Wind strength described by its effects
In 1805, Francis Beaufort, a British naval officer, devised a scale to describe wind by what it did to a sailing ship. It was later adapted to sea states and effects on land, and linked to measured wind speeds. Today, marine forecasts around the world still give wind in Beaufort forces, from 0, calm, to 12, hurricane force.
From speed to force
Each Beaufort force covers a range of wind speeds, measured as a 10-minute average at 10 meters above the surface. The ranges follow approximately:
v = 0.836 × B^1.5 m/s, so B = (v ÷ 0.836)^(2/3)
The calculator uses the official ranges in meters per second and converts them to mph, km/h or knots.
The scale at a glance
- 0 to 3, calm to gentle breeze: under about 12 mph, leaves and small twigs move.
- 4 and 5, moderate and fresh breeze: about 12 to 24 mph, small branches and small trees sway, whitecaps form.
- 6 and 7, strong breeze and near gale: about 25 to 38 mph, large branches move, walking into the wind is hard.
- 8 and 9, gale and strong gale: about 39 to 54 mph, twigs break and roofs can be damaged.
- 10 to 12, storm to hurricane force: 55 mph and above, trees uprooted and serious damage.
Example
A wind of 20 mph is force 5, a fresh breeze. Its exact Beaufort number is 4.9, the force 5 range runs from about 18 to 24 mph, and the typical wave height in the open sea is about 2 meters, or 6.6 feet.
Reading forecasts
- Small craft advisories in the US are typically issued from about 22 knots, force 6.
- Gusts can be 30 to 50% stronger than the mean wind: a force 6 can bring force 8 gusts.
- Wave heights are typical values for the open sea with a long fetch; near the coast or in sheltered water they are much lower.