Beaufort Scale

The Beaufort scale is a system for describing the strength of the wind by its observed effects rather than by instruments alone. Devised in the early nineteenth century by the naval officer Francis Beaufort, it runs from force 0 to force 12, with each step corresponding to a range of wind speeds and, importantly, to a vivid description of what that wind does, the state of the sea, the height and character of the waves, and the behaviour of smoke, trees and structures on land. This makes it possible for a mariner to estimate the wind force simply by looking at the sea, which was invaluable in the age before anemometers and remains a useful skill today.

The scale gives a shared vocabulary for conditions at sea. Force 0 is calm, with a mirror-like sea; light breezes in the low numbers raise small wavelets and then larger waves with breaking crests and white horses; force 6 is a strong breeze; force 7 a near gale; force 8 a gale; force 9 a strong gale; force 10 a storm; force 11 a violent storm; and force 12 is hurricane force, with the sea completely white with driving spray and visibility badly affected. Because each force is tied to concrete observations of sea state, it conveys not just wind speed but the seaway a ship will actually experience.

The Beaufort scale remains in everyday use in marine weather forecasts and shipping bulletins, where warnings are issued in its terms — a gale warning meaning force 8 or more, for instance. Mariners use it in reporting weather, in deciding when to reduce speed or alter course, and in relating forecasts to the conditions they see around them. It complements, rather than replaces, precise instrument measurements of wind speed, and it connects naturally to the assessment of sea state and swell that governs a ship's motion, safety and comfort. Simple, descriptive and practical, the Beaufort scale endures as one of the most familiar tools in the mariner's assessment of the weather.