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Wind force 0 to 12, described by what it does rather than measured.
Also known as: Wind force, Force 8
A scale defined by observable effects — smoke rises vertically, leaves stir, whole trees move — devised so that wind could be reported consistently from a ship with no instrument. It remains in the shipping forecast for the same reason it worked in 1805.
A scale of wind strength from 0, calm, to 12, hurricane force, each step defined by observable effects on the sea and on land as well as by a range of wind speeds.
Contexts: Navigation, Maps
Defined by effects rather than by instrument, which is why it was adopted: a scale anyone can apply from a deck by looking at the sea produces consistent reports from ships with no anemometer.
from 1805
Still used in the shipping forecast, where force 8 is a gale and triggers a warning. That threshold is why the number matters to a listener rather than the wind speed behind it.
United Kingdom
A numeral from 0 to 12, usually with a descriptive word beside it, in a table or spoken in a forecast.
The barb plots on a chart what the Beaufort number states in words.