Module 13 - Meteorology
The Beaufort Scale
What you will learn
Translate a forecast Beaufort force into its defined wind range and probable open-sea cues without turning the scale into a universal reefing rule.
The Beaufort scale describes wind intensity. It helps decode a marine forecast, but the yacht, crew, tide, fetch, sea room and observed sea determine the action.
The Met Office uses the Beaufort scale in marine forecasts. Force 4 is a moderate breeze of 11–16 knots, Force 6 is a strong breeze of 22–27 knots, Force 7 is a near gale of 28–33 knots and gale begins at Force 8, 34–40 knots.
The published wave cues are probable values for well-developed wind waves in the open sea. Actual sea state also depends on duration, fetch, depth, swell and current. Wind against tide can create a short steep sea, while a recently increased wind may not yet have produced the published probable height.
The force number does not prescribe one sail plan or go/no-go answer. Apply it to the yacht’s procedures, point of sail, crew, loading, visibility, route exposure and shelter. A conservative decision is made from the combined evidence, not from the scale alone.
Use the force to decode wind intensity; apply vessel, crew and route context before deciding what to do.
| Force | Description | Wind range | Decision boundary |
|---|---|---|---|
| 3 | Gentle breeze | 7–10 kn | Observe actual sea and local acceleration |
| 4 | Moderate breeze | 11–16 kn | Apply the yacht’s sail plan and route exposure |
| 5 | Fresh breeze | 17–21 kn | Reassess crew, sea state and refuges |
| 6 | Strong breeze | 22–27 kn | No universal reefing rule; act for the yacht and passage |
| 7 | Near gale | 28–33 kn | Demanding conditions need a wide safety margin |
| 8 | Gale | 34–40 kn | Gale warning threshold; reassess the whole passage |
Separate the force from the sailing decision
The forecast is westerly 5, increasing 6, and the route rounds a headland during an ebb running against the wind. One crew member is inexperienced.
1. Decode
Force 6 means a strong breeze of 22–27 knots.
2. Add local sea context
Allow for headland acceleration and a steeper sea where the ebb opposes the westerly wind.
3. Apply the yacht’s limits
Consider the sail plan, crew, sea room, visibility and the point at which the nearer refuge remains easy.
4. Choose early
Reef, reroute, delay or return before the exposed section if the combined evidence removes the planned margin.
Sense check: Two yachts in the same Force 6 may require different actions; neither can ignore the actual sea and route.
Common mistake or limitation
- Treating Force 6 as a universal reefing instruction rather than a defined wind range.
- Assuming the probable open-sea wave height exists immediately or applies unchanged in sheltered, shallow or tide-affected water.
- Comparing one gust directly with a forecast mean-force range without checking how each value is defined.
Recap
- Force 4 is 11–16 knots, Force 6 is 22–27 knots and gale begins at Force 8.
- Published sea cues assume developed open-sea wind waves.
- The scale describes wind; it does not dictate one action for every yacht.
Optional quick check
Section 2 of 15
Why does Force 6 not tell every yacht exactly when to reef?
Beaufort force lookup
| Force | Name | Knots | mph | km/h | Sailing meaning |
|---|---|---|---|---|---|
| 0 | Calm | <1 | <1 | <1 | Sea like a mirror. |
| 1 | Light air | 1-3 | 1-3 | 1-5 | Ripples; little drive under sail. |
| 2 | Light breeze | 4-6 | 4-7 | 6-11 | Small wavelets and gentle sailing. |
| 3 | Gentle breeze | 7-10 | 8-12 | 12-19 | Large wavelets; comfortable for many crews. |
| 4 | Moderate breeze | 11-16 | 13-18 | 20-28 | Small waves and whitecaps; good sailing. |
| 5 | Fresh breeze | 17-21 | 19-24 | 29-38 | More demanding; many yachts prepare to reef. |
| 6 | Strong breeze | 22-27 | 25-31 | 39-49 | Large waves and spray; reef early on many cruising yachts. |
| 7 | Near gale | 28-33 | 32-38 | 50-61 | Sea heaps up; most training and family cruising plans should seek shelter. |
| 8 | Gale | 34-40 | 39-46 | 62-74 | Dangerous for small craft. |
| 9 | Strong gale | 41-47 | 47-54 | 75-88 | Severe conditions. |
| 10 | Storm | 48-55 | 55-63 | 89-102 | Very high waves; avoid being at sea. |
| 11 | Violent storm | 56-63 | 64-72 | 103-117 | Exceptional waves and dangerous survival conditions. |
| 12 | Hurricane force | 64+ | 73+ | 118+ | Extreme danger. |
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Practice weather decisions
The meteorology module connects wind force, forecasts, pressure systems, fronts, and practical passage decisions.