Weather is the one thing a sailor cannot argue with. Learn to read the clouds, the pressure and the signs, learn what each wind strength feels like, then slide through the Beaufort scale yourself.
Weather is a sequence, not a surprise
Weather rarely arrives from nowhere. Clouds change, the barometer moves and the wind shifts before the big change comes. Sailors who notice the small signs have hours to reef, to turn back or to find shelter. This lesson teaches the signs.
Know where you are
Fronts and lows belong mainly to the temperate parts of the world, and this lesson describes the northern hemisphere (in the southern hemisphere, the wind turns the other way round lows). Near the equator the big dangers are different: fast-growing storm clouds, squalls and, in season, tropical cyclones. Always check an official forecast before you go, and remember that a forecast is a good guess, not a promise.
The Beaufort scale
Slide from calm to hurricane. Watch the flag and the sea, and read what it means for a small cruising boat.
Four skies
A weather map
Five wind strengths
Pressure on the barometer, and the wind turning on the compass
Study sheet
All the words in one place, by level.
Level 1: Clouds
Cirrus
High, thin, wispy clouds made of ice, like brush strokes. Often the first sign of a front still a day away. A sky full of thickening cirrus often means weather is coming.
Cumulus
Fluffy cotton-wool clouds with flat bottoms. Small ones mean fair weather. If they grow tall and dark, watch out, because they can turn into storm clouds.
Stratus
A low, flat, grey blanket of cloud. It brings dull weather, drizzle and poor visibility. It looks like a lid on the sea.
Cumulonimbus
A huge towering storm cloud with a flat anvil top and a dark base. It brings heavy rain, thunder and lightning, hail and sudden strong gusts. Keep well clear of it.
Level 2: Pressure and fronts
Low-pressure area
A region of low pressure, also called a depression. Air rises, clouds build, and there is rain and wind. In the northern hemisphere the wind circles anticlockwise around it. Lows bring the bad weather.
High-pressure area
A region of high pressure. Air sinks, skies clear and winds are light. In the northern hemisphere the wind circles clockwise around it. Highs bring fair, settled weather.
Isobars
Lines on a weather map joining places with the same pressure. Where they are close together, the wind is strong. Where they are far apart, the wind is light.
Warm front
Drawn in red with semicircles. Warm air slides over cold air. Cloud thickens and lowers, steady rain follows, and the wind often veers. Passing it, it gets warmer and muggy.
Cold front
Drawn in blue with triangles. Cold air pushes under warm air. A narrow band of heavy showers and squalls, a sudden wind shift, then clearer, cooler weather. Often the roughest part of a passing low.
Level 3: Wind strength
Calm (force 0)
Less than 1 knot. The sea is like a mirror and the flag hangs down. You will not sail, but you may need to watch for the weather that follows.
Gentle breeze (force 3)
7 to 10 knots. Large wavelets, a few whitecaps, and the flag lifts. Easy, pleasant sailing, good for beginners.
Fresh breeze (force 5)
17 to 21 knots. Moderate waves, lots of whitecaps and some spray. Time to reef. Not a day for a small boat or a beginner.
Near gale (force 7)
28 to 33 knots. The sea heaps up and white foam blows in streaks. Stay in harbour.
Strong gale (force 9)
41 to 47 knots. High waves, dense foam, the crests roll over and the air is full of spray. A serious storm at sea, dangerous for small boats.
Level 4: Warning signs
Falling pressure
The barometer is dropping. A slow fall means weather is turning worse. A fast fall (about 6 hectopascals in three hours) warns of strong wind or a gale. Take it seriously.
Rising pressure
The barometer is going up. The weather is improving, with clearer skies. After a front, a quick rise can bring strong, gusty wind first.
Steady pressure
The needle is not moving. The weather will probably stay the same for now.
Veering
The wind direction turns clockwise, for example from south-west to west. In the northern hemisphere it often means a front is passing and the weather is clearing.
Backing
The wind direction turns anticlockwise, for example from west to south-west. With falling pressure it often means a low is coming closer and the weather is getting worse.