knots
Only the Small Craft Advisory trigger changes with it.
| Force | Description | Wind | Waves | Sea state |
|---|---|---|---|---|
| 0 | Calm | WindUnder 1 kt | WavesSmooth | Sea stateSea like a mirror. |
| 1 | Light air | Wind1–3 kt | WavesScaly ripples | Sea stateRipples with glassy crests, no foam. |
| 2 | Light breeze | Wind4–6 kt | WavesSmall wavelets | Sea stateSmall wavelets with glassy crests that do not break. |
| 3 | Gentle breeze | Wind7–10 kt | WavesLarge wavelets | Sea stateLarge wavelets; crests begin to break, scattered white horses. |
| 4 | Moderate breeze | Wind11–16 kt | Waves1–4 ft | Sea stateSmall waves becoming longer; fairly frequent white horses. |
| 5 | Fresh breeze | Wind17–21 kt | Waves4–8 ft | Sea stateModerate waves, many white horses; some spray. |
| 6 | Strong breeze | Wind22–27 kt | Waves8–13 ft | Sea stateLarge waves with extensive white foam crests; some spray. |
| 7 | Near gale | Wind28–33 kt | Waves13–19 ft | Sea stateSea heaps up; white foam from breaking waves begins to blow in streaks. |
| 8 | Gale | Wind34–40 kt | Waves18–25 ft | Sea stateModerately high waves; edges of crests break into spindrift, marked foam streaks. |
| 9 | Strong gale | Wind41–47 kt | Waves23–32 ft | Sea stateHigh waves; crests topple and tumble, dense foam streaks, spray may reduce visibility. |
| 10 | Storm | Wind48–55 kt | Waves29–41 ft | Sea stateVery high waves with long overhanging crests; sea takes on a white appearance. |
| 11 | Violent storm | Wind56–63 kt | Waves37–52 ft | Sea stateExceptionally high waves; sea covered with long white patches of foam. |
| 12 | Hurricane | Wind64 kt and above | WavesOver 45 ft | Sea stateAir filled with foam and spray; sea completely white with driving spray. |
Wave heights are the probable height in the open sea, well clear of land, with the wind having blown long enough over enough fetch to raise a fully developed sea. In a bay, a river mouth or an ebb tide against the wind, the sea for a given force can be far worse than the table suggests.
The warnings, and what is flying
Gale, Storm and Hurricane Force thresholds are national. Only the Small Craft Advisory trigger moves with the region.
| Warning | Wind | By day | By night |
|---|---|---|---|
| Small Craft Advisory | WindRegional lower bound to 33 kt | By dayOne red pennant | By nightRed over white |
| Gale Warning | Wind34–47 kt | By dayTwo red pennants | By nightWhite over red |
| Storm Warning | Wind48–63 kt | By dayOne square red flag with a black centre | By nightRed over red |
| Hurricane Force Wind Warning | Wind64 kt and above | By dayTwo square red flags with black centres | By nightRed, white, red |
Three numbers worth memorising
34 kt
Force 8 — Gale
Where a Gale Warning is issued. Force 7 below it is a near gale, which is not a gale.
48 kt
Force 10 — Storm
Storm Warning, and the signal changes from pennants to a square red flag with a black centre.
64 kt
Force 12 — Hurricane
Hurricane Force Wind Warning. The top of the scale is open-ended, not capped at 71 kt.
Frequently asked
What is the Beaufort scale?
A thirteen-step scale, force 0 to force 12, that describes wind by what it does to the sea rather than by an instrument reading. Francis Beaufort drew it up in 1805 so that officers with no anemometer could log wind consistently — the definitions are observations of wavelets, white horses, foam streaks and spray. That is still its value: it lets you read the wind off the water in front of you, and it is why the sea-state column matters more than the numbers.
What force is a gale?
Force 8, which is 34 to 47 knots — and the number is worth committing to memory because it is exactly where the National Weather Service issues a Gale Warning. Force 7 at 28 to 33 knots is a near gale and not a gale; force 9 at 41 to 47 knots is a strong gale (NWS also prints 'severe gale') and still inside the Gale Warning band. Force 10 is a storm and triggers a Storm Warning at 48 knots.
At what wind speed is a Small Craft Advisory issued?
It depends where you are, which is why a single number on a chart is misleading. The upper bound is 33 knots everywhere, but the trigger is set regionally: 20 knots in the Southern region (Georgia to Texas and the Caribbean), 21 in the Western region, 22 in Central, 23 in Alaska and 25 in Eastern and Pacific. Pick your region in the lookup above and it applies the right one. Gale, Storm and Hurricane Force thresholds, by contrast, are national.
What do the warning flags mean?
One red pennant is a Small Craft Advisory. Two red pennants is a Gale Warning. One square red flag with a black centre is a Storm Warning, and two of them is a Hurricane Force Wind Warning. At night the same warnings are shown with lights: red over white for small craft, white over red for gale, red over red for storm, and red-white-red for hurricane force. The pennant-versus-square-flag distinction is the one to hold on to — pennants are advisories and gales, squares mean storm force or worse.
How accurate are the wave heights?
They are the probable height in the open sea, well clear of land, with the wind having blown long enough over enough fetch to raise a fully developed sea. Real conditions are often worse for the same wind: a short steep sea gets up in a bay, an ebb tide running against the wind stands the waves up, and shoaling water does the rest. Treat the column as a floor for open water, not a forecast for where you actually are.
Do I need the Beaufort scale for the USCG exam?
Wind force, sea state and the warning thresholds all appear in the deck general and safety material, and the gale/storm/hurricane boundaries at 34, 48 and 64 knots are the kind of specific figure that makes a clean exam question. The signal flags are classic material too. Working the numbers is easy; what gets tested is whether you know which force a gale is and which flag is flying for it.
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