Weather Tools
See what each Beaufort force looks like at sea, decode any Shipping Forecast into plain English and run a go / no-go check before you leave. Ready to go deeper? Play the Weather School game to learn clouds, fronts and synoptic charts.
Beaufort force 4
Moderate breeze
- Mean wind
- 11–16 kn
- Probable waves
- 1 m (max 1.5 m)
- What the sea looks like
- Small waves becoming longer; fairly frequent white horses.
- On a 10 m cruising yacht
- Full sail / 1 reef. Good sailing. Smaller yachts may take a first reef upwind.
Wave heights are for the open sea, well away from land. Near the coast, and with wind against tide, waves are shorter and steeper.
The Beaufort wind scale
| Force | Description | Wind | Waves | Sea |
|---|---|---|---|---|
| 0 | Calm | 0–1 kn | 0 m | Sea like a mirror. |
| 1 | Light air | 1–3 kn | 0.1 m (max 0.1 m) | Ripples like scales, no foam crests. |
| 2 | Light breeze | 4–6 kn | 0.2 m (max 0.3 m) | Small wavelets with glassy crests that do not break. |
| 3 | Gentle breeze | 7–10 kn | 0.6 m (max 1 m) | Large wavelets. Crests begin to break; scattered white horses. |
| 4 | Moderate breeze | 11–16 kn | 1 m (max 1.5 m) | Small waves becoming longer; fairly frequent white horses. |
| 5 | Fresh breeze | 17–21 kn | 2 m (max 2.5 m) | Moderate waves, many white horses, chance of some spray. |
| 6 | Strong breeze | 22–27 kn | 3 m (max 4 m) | Large waves form; white foam crests everywhere; probably some spray. |
| 7 | Near gale | 28–33 kn | 4 m (max 5.5 m) | Sea heaps up; white foam from breaking waves blows in streaks. |
| 8 | Gale | 34–40 kn | 5.5 m (max 7.5 m) | Moderately high waves; crests break into spindrift; foam in well-marked streaks. |
| 9 | Severe gale | 41–47 kn | 7 m (max 10 m) | High waves; dense foam streaks; crests topple and roll over; spray may reduce visibility. |
| 10 | Storm | 48–55 kn | 9 m (max 12.5 m) | Very high waves with long overhanging crests; the sea looks white; heavy tumbling. |
| 11 | Violent storm | 56–63 kn | 11.5 m (max 16 m) | Exceptionally high waves; sea covered in long white patches of foam. |
| 12 | Hurricane force | 64+ kn | 14 m+ | Air filled with foam and spray; sea completely white; visibility very seriously affected. |
How to read the Shipping Forecast
The Met Office Shipping Forecast covers 31 sea areas around the British Isles. After the gale warnings and the general synopsis (where the pressure systems are and how they are moving), each area gets four parts, always in this order: wind (direction and Beaufort force), sea state, weather and visibility. “Southwest 5 to 7, occasionally gale 8 later. Moderate or rough. Rain then showers. Good, occasionally poor” is a complete area forecast.
| Term | Wave height |
|---|---|
| Smooth | less than 0.5 m |
| Slight | 0.5 to 1.25 m |
| Moderate | 1.25 to 2.5 m |
| Rough | 2.5 to 4 m |
| Very rough | 4 to 6 m |
| High | 6 to 9 m |
| Very high | 9 to 14 m |
| Phenomenal | more than 14 m |
| Term | Visibility |
|---|---|
| Good | more than 5 nautical miles |
| Moderate | 2 to 5 nautical miles |
| Poor | 1000 metres to 2 nautical miles |
| Very poor | less than 1000 metres |
| Term | Meaning |
|---|---|
| Imminent | Expected within 6 hours of the time of issue. |
| Soon | Expected 6 to 12 hours after the time of issue. |
| Later | Expected more than 12 hours after the time of issue. |
| Term | Meaning |
|---|---|
| Steady | Change of less than 0.1 hPa in the last 3 hours. |
| Rising slowly / Falling slowly | Change of 0.1 to 1.5 hPa in the last 3 hours. |
| Rising / Falling | Change of 1.6 to 3.5 hPa in the last 3 hours. |
| Rising quickly / Falling quickly | Change of 3.6 to 6 hPa in the last 3 hours. |
| Rising very rapidly / Falling very rapidly | Change of more than 6 hPa in the last 3 hours. |
| Now rising / Now falling | Pressure has started rising after falling, or falling after rising, within the last 3 hours. |
| Term | Meaning |
|---|---|
| Slowly | A pressure system moving at less than 15 knots. |
| Steadily | Moving at 15 to 25 knots. |
| Rather quickly | Moving at 25 to 35 knots. |
| Rapidly | Moving at 35 to 45 knots. |
| Very rapidly | Moving at more than 45 knots. |
Deciding whether to sail
There is no single wind speed that is “too windy”. It depends on the boat, the crew, the direction of the wind relative to the tide and coast, and how far you are going. A common rule of thumb for a 10 m cruiser is that force 4 is comfortable for a new crew, force 5 needs a reef and experience, and force 6 is for well-practised crews only. A gale warning means stay in harbour. Read more in our guide when is it too windy to sail?