Wx Savvy | Climate

How Do Deserts Form?

All deserts form due to very little rain or snow over long periods of time. Yes, that's right...I said lack of rain or SNOW, in fact Antarctica is the largest desert on earth. Although lack of rain or snow is the paramount reason for a desert forming, there are various factors responsible for the lack of precipitation. Ultimately, it depends on the location of the desert to decipher the reasons why it became a desert in the first place. It's all about location, location, location.

Subtropical High Pressure:

The largest deserts on earth (other than the polar deserts) are largely influenced by a subtropical high pressure. Subtropical means out of the tropics, specifically it refers to an area of high pressure around 30 degrees North and 30 degrees South of the equator. If you look at a globe, you'll notice most of the major deserts are found in this "belt" near 30 South or 30 North. This includes the Australian desert, the Atacama in Chile (considered driest in the world), the deserts of the Western US and Mexico, the Sahara, and Arabian deserts among others (1).

In this map below you'll notice all the deserts in orange to reddish colors. Then, I have drawn a black line where 30 degrees North and 30 degrees South is located. Notice how many of the large deserts are all located in or around these belts. Again this is the location of the subtropical high pressure that helps to create atmospheric conditions ripe for desert formation.

Here's why these black lines above are ideal atmospheric conditions for deserts; the air from storms near the equator rises very high and eventually falls back down to earth creating the subtropical high pressure zone. This air in the subtropical high pressure zones is very dry because the air has dropped most of its moisture in the form of rain over the tropics. Also, as the air sinks around 30°North and 30°South it warms due to compressional heating. Warmer air encourages more evaporation from the surface which helps to further the dry climate. Along with a dominant high pressure system, there tends not to be any strong winds. Without a dominant wind blowing in storms or moisture off the ocean, the land further remains dry.

Did You Know: The region around 30° North and 30° South is called the horse latitudes. The name horse latitudes came about due to Spanish sailing ships headed for the West Indies. As the ships encountered the calm winds around 30° degrees North, the ships wouldn't move fast enough. So, sailors often threw horses overboard in order to preserve fresh water for themselves.

The Rainshadow Effect

The rainshadow effect is another influence on the formation of deserts and it has to do with being located near mountains (like I said, location, location, location). The deserts in the US are all created largely from the rainshadow effect.

What happens is that winds flow towards a mountain range (the windward side) and then the winds are forced up the mountain, this is called orographic lifting. In the Western US the wind typically comes in from the West, up the mountain's West side (windward side) and then down on the eastern side of the mountains called the leeward side. As the air is forced up the windward side of the mountain it cools and this helps the moisture in the air condense into rain or snow. When the air reaches the top of the mountain range it has dumped most of it's moisture. So when the air slides down the other side of the mountain it's a very dry air. Also, when the air slides down the mountain slope (the leeward side), it warms as it sinks back down. This warming causes more evaporation, removing moisture from the soil.

It is on the leeward side of mountains that you'll find the rainshadow effect influencing the development of a desert. Although not as large an influence as the subtropical high pressure talked about above, the rainshadow effect is evident in helping to from the North American deserts (from the Rocky Mountains) and the Gobi (from the Himalayans)(2).

(1) http://www.factmonster.com/ipka/A0778851.html

(2) http://horizon.nmsu.edu/ddl/database/overview.html#desert