If you have ever launched a balloon from a large field, away from many obstacles on a day with a modest wind, you will probably see that as the balloon rises, it makes a sweeping motion toward the right of the direction of surface wind flow as you look downwind. You will also note that the balloon will be carried along horizontally at faster speeds aloft.
What you have witnessed is an example of how friction changes the direction and speed of the wind as one leaves the surface. When an altitude of approximately 1000 meters is reached, the wind often has arrived at the so-called gradient wind flow. At this level the winds would parallel the isobars drawn on the current surface weather chart, with low pressure to the left of the motion in the Northern Hemisphere.
A mathematical model of this wind change with height is called the "Ekman spiral", named for the Swedish physicist, Vagn Walfrid Ekman (1874-1954). This model describes the winds in the region between the surface and the free atmosphere, where frictional effects become negligible. A simple analogue would be a very tall flag pole that has pennants or streamers located at evenly spaced intervals up the pole subject to the wind at a series of altitudes. The direction of each pennant is turned slightly to the right of the one below, forming a spiral that turns toward the right. Moving up the pole, the pennants would become more unfurled and stand out more from the pole because the strength of the winds near the top would be stronger.
The increase in wind speed and the turning of the winds with height result from the reduction in frictional effects with height. At the top of this friction layer, the pennant would show the gradient (or geostrophic) wind based on that pressure gradient and Coriolis effect. If one connected the tips of each pennant with a curve in space, the resultant curve would form a smooth open spiral, similar to that traced out by the balloon.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological
Society.