Weather

Beaufort scale

Wind force from 0 (calm) to 12 (hurricane).

The Beaufort scale matches wind speed to numbered forces and the appearance of the sea:

• 0 calm (< 1 kn)
• 1–3 light air to gentle breeze (1–10 kn)
• 4 moderate breeze (11–16 kn)
• 5 fresh breeze (17–21 kn)
• 6 strong breeze (22–27 kn)
• 7 near gale (28–33 kn)
• 8 gale (34–40 kn)
• 9 strong gale (41–47 kn)
• 10 storm (48–55 kn)
• 11 violent storm (56–63 kn)
• 12 hurricane force (≥ 64 kn).

See alsoSea state & swell · Preparing for heavy weather

Highs and lows

Rotation and wind flow around pressure systems.

In the northern hemisphere the wind circulates anticlockwise and slightly inwards around a low, clockwise and outwards around a high. The closer the isobars (lines of equal pressure), the stronger the wind.

Rule of thumb

Stand with your back to the wind and the low lies roughly ahead to your left (northern hemisphere) — useful for sizing up the weather situation.

See alsoWarm, cold and occluded fronts · Air pressure & barometer

Warm, cold and occluded fronts

What you see as a low passes through.

• Warm front — warm air riding up, cloud lowering steadily, prolonged rain; afterwards milder, often hazy air.
• Cold front — colder air pushing in quickly beneath the warm air: showers, squalls, a sharp veer of the wind, then clearing and good visibility.
• Occlusion — the cold front catches up with the warm front; a mixture of both, with precipitation.

See alsoHighs and lows · Veering & backing wind

Land and sea breeze

Thermal coastal winds over the course of the day.

In sunny weather with a weak gradient, the land warms up faster than the water during the day: air rises over the land and cooler air flows in from the sea — the onshore sea breeze, strongest in the afternoon. At night the effect reverses: the land cools faster and an offshore land breeze sets in.

See alsoHighs and lows

Sea state & swell

Wind sea, swell and dangerous waves.

Wind sea is raised by the wind blowing now; its height depends on wind strength, duration and fetch. Swell is the regular, decayed wave train left by a distant or earlier wind field.

Wind against tide

When the tidal stream runs against the wind, the sea becomes short, steep and breaking — a particularly unpleasant and dangerous situation, for instance in tidal waters or off river mouths.

See alsoBeaufort scale · Breaking seas & broaching

Air pressure & barometer

The pressure trend as an early warning.

Air pressure is measured in hectopascals (hPa). What matters is the trend: a rapid, marked fall announces an approaching storm depression — the faster the fall, the stronger the wind that usually follows. Reading and noting it regularly makes the development visible.

See alsoHighs and lows · Marine weather forecast

Veering & backing wind

Veering and backing — and what they tell you.

If the wind shifts clockwise (e.g. south → west) it is veering — typical after a cold front has passed. If it shifts anticlockwise it is backing, which in the northern hemisphere often announces a deterioration or an approaching low.

See alsoWarm, cold and occluded fronts · Highs and lows

Marine weather forecast

Sources and structure of the forecast.

Marine forecasts come from the national weather service (bulletins and marine radio), via NAVTEX and from apps and websites. The structure is usually: current warnings (strong wind/gale), general situation, forecast for the sea areas and a further outlook. Check before departure and regularly while under way.

See alsoAir pressure & barometer · Preparing for heavy weather

fog

Cloud at ground level — how it forms and why it is dangerous.

Fog is cloud at ground level: the air cools below its dew point. Advection fog forms when warm, moist air moves over colder water — typical along coasts and often persistent. Radiation fog forms overnight over land in clear, calm air and drifts out over the water in the morning.

The danger is the loss of visibility: take the speed off, keep the position up to date continuously, sound fog signals, use radar and AIS, strengthen the look-out — see Rule 19.

See alsoRestricted visibility (Rule 19) · Sound signals · Air pressure & barometer

Thunderstorms & squalls

Squall line, torrential rain, lightning — and what to do.

Thunderstorms grow in towering convective cloud (cumulonimbus). The signs are building towers, a dark squall front and rapidly falling pressure. With the front come violent, shifting gusts, torrential rain and a loss of visibility, often all at once.

Preparation: reef or hand sail in good time, crew clipped on and in lifejackets, hatches shut, course and position clear. Secure loose gear. After it has passed the wind usually shifts and freshens in gusts.

See alsoReefing · Marine weather forecast · Preparing for heavy weather

Clouds as weather signs

The sequence of cloud betrays an approaching front.

An approaching warm front announces itself through a typical sequence of cloud: first high, fibrous cirrus, then milky cirrostratus (a halo around sun or moon), lowering altostratus, and finally steady rain from nimbostratus.

Convective cloud (cumulus) shows instability. If it builds up into cumulonimbus, a thunderstorm is on the way.

See alsoWarm, cold and occluded fronts · Thunderstorms & squalls · Air pressure & barometer