Moving air is called wind. Slow winds are called gentle breezes whereas strong winds are known as gales. You can see the speed of the wind by its effect on trees and buildings, but cannot measure its velocity easily because it is invisible. The instrument used for measuring the velocity of wind is called the ‘anemometer’ or wind gauge. An English scientist called Robert Hooke invented anemometer in 1667. There are many kinds of anemometers. Generally, they consist of three or four small aluminium cups attached to an axle. These cups can freely rotate when kept in the open, if wind is blowing. The higher the velocity of the wind, the faster is the speed of rotation of these cups. The rate of rotation of these cups in a fixed time is used to calculate the velocity of wind. This instrument has a meter with a pointer, which gives the speed of rotation of the cups. This meter is so calibrated that it directly gives the velocity of the wind. It is very necessary to measure the velocity of wind and its direction. When man started flying in aeroplanes, it became essential for him to know the wind velocity and wind direction. Initially he used to measure the speed of wind by observing the movement of a balloon in the sky, but with the invention of anemometers, the task has become much easier. Scientists have succeeded in developing small size anemometers. Knowledge of wind direction and velocity has proved very useful in meteorological studies, especially in weather forecasting. Sailors, like pilots, need to know the wind direction and speed.