Wind direction and speed

Wind is the movement of air across the earth’s surface due to differences in pressure between two regions. Air moves from high pressure regions to low pressure regions. The differences in pressure can be caused by differential heating of the earth’s surface or by earth and sea breezes.

Two aspects of wind are measured at weather stations:

1. Wind direction- this is done using a wind wane-the red-tailed instrument in the picture and the information is plotted given in terms of the cardinal points of a compass.
2. Wind speed- this is done using an cup anemometer and the speed is given in knots or as is increasingly becoming the case in km/hr.

Points of a compass

The points of a compass. Image by MediaWiki.

• The four points N,S,E and W are referred to as the cardinal points.
• The additional four points SW,SE,NE and NW are referred to as the inter-cardinal or ordinal points.
• The additional points e.g. SSW are called the secondary-inter-cardinal points.
• Wind direction is typically given in using either the cardinal points or sometimes in terms of the ordinal points.

Knots

• A note is a speed equal to 1.852 Km/hr.
• It used to be the most popular unit to measure air speed and ZIMSEC examinations and textbooks still use it.
• It has largely been superseded in most reports by the more familiar metric unit km/hr.

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