The Beaufort Wind Force Scale, or Beaufort Scale for short, is a standardized method of estimating and reporting wind speed. By observing the visible effects of the wind upon the open sea surface or land objects, a person can determine the approximate wind speed. Once the wind speed is determined, the wind conditions are given a Force number and designation on the Beaufort scale for use in logging weather conditions on sea or land.
The scale was devised by Rear Admiral Sir Francis Beaufort (1777-1857) of the British Navy in 1805. It is probably inaccurate to state that Beaufort "invented" the scale. Rather, he standardized terms that were already in use but were being used subjectively rather than objectively in recording weather conditions in ship's logs. For example, one captain might record a "gentle breeze" in his log while another, experiencing the same conditions, might use the term "moderate breeze" or "light breeze." Thus, without some standardized system of empirical observation and notation, the logs could be misleading or confusing.
Beaufort's original scale was designed to take account of the observed effects of the wind on the sails of British Naval ships and how those ships would act under the force of that wind rather than observations of the wind's effect on the sea itself. The British Royal Navy mandated the use of the scale as the standard form of log entries for all naval vessels in the late 1830's and it was widely adopted for non-naval use by the 1850's.
With the advent of steam power, the scale descriptors were changed to reflect the effect of wind on the open sea itself rather than on the vessel. Land-descriptors were added later to make the scale more versatile and useful for areas other than sailing.
In the 1940's the scale was further extended to include Forces 13-17 but is only used by a few countries today. Using this updated scale, the Forces 12-17 are roughly equivalent to the Saffir-Simpson Hurricane Scale but are not a complete match. Most countries and weather forecasting agencies use Saffir-Simpson when describing Hurricane force.
The Beaufort Scale can be found in numerous places on the internet and in various books such as weather field guides. Since it is standardized, most charts that give details of the scale are virtually identical in the information they contain. I have consulted many of those charts at various weather sites on the internet such as www.spc.noaa.gov and a few field guides. Listed below you will find a summary of the Scale and the conditions inherent to each.
Please note that each set of conditions is denoted by a Force Number of 0-12 and a corresponding designation as given by the World Meteorological Organization. Basically, whether you are on land or sea, you observe the effect the wind is having on your surroundings and then logically work backward to determine the Force number and WMO designation. For example: If you are on land and the wind is only slightly affecting the drift of smoke as it rises, but it is not strong enough to move a weather vane, you can deduce that this is Force 1 on the Beaufort Scale and should be referred to as Light Air in the WMO designation.
Force 0/Calm - winds under 1 knot (<1 kph). Open Sea: The surface of the water is smooth or mirror-calm . Land: Smoke rises vertically and appears undisturbed by the wind. Tree leaves do not move.
Force 1/Light Air - winds of 1-3 knots (1-3 mph/1-5 kph). Open Sea: The water surface has scaly ripples without crests. Land: Wind direction is indicated in the drift of smoke. However, the wind is not strong enough to move a weather vane.
Force 2/Light Breeze - winds of 4-6 knots (4-7 mph/6-11 kph). Open Sea: Small wavelets are observed with crests that take on a glassy appearance but still do not break. Land: Wind can be felt on the face or exposed skin. Tree leaves rustle and weather vanes begin to move. Light flags may wave slightly.
Force 3/Gentle Breeze - winds of 7-10 knots (8-12 mph/12-19 kph). Open Sea: Large wavelets are observed. Crests begin to break and there are scattered whitecaps. Land: Leaves and twigs are in constant motion while the wind will extend light flags.
Force 4/Moderate Breeze - winds of 11-16 knots (13-18 mph/20-29 kph). Open Sea: Small waves of 1-4 feet in length are observed and whitecaps are more numerous. Land: The wind will raise dust, leaves, and loose paper. Small branches in trees begin to move.
Force 5/Fresh Breeze - winds of 17-21 knots (19-24 mph/30-38 kph). Open Sea: Moderate waves of 4-8 feet are observed with many whitecaps and some spray and foam. Land: Smaller trees in leaf begin to sway.
Force 6/Strong Breeze - winds of 22-27 knots (25-31 mph/39-50 kph). Open Sea: Larger waves of 8-13 feet are observed. Whitecaps are now common with more foam and spray. Land: Large tree branches move, the wind whistles in overhead wires, and the use of an umbrella becomes difficult.
Force 7/Near Gale - winds of 28-33 knots (32-38 mph/51-61 kph). Open Sea: The surface heaps up with waves of 13-20 feet and white foam streaks can be observed off breakers. Land: Whole trees are moving in the wind while walking becomes affected by the wind.
Force 8/Gale - winds of 34-40 knots (39-46 mph/62-74 kph). Open Sea: Waves are moderately high at 13-20 feet and of greater length. Breaking crests begin to form spindrift and foam is blown in streaks. Land: Whole trees are in motion with twigs being broken from them. Walking in the wind becomes very difficult and cars veer on the road due to wind.
Force 9/Strong Gale - winds of 41-47 knots (47-54 mph/75-86 kph). Open Sea: High waves consistently 20 feet or more are observed. The sea begins to roll and has dense streaks of foam while considerable spray may reduce visibility. Land: Light structural damage will occur such as the removal of slate or shingles from roof tops. Tree branches break due to wind force.
Force 10/Storm - winds of 48-55 knots (55-63 mph/87-101 kph). Open Sea: Very high waves of 20-30 feet with overhanging crests are observed. The sea is white with densely blown foam and is rolling heavily. Visibility is lowered. Land: Trees are broken or uprooted. Structural damage is considerable.
Force 11/Violent Storm - winds of 56-63 knots (64-74 mph/102-120 kph). Open Sea: Waves are 30-45 feet in height and the sea is covered with white foam patches. Visibility is further reduced. Land: Structural damage is widespread and considerable.
Force 12/Hurricane - winds in excess of 64 knots (75+ mph/120+ kph). Open Sea: Waves exceed 45 feet, the air is filled with foam and the sea is completely white with driving spray. Visibility is greatly reduced. Land: Structural damage is severe and extensive.
Although the Beaufort Scale has largely fallen out of use in recent years, the scale and its designations still roughly correspond to the severe weather warnings given out by various weather reporting agencies.
The scale can be useful to anyone outdoors, whether on sea or land, to observe the wind conditions and how they may be changing, to make preparations or take appropriate action to prepare yourself to seek shelter or take other precautionary measures. It is still a good idea for sailors, campers, hikers, and outdoor lovers of all types to familiarize themselves with the Beaufort Scale as there may be times in our outdoor adventures when the scale may be the only way of determining the type of weather that is around the corner.