Forecast terms: wind, sea and visibility
1The lesson in plain words
Check the issuing service’s definitions. A marine forecast uses terms for mean wind, significant wave height and visibility.
Wind force
Marine forecasts give wind on the Beaufort scale, from force 0, calm, to force 12, hurricane force. Each force is a band of mean wind speed: force 4 is 11–16 knots, force 6 is 22–27 knots, force 8 is 34–40 knots. A knot is one nautical mile per hour. The Beaufort scale page in this module shows all thirteen forces with sea clues.
Gusts and gales
The forecast force is a mean wind. A gust is a short burst, lasting seconds, above the mean. A gale is force 8. The Met Office also issues a gale warning for gusts of 43–51 knots. Many coastal services warn earlier for small craft, for example at force 6 or 7. France warns its coastal strip from force 7.
Direction words
Wind direction is where the wind comes from: a westerly blows from the west. Veering means the direction turns clockwise, such as south-west to west. Backing means anticlockwise, such as west to south-west. Cyclonic means winds circulate around a low-pressure system within the forecast area, so directions differ across the area.
Sea state
Sea state describes wave height; some products give swell separately. Heights are significant wave heights: the average height of the highest third of waves. Individual waves can be much higher. UK/Spanish terms: smooth/marejadilla 0.1–0.5 m; slight/marejada 0.5–1.25 m; moderate/fuerte marejada 1.25–2.5 m; rough/gruesa 2.5–4 m; very rough/muy gruesa 4–6 m.
Visibility (UK Met Office)
Good: over 5 nautical miles (nm). Moderate: 2–5 nm. Poor: 1,000 m to 2 nm. Very poor: below 1,000 m. Fog can cause very poor visibility; heavy rain or snow can too. Check the issuing service’s definitions. "Occasionally poor" means poor for part of the forecast period.
2Key data
- Gale (force 8)
- 34–40 knots mean; UK warning also for gusts of 43–51 knots
- Veering / backing
- Clockwise / anticlockwise change of wind direction
- Significant wave height: slight / moderate
- Slight/marejada 0.5–1.25 m; moderate/fuerte marejada 1.25–2.5 m
- Significant wave height: rough / very rough
- Rough/gruesa 2.5–4 m; very rough/muy gruesa 4–6 m
- UK visibility: good / moderate
- Over 5 nm / 2–5 nm
- UK visibility: poor / very poor
- 1,000 m to 2 nm / below 1,000 m
3Key points
- 1
Beaufort force is a mean wind band; gusts are stronger.
- 2
A gale is force 8, 34–40 knots. Coastal warnings may start at force 6 or 7.
- 3
Wind is named from where it comes. Veering is clockwise, backing anticlockwise.
- 4
Moderate sea: significant wave height 1.25–2.5 m; rough: 2.5–4 m. Individual waves can be higher.
- 5
UK visibility very poor is below 1,000 m. Fog, heavy rain or snow can cause it.
4Frequent mistakes
Wrong: Force 5 means the wind will never be stronger than 21 knots.
Right: Force 5 is a mean wind of 17–21 knots. Gusts can be well above it, and the forecast may say "occasionally 6".
Wrong: A northerly wind blows towards the north.
Right: Wind is named from where it comes. A northerly blows from the north towards the south.
Wrong: "Moderate" means pleasant conditions for any small boat.
Right: A moderate sea has significant wave heights of 1.25–2.5 m. Individual waves can be higher. That can be hard work or dangerous for a small open boat.
5Practice case & exam keys
The UK forecast reads: "Southwest 4 or 5, veering west 6. Slight or moderate. Showers. Good, occasionally poor." Your crew expects steady wind, no waves above the stated range and good visibility throughout. What needs correcting?
Show the answer
The mean wind is from the south-west at 11–21 knots, turning clockwise to the west at 22–27 knots. Gusts can be stronger. Significant wave height is 0.5–2.5 m; individual waves can be higher. Good visibility is over 5 nm, but occasionally poor means it can fall to 1,000 m–2 nm for part of the period. The wind, individual waves and visibility will not stay at one value.
Exam keys
- Beaufort force = mean wind band; force 8 (34–40 knots) is a gale.
- Veering = clockwise; backing = anticlockwise; wind is named from where it blows.
- Sea state terms give significant wave heights; UK very poor visibility is below 1,000 m.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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