What Is a Squall?
A sudden, sharp burst of wind under a dark cloud — a few minutes can take you from 15 knots to 35, but a well-prepared boat will ride it out with no fuss.
A squall is a sudden and strong increase in wind, usually carried under a rain cloud: within minutes the wind can double or treble, and may shift by up to 90 degrees. It is often short-lived, passing in 15-30 minutes.
The danger is in the speed of arrival rather than the strength itself. The sky will warn you: a darkening base on the horizon, a curtain of rain beneath it, and a dark, rippled band moving across the water towards you. The procedure is straightforward.
Squall procedure
Watch the cloud (darkening base on the horizon, rain curtain, dark band on the water).
Reef or stow sail early; if it only brushes past, you lose five minutes — if it does not, you keep the boat safe.
Secure loose gear on deck and put on waterproofs.
Check your course and anchor; rig a preventer on the boom, set an anchor alarm in a bay, and have the engine ready.
After the squall has passed, wait one minute: a second burst of wind and a brief lull with a wind shift are both common on its trailing edge. Do not hoist sail until the cloud has fully cleared. For boats at anchor, squall-induced veering is the classic trigger for dragging.