The Beaufort scale was long in use as a system for estimating wind speeds. It was introduced in 1806 by Adm. Sir Francis Beaufort (1774–1857) of the British navy to describe wind effects on a fully rigged man-of-war sailing vessel, and it was later extended to include descriptions of effects on land features as well. Today the accepted international practice is to report wind speed in knots (1 knot equals about 1.85 km, or 1.15 miles per hour).

The Beaufort scale is divided into a series of values, from 0 for calm winds to 12 and above for hurricanes. Each value represents a specific range and classification of wind speeds with accompanying descriptions of the effects on surface features, as follows:

  • 0 (calm): 0–1 knot, smoke rises vertically, and the sea is mirror smooth.
  • 1 (light air): 1–3 knots, smoke shows the direction of the wind.
  • 2 (light breeze): 4–6 knots, wind is felt on the face and leaves rustle in the trees.
  • 3 (gentle breeze): 7–10 knots, wind extends a light flag.
  • 4 (moderate breeze): 11–16 knots, loose paper blows around, and fairly frequent whitecaps occur.
  • 5 (fresh breeze): 17–21 knots, small trees sway.
  • 6 (strong breeze): 22–27 knots, wind causes whistling in telephone wires and some spray on the sea surface.
  • 7 (moderate gale): 28–33 knots, large trees sway.
  • 8 (fresh gale): 34–40 knots, twigs break from trees, and long streaks of foam appear on the ocean.
  • 9 (strong gale): 41–47 knots, branches break from trees.
  • 10 (whole gale): 48–55 knots, trees are uprooted, and the sea takes on a white appearance.
  • 11 (storm): 56–63 knots, widespread damage.
  • 12 (hurricane): 64 knots and higher, structural damage on land and storm waves at sea.

by Roger A. Pielke