Weather for Sailors – Beaufort, Fronts, Clouds, Sea Breezes

Pressure Systems & Fronts

Weather in the UK is driven by the interaction of high-pressure (anticyclone) and low-pressure (depression/cyclone) systems moving across the Atlantic.

	High Pressure	Low Pressure
Wind direction	Clockwise (N. hemisphere)	Anticlockwise (N. hemisphere)
Typical weather	Settled, dry, light winds	Unsettled, rain, strong winds
Isobars	Widely spaced (light winds)	Closely spaced (strong winds)
Pressure (mb)	Above ~1013 mb	Below ~1013 mb

Fronts are boundaries between air masses of different temperatures:

Warm front — warm air slides over cold air. Brings gradual cloud thickening (cirrus → stratus), steady rain, poor visibility. Wind veers (shifts clockwise) as it passes. Pressure falls then steadies.
Cold front — cold air undercuts warm air. Brings a narrow band of heavy rain, possible thunderstorms, then rapid clearing. Wind veers sharply, pressure rises. Often gusty near the front.
Occluded front — when a cold front catches up with a warm front. Mixed weather, often persistent rain.

In the Northern Hemisphere, if you stand with the wind at your back, low pressure is to your LEFT (Buys Ballot's Law). This helps you locate approaching weather systems.

Activity: Front Passage

A ___ front brings gradual cloud thickening and steady rain. As it passes, the wind ___ (shifts clockwise). A ___ front brings a narrow band of heavy rain followed by rapid ___.

cold warm veers clearing backs

Correct!

The Beaufort Scale

Devised by Admiral Sir Francis Beaufort in 1805, the Beaufort Scale relates wind speed to observed conditions at sea. It is used in all UK shipping forecasts and is essential knowledge for sailors.

Force	Knots	Description	Sea State
0	<1	Calm	Mirror-like sea
1	1–3	Light air	Ripples, no crests
2	4–6	Light breeze	Small wavelets, crests not breaking
3	7–10	Gentle breeze	Large wavelets, scattered whitecaps
4	11–16	Moderate breeze	Small waves, frequent whitecaps
5	17–21	Fresh breeze	Moderate waves, many whitecaps, some spray
6	22–27	Strong breeze	Large waves, extensive whitecaps, spray
7	28–33	Near gale	Sea heaps up, foam blows in streaks
8	34–40	Gale	Moderately high waves, crests break into spindrift
9	41–47	Strong gale	High waves, dense foam streaks, spray reduces visibility
10	48–55	Storm	Very high waves, sea white with foam, tumbling heavy
11	56–63	Violent storm	Exceptionally high waves, visibility severely affected
12	64+	Hurricane	Air filled with foam and spray, sea white

For dinghy sailing, Force 3–4 is ideal for learning. Force 5+ requires experience. Most sailing clubs will cancel dinghy racing at Force 6. Keelboats can handle more, but even experienced crews take care above Force 7.

Activity: Match Beaufort Force to Description

Force

Force 0

Force 4

Force 6

Force 8

Force 12

Description

?

All matched!

Cloud Types & What They Tell You

Clouds are visible indicators of what the atmosphere is doing. Learning to read them gives you a free, real-time weather forecast.

Clouds are classified by altitude and shape:

Cloud	Altitude	Appearance	What It Means
Cirrus	High (6–12 km)	Wispy, thin streaks	Fair weather, but if thickening → warm front approaching (rain in 12–24 hrs)
Cirrostratus	High	Thin veil, halo around sun/moon	Warm front approaching — rain within 12 hrs
Altostratus	Mid (2–6 km)	Grey sheet, sun barely visible	Rain imminent (warm front arriving)
Altocumulus	Mid	White/grey puffs in rows	Fair, but can precede thunderstorms if on a warm morning
Cumulus	Low (0–2 km)	White fluffy "cauliflower" puffs	Fair weather (if small and flat-bottomed)
Cumulonimbus	Low–High	Towering, anvil-shaped	Thunderstorms, heavy rain, strong gusts, lightning — avoid!
Stratus	Low	Grey, uniform layer	Drizzle, poor visibility, overcast
Nimbostratus	Low–Mid	Dark, thick layer	Prolonged steady rain or snow

The cloud sequence Cirrus → Cirrostratus → Altostratus → Nimbostratus is the classic warm front approach. If you see this pattern developing, expect rain within 12–24 hours and plan accordingly.

Activity: Cloud Type Flashcards

Tap each card to reveal what the cloud type means for sailors.

Cirrus

Fair now, but thickening = warm front in 12-24 hrs

Cumulus

Fair weather if small and flat-bottomed

Cumulonimbus

Thunderstorms, heavy rain, strong gusts — avoid!

Nimbostratus

Prolonged steady rain or snow, dark thick layer

Stratus

Drizzle, poor visibility, grey overcast

Cirrostratus

Halo around sun/moon — rain within 12 hrs

Tap a card to flip it

All clouds learned!

Sea Breezes & Land Breezes

Thermal breezes are caused by the different rates at which land and sea heat up and cool down. They are a major feature of coastal sailing, especially in summer.

Sea breeze (daytime):

The land heats up faster than the sea.
Hot air over the land rises, creating low pressure.
Cooler air from the sea flows in to replace it — this is the sea breeze.
Typically develops late morning, peaks mid-afternoon, dies at sunset.
Blows onshore (sea to land), often Force 2–4.
Look for a line of cumulus clouds forming inland (the "sea breeze front").

Land breeze (night-time):

The land cools faster than the sea at night.
Air flows from land to sea — the reverse of the sea breeze.
Usually much lighter (Force 1–2) and occurs in early morning.
Blows offshore (land to sea).

The sea breeze can add to or oppose the gradient (synoptic) wind. If the gradient wind is already onshore, the sea breeze will strengthen it. If offshore, the sea breeze may cancel it out, creating a calm period before building from a new direction.

Activity: Sea Breeze vs Land Breeze

Feature

Blows onshore

Blows offshore

Peaks mid-afternoon

Occurs early morning

Breeze Type

?

All matched!

Practical Forecasting

Before every sail you should check multiple weather sources. No single forecast is perfect — cross-referencing gives you the best picture.

Key sources for UK sailors:

Met Office Inshore Waters Forecast — 5-day forecast for UK coastal waters, updated twice daily.
Shipping Forecast (BBC Radio 4) — broadcast at 0048, 0520, 1201, 1754. Covers 31 sea areas around the UK.
GRIB files — gridded weather data you can view on chart plotters or apps (e.g. PredictWind, Windy).
Navtex — text-based weather and safety info received automatically on an onboard receiver.
Visual observation — clouds, barometer, wind shifts, sea state.

Barometer reading tips:

Steady or rising pressure = generally improving weather.
Falling pressure = deteriorating weather approaching.
A fall of more than 5 mb in 3 hours = gale likely.
Tap the barometer — if the needle moves, note the trend direction.

The shipping forecast uses a standard format: area name, wind direction and force, weather, visibility. Learn to decode it: "Viking, North Utsire: Southwesterly 5 to 7, occasionally gale 8, rain then showers, moderate or good."

Personal weather log: Keep a log at fixed times (e.g. 0800, 1200, 1800) recording: barometric pressure, wind direction and strength, cloud type and cover, visibility, and sea state. Comparing your observations with the forecast improves your skill over time.

Activity: Forecasting Flashcards

Pressure falling fast?

Fall of 5+ mb in 3 hrs = gale likely

Shipping Forecast times?

0048, 0520, 1201, 1754 on BBC Radio 4

Buys Ballot's Law?

Wind at your back: low pressure is to your left (N. hemisphere)

Wind "veers"?

Shifts clockwise (e.g. SW to W to NW)

Wind "backs"?

Shifts anticlockwise (e.g. NW to W to SW)

What are GRIB files?

Gridded weather data for chart plotters and apps

Tap a card to flip it

All revealed!