one coming
- Cloud: a dark, low, hard-edged base, sometimes with a roll or shelf cloud along the front. See cloud types for sailors.
- Water: a dark line of ruffled water and white caps approaching across the surface.
- Rain curtain: a grey veil hanging below the cloud, often with the strongest wind just ahead of it.
- Temperature: a sudden drop as cold downdraught air arrives.
- Radar: heavy rain shows as dense patches; track them to see if they will cross you.
- Barometer: a short jump in pressure as the squall passes.
| Gust | Squall | |
|---|---|---|
| Duration | Seconds | Typically a minute or more, often several |
| Cause | Turbulence in the wind | Cumulonimbus or a front |
| Warning | Little | Visible cloud and water, often minutes ahead |
| Typical extras | None | Rain, hail, wind shift, poor visibility |
Handling a squall
- Reef early. As soon as you see a squall that will hit you, shorten sail. At night, reef before it arrives on the basis of radar or a dark sky.
- Get the crew ready. Lifejackets and harnesses on, hatches closed, everyone briefed.
- Decide your heading. Many crews bear away and run off under reduced sail; others luff and depower. Avoid being beam-on with full sail when the first gust strikes.
- Watch the shift. Be ready for the wind to come round suddenly, which can cause an accidental gybe. Rig a gybe preventer when running.
- Keep a lookout. Heavy rain can cut visibility sharply and clutter radar.
- Wait it out. Shake out reefs only once the squall has clearly passed.
The heavy weather sailing lesson and weather awareness lesson cover squalls and showery airstreams in more depth.