# Meteorological instruments --Wind

## Measurement of Surface Wind

Cup Anemometer
and Wind Vane

### Definition of wind velocity

Wind velocity is a three-dimensional vector quantity with small-scale random fluctuations in space and time superimposed upon a larger-scale organized flow.  In general, surface wind is considered mainly as a two-dimensional horizontal vector quantity specified by two numbers representing direction and speed.

Surface wind is usually measured by a wind vane and a cup or propeller anemometer.

### Meteorological requirements

Due to the effects of friction, wind speed increases considerably with height.  For this reason, a standard height is defined for the exposure of anemometers above open terrain. The standard exposure of anemometers over level and open terrain is 10 m above the ground.  Open terrain is defined as an area where the distance between the anemometer and any obstruction is at least 10 times the height of the obstruction.

### Units of measurement

Wind speed: metres per second (m/s)
kilometres per hour (km/h)
nautical miles per hour (knots)

Conversion factors:

 1 m/s = 3.6 km/h= 1.944 knots 1 km/h = 0.278 m/s= 0.540 knot 1 knot = 0.514 m/s= 1.852 km/h

Wind direction:

Wind direction is defined as the direction from which the wind blows, and is measured clockwise from geographical north.
Compass points (e.g. N, NE, E, S, W)
Angular bearing (e.g. 360o, 045o, 090o, 180o, 270o)