March 30, 1998 transcript #: 227-1 
Subject(s): desert, equator 

There are plenty of dry places around the world, but not every one qualifies as a desert.  Hi, I’m Dave Thurlow from the Mount Washington Observatory for The Weather Notebook.  A desert, to qualify as a true desert, has to get less than ten inches of rain in a typical year.  Even the windblown Great Plains usually get more than that.  

The major deserts of North America are in the southwest U.S. and Mexico, clustered around 30 degrees north latitude.  Take a look around the globe, and you’ll find that deserts seem to like this latitude.  The Sahara in Africa, the Great Indian Desert, and the Saudi Arabian desert all lie about a third of the way from the equator to the North Pole.  The same rule applies at 30 degrees south.  So why do deserts like to hang out at a certain distance from the equator?  It starts with simple physics.  The equator gets more sunshine, therefore, more heat than the poles, so air rises near the equator and sinks near the poles.  Sinking air dries things out and in fact, the North and South Poles are both dry enough to qualify as deserts.  

But the world is a complex and puzzling place that spins and curves.  Because of this, you actually have rising air near the equator, then sinking near 30 degrees north and south, rising near 60 degrees north and south, and sinking air at the poles.  Now, the upshot of all this rising and sinking is that the earth’s circulation causes air to sink, thus get warmer and drier right where we find the world’s great deserts.  It’s no coincidence.  And those deserts are found in between the soggy equator and the stormy midlatitudes.  The Weather Notebook today is written by Bob Henson.  Our show is underwritten by Subaru, the beauty of all wheel drive with major funding provided by the National Science Foundation.