Wind direction

An example of a wind vane

Wind direction is reported by the direction from which it originates. For example, a northerly wind blows from the north to the south. Wind direction is usually reported in cardinal directions or in azimuth degrees. So, for example, a wind coming from the south is given as 180 degrees; one from the east is 90 degrees.

Origins

Winds are reported as blowing from a direction as the source of a wind is important for the properties of the air that is conveyed ('convected') by the wind. For example, a north wind in Britain frequently brings cooler air, while a 'northeaster' during the winter brings cool, dry air from the north of Europe. For more discussion of this, see Air masses.

Other examples of this phenomena are seen worldwide, for example southerly winds into Europe occasionally transport sand from north Africa.

Measurement Methods

There are a variety of instruments used to measure wind direction, such as the windsock and wind vane. Both of these instruments work by moving to minimize air resistance. The way a weather vane is pointed by prevailing winds indicates the direction from which the wind is blowing. The larger opening of a windsock faces the direction that the wind is blowing from; its tail, with the smaller opening, points in the direction the wind is blowing.

Modern instruments used to measure wind speed and direction are called anemometers and wind vanes respectively. These types of instruments are used by the wind energy industry, both for wind resource assessment and turbine control.

In primitive situations where these modern instruments are not available, a person can use the index finger to test the direction of wind. This would be done by wetting the finger and pointing it upwards. Thus, the side of the finger which feels cool is the direction from which wind is blowing. The coolness is caused by an increased rate of evaporation of the moisture on the finger due to the air flow across the finger, and thus the "finger technique" of measuring wind direction does not work well in either very humid or very hot conditions. The same principle is used to measure the dew point (using a sling psychrometer, a more accurate instrument than the human finger). One may also take a pinch of grass and drop it, the direction that the grass falls is the direction the wind is blowing. You can often see this last technique done by golfers because it allows them to gauge the strength of the wind as well.

See also

• Yamartino method for calculating the standard deviation of wind direction
• Apparent wind
• Weather vane
• Wind power
• Air masses