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From: dipper@utastro.UUCP (Debbie Byrd)
Newsgroups: net.astro
Subject: StarDate: July 11 Wind
Message-ID: <333@utastro.UUCP>
Date: Thu, 11-Jul-85 02:00:23 EDT
Article-I.D.: utastro.333
Posted: Thu Jul 11 02:00:23 1985
Date-Received: Mon, 15-Jul-85 07:14:03 EDT
Organization: U. Texas, Astronomy, Austin, TX
Lines: 31


The air surrounding our planet Earth is always in motion.  We'll talk
more about the wind -- when we come back.

July 11  Wind

On our planet Earth, the wind is always blowing somewhere.  So the wind
must ve a constantly renewable source of energy.  It turns out that the
wind stems from the ultimate energy source of almost everything on
Earth -- the sun.

Here's why the wind blows.  Sunlight shines on the Earth, and warms the
air and the ground.  Warm air expands -- and pretty soon the air near
the ground gets warm enough to expand and rise.  New air flows in along
the ground to take its place -- and that flow of air is what we call
wind.

In the lower atmosphere, where we are, wind speed tends to increase
with altitude.  If you live in the city, you probably know that the
wind blows harder at the top of tall buildings than it does near the
ground.  The wind near the ground is slowed down by friction with the
surface of the Earth.

That's the general rule -- but it's actually the case that on summy or
rainy days, fast-moving air from the upper atmosphere moves down toward
the ground to increase the speed of the wind.  If the atmosphere is
stable, the fast-moving air stays up high.  That's why, in a fog or at
night, the wind usually blows very softly -- or not at all.

Script by Deborah Byrd.
(c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin