Marine weather is ever-changing, but the Beaufort Scale of Wind Force helps boaters gauge wind speed and wave height, and gives the National Weather Service a standardized way to communicate weather advisories in their marine weather forecasts. Developed in 1805 by Admiral Sir Francis Beaufort, the Beaufort Scale of Wind Force was designed to gauge wind speeds through visual observations of the sea state. The scale runs from force 0 (calm) to force 12 (Hurricane). Forces 7 through 12 correspond to the tropical storm cycle of tropical depression, tropical storm and hurricane.
The National Weather Service issues Gale Warnings for sea areas when winds of force 8 or greater are forecast, and, locally, strong wind warnings may be issued for winds of force 5 to 7.
Calm - 0
Wind speed is 0-1 mph/kts; wave height is 0 ft.
The sea is calm, like a mirror.
Light Air - 1
Wind speed is 1-3 mph/kts; wave height is >.5 ft.
Ripples with the appearance of scales are formed, but without foam crests.
Light Breeze - 2
Wind speed is 4-7 mph/4-6 kts; wave height is >1 ft.
Small wavelets, still short, but more pronounced. Crests have a glassy appearance and do not break.
Gentle Breeze - 3
Wind speed is 8-12 mph/7-10 kts; wave height is 2-3 ft.
Large wavelets. Crests begin to break. Foam of glassy appearance. Some whitecaps.
Moderate Breeze - 4
Wind speed is 13-18 mph/11-16 kts; wave height is 3.5-5 ft. Small waves, becoming larger; fairly frequent whitecaps.
Fresh Breeze - 5
Wind speed is 19-24 mph/17-21 kts; wave height is 6-8 ft.
Moderate waves, taking a more pronounced long form; many whitecaps are formed. Chance of some spray.
Strong Breeze - 6
Wind speed is 25-31 mph/22-27 kts; wave height is 9-13 ft.
Large waves begin to form; the white foam crests are more extensive everywhere. Probably some spray. If the storm forms in the tropics, a 6 or 7 on the Beaufort Scale corresponds to a Tropical Depression's wind speed as it develops first into a Tropical Storm, and if it strengthens, a hurricane.
Near Gale - 7
Wind speed is 32-38 mph/28-33 kts; wave height is 13-19 ft.
Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind.
Gale - 8
Wind speed is 39-46 mph/34-40 kts; 19-25 ft. wave height is 18-25 ft.
Moderately high waves of greater length; edges of crests begin to break into spindrift. The foam is blown in well-marked streaks along the direction of the wind. If the storm forms in the tropics, once the wind reaches speeds above 38 mph, it is categorized as a Tropical Storm
and given a name.
Severe Gale - 9
Wind speed is 47-54 mph/41-47 kts; wave height is 23-32 ft.
High waves. Dense streaks of foam along the direction of the wind. Crests of waves begin to topple, tumble and roll over. Spray may affect visibility.
Storm - 10
Wind speed is 55-63 mph/48-55 kts; wave height is 29-41 ft.
Very high waves with long overhanging crests. The resulting foam, in great patches, is blown in dense white streaks along the direction of the wind. On the whole the surface of the sea takes on a white appearance. The 'tumbling' of the sea becomes heavy and visibility is affected.
Violent Storm - 11
Wind speed is 64-72 mph/56-63 kts; wave height is 37-52 ft.
Exceptionally high waves (small and medium-size ships might be for a time lost to view behind the waves). The sea is completely covered with long white patches of foam lying along the direction of the wind. Everywhere the edges of the wave crests are blown into froth. Visibility affected.
Hurricane - 12
Wind speed is 73-83 mph/64-71 kts; wave height is over 45 ft.
The air is filled with foam and spray. Sea completely white with driving spray; visibility very seriously affected. A 12 on the Beaufort Scale corresponds to a Category 1 Hurricane on the Saffir-Simpson Scale
, the scale by which hurricanes are measured.