Air speedThe rate of progress of a bird in relation to the air in which it is flying. The bird's speed in relation to the ground ('ground speed') will vary according to the wind, in that it will consist of its air speed plus or minus the wind speed (depending on whether the bird is flying wholly or partly against or with the wind). Therefore, a bird with an air speed of 20 kph flying into a head wind of 12 kph will have a ground speed of 8 kph.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver