The Beaufort scale: wind forces, knots and the Spanish sailing names

If you have ever listened to a marine forecast, you have heard lines like "force 5, moderate sea". That number is the Beaufort scale, the way sailors have measured wind at sea for more than two centuries. Here is the full scale, with the Spanish names you will hear in forecasts in Spain, and what each force means when you are on board.

What the Beaufort scale is

The Beaufort scale ranks wind strength in 13 steps, from 0 (calm) to 12 (hurricane). It was devised in 1805 by Francis Beaufort, an Irish hydrographer in the British Royal Navy, and the Navy adopted it officially in the 1830s. At first it gave no speeds at all: each step was recognised by the effect of the wind on the sails of a frigate.

Today each step has a speed range. In Spain, the national weather agency, AEMET, gives the wind in its marine forecasts on the Beaufort scale, and that wind is an average: the mean over 10 minutes, 10 metres above the sea surface. Gusts, the short bursts above that average, are given separately and in knots.

The full scale, with its Spanish names

A knot is one nautical mile per hour, about 1.85 km/h. The Spanish names are the ones used by AEMET, and the ones you will find on our Beaufort tee.

How many knots is a lot of wind?

There is no single number, but as a rule of thumb wind starts to feel like a lot at about 17 knots (force 5), and at 25 knots or more many small boats have already reduced sail or stayed in harbour.

• 10 knots: force 3, flojo or gentle breeze (7 to 10 knots). The top of force 3, with small waves and the odd patch of foam. It is not a lot of wind: a comfortable breeze for almost any boat, beginners included.
• 12 knots: force 4, bonancible or moderate breeze (11 to 16 knots). Whitecaps appear across the water. It is good sailing wind for most crews; in a small boat you will feel it, and it can be lively for a beginner.
• 16 knots: force 4, the top of bonancible. At 17 knots it is already force 5. A small boat is working hard, and on a cruising yacht many skippers start thinking about reducing sail if the wind keeps rising or the gusts run well above the average.
• 20 knots: force 5, fresquito or fresh breeze (17 to 21 knots). Whitecaps everywhere and some spray. This is what many sailors call a lot of wind: small boats and beginners often stay ashore, and a cruising yacht usually takes its first reef somewhere around 15 to 20 knots of apparent wind.
• 25 knots and above: force 6, fresco or strong breeze (22 to 27 knots), and up. Large waves and frequent spray. From 28 knots it is force 7, where AEMET begins to issue coastal warnings, and many leisure boats stay in harbour.

These are rules of thumb, not rules. They depend on the boat, the crew, the sea and whether the wind is steady or still rising. The forecast wind is also an average, and gusts can be stronger. For your own boat, follow its manual and the advice of your instructor or skipper.

What the sea looks like at each force

• Forces 0 to 3, calm to gentle breeze. From a sea like a mirror to small waves that begin to break, with the odd patch of foam.
• Force 4, bonancible. The waves grow longer and whitecaps appear all over the water.
• Force 5, fresquito. Moderate, longer waves, whitecaps everywhere and some spray.
• Force 6, fresco. Large waves begin to form, with white foam crests and frequent spray.
• Force 7, frescachón. The sea heaps up and the wind blows the foam from the waves into streaks.
• Force 8 and above, temporal. Ever higher waves, spray blown off the crests and reduced visibility.

Why "fresco" is not a nice breeze

In everyday Spanish, "hace fresco" means it is a little cool, almost pleasant. At sea, fresco is force 6: 22 to 27 knots, large waves and breaking crests. The old names can fool anyone who does not know them. Bonancible, which sounds like calm weather, is already 11 to 16 knots, and fresquito, 17 to 21 knots, is wind that asks for your full attention. When someone on a Spanish pontoon says "hoy sopla fresquito", they do not mean a gentle breeze.

Beaufort and Douglas: wind and sea

The Beaufort scale measures wind. The state of the sea uses a different scale, the Douglas scale, which is why Spanish forecasts pair the two: "fuerza 5, marejada" means 17 to 21 knots of wind and waves of 0.5 to 1.25 metres. These are the Douglas steps you will hear most in a coastal forecast:

• Rizada (rippled): up to 0.1 m.
• Marejadilla (smooth): 0.1 to 0.5 m.
• Marejada (slight): 0.5 to 1.25 m.
• Fuerte marejada (moderate): 1.25 to 2.5 m.
• Gruesa (rough): 2.5 to 4 m.
• Muy gruesa (very rough): 4 to 6 m.

Above those come arbolada (high), montañosa (very high) and enorme (phenomenal). The Douglas scale describes the wind sea, the waves raised by the wind blowing in the area. Swell that arrives from far away, mar de fondo, is given separately.

Force 7: where the warnings start

In coastal waters, AEMET issues a warning when it expects the wind to reach force 7, or the total sea to reach 4 metres in the Atlantic or 3 metres in the Mediterranean. It is a useful number to remember: frescachón is the step at which the forecast is already warning you. You will also meet two old sailing verbs in AEMET bulletins: arreciar, when the wind rises by one step or more, and amainar, when it drops.

Dressing for the wind

Wind makes it feel colder on the water than on the dock, and from force 4 upwards spray starts to reach the deck. Our layering guide for sailing explains how to dress in layers, and softshell or rain jacket helps you choose the outer layer for the day.

The scale on your back

Our Beaufort tech tee carries the Beaufort scale on the back with one force marked, and you choose which: 4 Bonancible, 5 Fresquito, 6 Fresco or 7 Frescachón, each with its knots and its own line, from «El día que esperabas» ("the day you were waiting for") to «Hoy se mira desde el muelle» ("today you watch from the dock"). It is white, in a quick-dry, breathable fabric, with the NewPortVela logo on the chest. If you are not sure which force is yours, think of the day you most like to go out.

Common questions

Is 10 knots a lot of wind?

No. 10 knots (10 kt) is the top of force 3, flojo or gentle breeze (7 to 10 knots): a light breeze with small waves that almost any boat and crew handle well. 11 knots is already force 4.

Is 12 knots a lot of wind?

12 knots is force 4, bonancible or moderate breeze (11 to 16 knots), the force at which whitecaps appear across the water. Many crews call it a good sailing breeze; in a small dinghy, for a beginner, it can feel lively. How much is too much depends on the boat, the crew and the sea, so read the whole forecast, gusts included. 13 knots is still force 4.

Is 16 knots a lot of wind?

16 knots is the top of force 4, bonancible or moderate breeze (11 to 16 knots). For a small boat or a beginner it can be a lot; on a cruising yacht it is usually a day for sailing with attention and watching whether the wind rises. 17 knots is already force 5.

What force is 20 knots?

20 knots is force 5, fresquito or fresh breeze (17 to 21 knots). 22 knots is already force 6, fresco or strong breeze.

What does force 7 mean?

Force 7 is near gale, frescachón in Spanish: 28 to 33 knots, about 50 to 61 km/h. The sea heaps up and the wind blows the foam from the waves in streaks. It is the step at which AEMET starts issuing coastal warnings, and many leisure boats stay in harbour.

What is force 5 in knots?

Force 5, fresh breeze or fresquito, is 17 to 21 knots, about 29 to 38 km/h.

How do I convert knots to km/h?

A knot is one nautical mile per hour, and a nautical mile is 1,852 metres, so multiply the knots by 1.852. 10 knots is about 18.5 km/h.

Is the forecast wind an average or a gust?

In AEMET marine forecasts the Beaufort force is an average over 10 minutes, 10 metres above the sea. Gusts can be stronger and are given separately, in knots.

What is the difference between the Beaufort and Douglas scales?

Beaufort measures the wind; Douglas measures the state of the sea raised by that wind. Spanish forecasts give both, as in "fuerza 5, marejada".

Before you sail, always check the official marine forecast for your area. This guide explains the scale; it does not replace the forecast.

Sources: AEMET, "Escalas de viento y oleaje" and marine forecast glossary; Royal Meteorological Society, "The Beaufort Wind Scale"; Met Office, Beaufort wind force scale; Yachting Monthly, "Essential reefing tips for cruisers"; Wikipedia, "Beaufort scale".