From: Subject: Wednesday Weather Topic in Greater Depth Date: Fri, 30 Oct 2009 14:15:35 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/online/archive/course/09_fall/f09w08w_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Wednesday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Wednesday, 28 = October=20 2009

THE EKMAN SPIRAL

If you have ever launched a balloon from a large field, away from = many=20 obstacles on a day with a modest wind, you will probably see that as the = balloon=20 rises, it makes a sweeping motion toward the right of the direction of = surface=20 wind flow as you look downwind. You will also note that the balloon will = be=20 carried along horizontally at faster speeds aloft.

What you have witnessed is an example of how friction changes the = direction=20 and speed of the wind as one leaves the surface. When an altitude of=20 approximately 1000 meters is reached, the wind often has arrived at the=20 so-called gradient wind flow. At this level the winds would parallel the = isobars=20 drawn on the current surface weather chart, with low pressure to the = left of the=20 motion in the Northern Hemisphere.

A mathematical model of this wind change with height is called the = "Ekman=20 spiral", named for the Swedish physicist, Vagn Walfrid Ekman = (1874-1954). This=20 model describes the winds in the region between the surface and the free = atmosphere, where frictional effects become negligible. A simple = analogue would=20 be a very tall flag pole that has pennants or streamers located at = evenly spaced=20 intervals up the pole subject to the wind at a series of altitudes. The=20 direction of each pennant is turned slightly to the right of the one = below,=20 forming a spiral that turns toward the right. Moving up the pole, the = pennants=20 would become more unfurled and stand out more from the pole because the = strength=20 of the winds near the top would be stronger.

The increase in wind speed and the turning of the winds with height = result=20 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=20 on that pressure gradient and Coriolis effect. If one connected the tips = of each=20 pennant with a curve in space, the resultant curve would form a smooth = open=20 spiral, similar to that traced out by the balloon.

Return to the Wednesday=20 Daily Weather Summary

Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu =
=A9=20 Copyright, 2009, The American Meteorological Society. =