The Beaufort scale: wind force from 0 to 12

The Beaufort scale chart, from 0 to 12

There are 13 steps, from force 0 (calm) to force 12 (hurricane force). Each force has a name, a speed range and a description of the sea, so you can judge the wind by eye even without an anemometer. The speed ranges, mph included, are the World Meteorological Organization’s (WMO), and the open-sea wave heights are the UK Met Office’s, converted to feet:

Force	Name	Knots	mph	m/s	Waves (ft)	What the sea looks like
0	Calm	under 1	under 1	0–0.2	0	Flat, like a mirror
1	Light air	1–3	1–3	0.3–1.5	0.3	Ripples like scales, no foam
2	Light breeze	4–6	4–7	1.6–3.3	0.7–1	Small wavelets that don’t break
3	Gentle breeze	7–10	8–12	3.4–5.4	2–3	Some crests break; scattered whitecaps
4	Moderate breeze	11–16	13–18	5.5–7.9	3–5	Frequent whitecaps
5	Fresh breeze	17–21	19–24	8–10.7	6.5–8	Moderate waves; some spray
6	Strong breeze	22–27	25–31	10.8–13.8	10–13	Large waves with white foam crests
7	Near gale	28–33	32–38	13.9–17.1	13–18	Sea heaps up; foam blows in streaks
8	Gale	34–40	39–46	17.2–20.7	18–25	Crests break into spindrift
9	Strong gale	41–47	47–54	20.8–24.4	23–33	Dense foam; spray cuts visibility
10	Storm	48–55	55–63	24.5–28.4	30–41	The whole sea looks white
11	Violent storm	56–63	64–72	28.5–32.6	38–52	Medium ships hidden in the troughs
12	Hurricane force	64+	73+	32.7+	46+	Air full of foam and spray

A few things to keep in mind when you read it:

• The speeds are average winds, not gusts, as we explain below.
• The waves are open-ocean waves, with the wind blowing for many hours over a long stretch of water. The first number is the probable height and the second the probable maximum. Near shore, in a bay or with an offshore wind, waves are much smaller, and when the wind has just picked up, the sea hasn’t had time to build.
• The names vary a little. The UK Met Office and the NWS call force 9 a “severe gale,” and some tables call force 12 simply “hurricane.” The numbers are the same.

Force 7 and other forces people ask about

• Force 4 (moderate breeze): 13 to 18 mph. Whitecaps everywhere; on land, the wind raises dust and loose paper.
• Force 6 (strong breeze): 25 to 31 mph. Large waves with foam crests; on land, wires whistle and umbrellas are hard to use.
• Force 7 (near gale): 32 to 38 mph. The sea heaps up, foam blows in streaks and walking into the wind is hard.
• Force 8 (gale): 39 to 46 mph. This is where an NWS gale warning starts, at 34 knots, and on land twigs break off trees.

If you think in mph rather than knots, our guide to knots to mph has a full conversion chart and shortcuts you can do in your head.

Average wind vs. gusts

The Beaufort force is based on the average wind. By international convention, that’s a 10-minute average measured 33 feet (10 m) above the ground. Gusts are the bursts on top of that average; the WMO recommends measuring a gust as the highest 3-second average wind speed.

The gap can be big. Australia’s Bureau of Meteorology says gusts can run 40% stronger than the forecast average wind, and stronger still in squalls and thunderstorms. With force 5, about 20 knots on average, you can get gusts near 28 knots, which would be force 7 if they lasted.

That’s why forecasts give both numbers, and why gusts count toward warnings too. NWS gale and storm warnings cover sustained winds or frequent gusts, and the UK Met Office calls it a gale when the average wind reaches force 8 or gusts reach 43 to 51 knots.

Two more details:

• US airport observations (METARs) average the wind over 2 minutes, not 10, and report a gust when the speed swings 10 knots or more between peaks and lulls. Keep that in mind if you compare US data with other countries.
• The scale uses the wind 33 feet up. According to the Royal Meteorological Society, at about 6.5 feet (2 m), roughly head height on a beach, the wind may be only 50 to 70% of that.

On TidePlanet’s wind page for each place, bars show the average wind and a dashed line shows the gusts. Check the wind at Hatteras, for example: the 3-hour table gives the Beaufort force for each slot, and the scale at the bottom highlights today’s force and the week’s peak.

Where the scale came from

The scale was devised by Francis Beaufort (1774-1857), an Irish hydrographer in Britain’s Royal Navy, in 1805 and 1806, while he commanded HMS Woolwich. Officers logged the weather every day, but each in his own way, and a “fresh breeze” didn’t mean the same thing to everyone.

His scale said nothing about speed, because ships had no way to measure it. In his 1810 version, each force also described its effect on a Royal Navy frigate.

In the 1830s, as Hydrographer of the Navy, Beaufort got the scale officially adopted. One of the first ships to use it was HMS Beagle, the ship of Charles Darwin’s voyage, under Captain Robert FitzRoy, who later founded Britain’s weather service, the Met Office.

Over time, the sails, which differed from ship to ship, gave way to the look of the sea and effects on land. Wind speeds in knots came in the 20th century, with anemometers. Today the WMO keeps the scale from 0 to 12.

Sea state: the Douglas scale

Beaufort describes the wind. To describe the sea, British and European forecasts use the Douglas scale, which rates the height of the wind sea, the waves raised by the local wind:

Degree	Term	Wave height
0	Calm (glassy)	0
1	Calm (rippled)	0–0.3 ft (0–0.1 m)
2	Smooth	0.3–1.6 ft (0.1–0.5 m)
3	Slight	1.6–4 ft (0.5–1.25 m)
4	Moderate	4–8 ft (1.25–2.5 m)
5	Rough	8–13 ft (2.5–4 m)
6	Very rough	13–20 ft (4–6 m)
7	High	20–30 ft (6–9 m)
8	Very high	30–46 ft (9–14 m)
9	Phenomenal	over 46 ft (14 m)

The height is the significant wave height: the average of the highest third of the waves, roughly what an observer would estimate by eye. Some individual waves will always be quite a bit bigger.

When the wind blows for a long time over open water, the two scales move together. In the Met Office’s table, force 3 usually goes with a slight sea, force 5 with moderate, force 6 with rough and force 8 with very rough to high.

They don’t always match, though. A wind that has just picked up hasn’t built the sea yet, and an offshore wind barely makes waves near the beach. And swell, the long waves from distant storms, can make the sea rough with almost no wind at all. That’s why forecasts list swell separately. On the waves page for Hatteras you’ll see the day’s sea state with its Douglas degree, and the swell on its own.

How marine forecasts use it

United States. The NWS doesn’t use Beaufort forces in its marine forecasts. A coastal waters forecast reads more like “SW winds 10 to 15 kt. Seas 2 to 3 ft.” Its warnings line up with Beaufort, though:

NWS product	Wind	Beaufort
Small Craft Advisory	starts at 20 to 25 kt, by region	5 to 6
Gale Warning	34–47 kt (39–54 mph)	8 to 9
Storm Warning	48–63 kt (55–73 mph)	10 to 11
Hurricane Force Wind Warning	64 kt (74 mph) or more	12

Small craft advisory thresholds vary by region, from 20 knots on the Gulf Coast to 25 knots on the East Coast, and some also kick in for high seas. The NWS itself notes there is no precise definition of a small craft. The last three warnings are for storms that aren’t tropical cyclones, which have their own watches and warnings.

United Kingdom. The Met Office’s famous Shipping Forecast goes out four times a day for 31 sea areas, giving wind direction and Beaufort force, weather and visibility in clipped lines like “Southwest 5 to 7, occasionally gale 8 later.” Gale warnings start at force 8, and the inshore waters forecast, out to 12 miles, flags winds of force 6 or more.

Elsewhere. Australia’s Bureau of Meteorology issues marine wind warnings once average winds are expected to reach 26 knots. Spain’s weather agency, AEMET, gives wind in Beaufort forces and issues coastal warnings from force 7.

How much wind is OK for kayaking, sailing, fishing or surfing?

There’s no single number for everyone: it depends on your experience, your boat, whether the wind is blowing onshore or offshore, and the current. Official agencies offer these benchmarks:

• Kayaking and paddleboarding. Australia’s Bureau of Meteorology advises novice paddlers to stay under 10 knots (force 3 or less) and advanced paddlers to stay at or under 20 knots (force 5). Watch out for offshore wind, which can carry a kayak or paddleboard away from the beach.
• Small-boat sailing. As with paddling, beginners should look for calm conditions: a light wind is the most comfortable for learning. At force 4, whitecaps appear and gusts call for quick reactions.
• Fishing from a small boat. The Bureau of Meteorology warns that wind can be difficult or dangerous for small boats below the 26 knots at which it issues warnings, and by force 5 or 6 you’re at Small Craft Advisory levels. When the wind blows against the tide, it stacks up short, steep waves at inlets and bars, as we explain in tidal currents.
• Surfing. Direction matters more than strength. An offshore wind smooths the surface and the waves break cleanly; an onshore wind makes them choppy. On each beach’s surf page, like surf at Montauk, TidePlanet rates calm and light offshore winds best.

Whatever you do, check the gusts as well as the average wind, and look at the official warnings before you head out.

TidePlanet’s nautical converter

In the nautical converter, you type a speed in any box (knots, mph, km/h or m/s) and the others update instantly, with no button to press. Below them you get the matching Beaufort force, its name, what the sea looks like and the wave height in the open sea. It also works the other way: pick a force from the drop-down and it fills in the speed at the middle of that force’s range.

The same converter also handles depth, pressure and temperature, and further down there’s a printable Beaufort table in knots, km/h and m/s, plus the exact formulas. Wave heights show in feet or meters, depending on what you pick in the Units panel (the gear icon in the header).