Safety guide
Squalls, thunderstorms and CAPE at sea
A squall can triple the wind in two minutes. Knowing how to anticipate them means reefing before it's too late.
What is a squall?
A squall is a convective cell: warm, moist air rises violently, forming a billowing cloud (cumulus then cumulonimbus). Beneath the cloud you find brutal gusts, wind shifts, heavy rain and sometimes lightning. The wind can veer 90° and double in an instant.
Signs on board: a line of dark, low clouds, a base that seems to "smoke", the sea whitening in the distance, a sudden drop in temperature. If you see this coming: reduce sail beforehand.
CAPE: the fuel of instability
CAPE (Convective Available Potential Energy, in J/kg) measures the energy available for convection. The higher it is, the more "explosive" the atmosphere. Read it as a potential, not a certainty:
| CAPE (J/kg) | Risk |
|---|---|
| < 300 | low — stable atmosphere |
| 300 – 1000 | showers and squalls possible |
| 1000 – 2500 | thunderstorms likely, sometimes strong |
| > 2500 | violent thunderstorms possible |
High CAPE combined with a trigger (front, sea breeze, terrain) points to thunderstorms. Keep an eye on it with the dedicated layer and cross-check it against the forecast gusts.
Example
Summer afternoon, CAPE 1800 J/kg and a showery sky: mean wind 12 knots, but isolated squalls bring peaks of 30–35 knots. Keep a "precautionary" reef in and watch the windward horizon.
The right approach
- Reduce sail as it approaches, not during the gust.
- Prepare the crew (harnesses, sheets ready to ease).
- Note the direction it arrives from: the wind often shifts as the squall clears.
- In marked thunderstorm risk, consider staying in port.