The correct answer is: A. Beaufort scale
The Beaufort scale is an empirical measure that relates wind speed to observed conditions at sea or on land. It was devised in 1805 by Sir Francis Beaufort, then a Royal Navy captain, as an aid to navigation. The scale is based on the observed effects of the wind on the sea, on land, and on trees.
The Beaufort scale is divided into 12 categories, from 0 to 12, with increasing wind speed. The scale is not linear, so a change of one Beaufort number does not correspond to a constant change in wind speed. For example, a wind of force 6 is twice as strong as a wind of force 4, but a wind of force 10 is only four times as strong as a wind of force 4.
The Beaufort scale is widely used by sailors, fishermen, and other people who need to be aware of the wind conditions. It is also used by meteorologists to provide information about the wind speed.
The other options are incorrect because:
- A wind indicator is a device that measures the wind speed.
- A barometer is a device that measures atmospheric pressure.
- None of these options is a measure of wind speed.