The wind is one of the most important aspects of sailing, surfing, and weather. One of the first scales to estimate wind speeds and their effects was created by Britain’s Admiral Sir Francis Beaufort (1774-1857). He developed the scale in 1805 to help sailors estimate the winds via visual observations. The Beaufort scale is still used today to estimate wind strengths. We use the Beauford scale in our sailing, surfing, and weather reports.
What is a Knot?
A ‘knot’ is a unit used to measure speed in nautical miles per hour. The term originated in the 17th century when sailors measured the speed of their ships by means of a device called a ‘common log.’ This device was a coil of rope with uniformly spaced knots tied in it, attached to a piece of wood shaped like a slice of pie. The line was allowed to pay out freely from the coil as the piece of wood fell behind the ship for a specific amount of time. When the specified time had passed, the line was pulled in and the number of knots on the rope between the ship and the wood was counted. The speed of the ship was said to be equal to this number.
0-1 Knots Calm – Sea like a mirror.
Calm; smoke rises vertically.
1-3 Knots Light Air – Ripples with the appearance of scales are formed but without foam crests.
The direction of the wind is shown by smoke drift, but not by wind vanes.
4-6 Knots Light Breeze – Small wavelets, still short, but more pronounced. Crests have a glassy appearance and do not break.
The wind is felt on your face; leaves rustle; ordinary vanes are moved by the wind.
7-10 Knots Gentle Breeze – Large wavelets. Crests begin to break. Foam of glassy appearance. Perhaps scattered white horses.
Leaves and small twigs are in constant motion; the wind extends a light flag.
11-16 Knots Moderate Breeze – Small waves, becoming larger; fairly frequent white horses.
The wind raises dust and loose paper; small branches are moved.
17-21 Knots Fresh Breeze – Moderate waves, taking a more pronounced long form; many white horses are formed.
Small trees in leaf begin to sway; crested wavelets form on inland waters.
22-27 Knots Strong Breeze – Large waves begin to form; the white foam crests are more extensive everywhere.
Large branches are in motion; whistling is heard in telegraph wires; umbrellas are used with difficulty.
28-33 Knots Near Gale – Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind.
Whole trees in motion; inconvenience felt when walking against the wind.
34-40 Knots Gale – 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.
Breaks twigs off trees; generally impedes progress.
41-47 Knots Severe Gale – High waves. Dense streaks of foam along the direction of the wind. Crests of waves begin to topple, tumble and roll over. Water spray may affect visibility
Slight structural damage occurs (chimney pots and slates removed)
48-55 Knots Storm – 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 shock-like. Visibility affected.
Seldom experienced inland; trees uprooted; considerable structural damage occurs.
56-63 Knots Violent Storm – Exceptionally high waves (small and medium-size ships might be for 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.
Very rarely experienced; accompanied by wide-spread damage.
64-71 Knots Hurricane – The air is filled with foam and spray. Sea is completely white with driving spray; visibility is very seriously affected.