Saturday, June 5, 2010

The Beaufort Scale

If someone told you there was a "storm" coming, how fast would the winds be, and what would the effect be on objects on the ground? What about a "breeze" or a "stiff breeze?" In our everyday conversation, these are inexact terms and often two people using the same term will mean two very different things.

This is the problem the Royal Navy had in the early 1800s. The more precise the weather observations the navy's officers could record in their logs, the easier it would be to predict the weather long-term in a given location, an important advantage to trade and defense for a sea-going empire like Britain's. Enter Admiral Sir Francis Beaufort. In 1809, Beaufort developed the scale that bears his name today, But this early version didn't measure wind speed at all, simply the effect of the wind on the sails of a square-rigged man-of-war. Consisting of thirteen "force" levels numbered from zero to thirteen, Beaufort's scale made it easy for officers on ships to record the wind: simply subtract the number of sails the ship had up from thirteen. The scale was adopted by the Royal Navy in the 1830, and by the 1850s non-military ships began to use. Gradually, it was adopted by ships of other maritime nations.

In the early 1900s, as sails were replaced by steam engines, the Beaufort scale was changed to reflect the effect of the wind on the seas. Thus a Force 0 indicated flat calm seas, while Force 12 winds were called hurricane force winds generating seas of 46 feet or more. While Beaufort's original scale stopped at Force 13 (later 12), force levels up to 17 were added as the system was adopted by maritime nations dealing with tropical typhoons in the western Pacific.

With the switch to the metric system, many countries dispensed with the Beaufort scale in favor of meters/second or kilometers/hour descriptions. The UK still uses the scale in civilian weather advisories, and many other countries issue weather warning based on the Beaufort scale.

Force 0, calm. Winds are less than 1 knot. Seas are flat. On land, smoke rises vertically.

Force 1, light air. Winds 1-2 knots. Seas begins to ripple. Smoke drifts downwind, but solid objects are unaffected.

Force 2, light breeze. Winds 3-6 knots. You can feel the wind on your bare skin and light objects like leaves and paper litter start to rustle.

Force 3, gentle breeze. Winds 7-10 knots. Seas begin to form some whitecaps. Small objects likes twigs start to move.

Force 4, moderate breeze. Winds 11-15 knots. Small waves with breaking crests, tree branches moving.

Force 5, fresh breeze. Winds 16-20 knots. Some sea spray, small trees start to sway.

Force 6, strong breeze. Winds 21-26 knots. Long waves, white foam crests, some airborne spray. On land, you hear whistling in overhead wires and empty plastic garbage cans turn over. In Canada this is called a strong wind; in the US winds at this level will result in a small craft advisory.

Force 7, high wind/moderate gale/near gale. Winds 27-33 knots. The seas begin to pile up. On land, it's tough to walk against the wind.

Force 8, gale/fresh gale. Winds 34-40 knots. Seas approaching 20 feet or more, the wind starts pushing cars around on the road. In Canada, this is called gale force; in the US, winds at this speed will result in a gale warning.

Force 9, strong gale. Winds 41-47 knots. Seas approaching 30 feet or more. Small trees blow over.

Force 10, storm/whole gale. Winds 48-55 knots. Seas 40 feet or more, large patches of form give the water a white appearance. Trees are uprooted, shingles are torn from roofs. Canada calls winds this speed storm force; the US will issue a storm warning at this level.

Force 11, violent storm. Winds 56-63 knots. Seas may reach 50 ft or more, and spray reduces visibility. Many plants, trees, and structures are damaged.

Force 12, hurricane force. Winds more than 64 knots. Huge seas. Lots of damage and debris on land. Corresponds to a Category One (on the Saffir-Simpson Scale) hurricane.

Hurricanes. Forces 13-17 roughly correspond to the higher levels on the Saffir-Simpson hurricane scale. Thus Hurricane Katrina, which was a Category 3 hurricane when it came ashore in Louisiana, would be expected to have Force 14 or 15 winds.