What are the Characteristics of Wind
Wind has two important characteristics—direction and speed. The direction of wind can be gauged using an instrument called the wind vane. It is also called a weather vane. Every wind vane has two parts, the front and the rear. A very common shape for a wind vane is the shape of an arrow. The tip of the arrow is the ‘front’, and the tail of the arrow is the ‘rear’. This is then mounted on a vertical column that is free to move when the wind blows.The front part has a smaller surface area than the rear.
When wind blows, it presses more on the rear part of the wind vane as it has a greater surface area. The arrow therefore aligns itself such that its tip points in the direction from which the wind is blowing. Directions, namely, north, south, east, west, etc., will be marked on the wind vane. Another common shape used in a wind vane is that of a rooster. The head has a smaller surface area than the tail, and points in the direction from which the wind blows. For an accurate reading, the wind vane should be located well above the ground, beyond trees, and other buildings which may interfere with wind direction.
The speed of wind is usually measured with an instrument called the anemometer (anemos, wind). In a type of anemometer, called cup anemometer, there are three or four cups mounted symmetrically at right angles to a vertical axis. The force exerted by the wind is greater on the inside surface of the cup than on the outside. Due to this, the cups rotate. The rate of rotation is directly proportional to the wind speed. The faster the wind speed, the faster the cups rotate. Cup anemometers are mainly used by meteorological stations.