Deserts, Not Desserts...Hmmmm...Desserts

Hey Steve,

How are deserts formed?

I live in Australia, an island surrounded by oceans but for the most part a vast sandy wilderness. Coastal areas are the only areas not subject to drought and support the vast majority of it's population. As deserts always seem to get bigger and never smaller,what factors tend to increase their size.?

Regards,

Allan Wilson
Victoria, Australia

That's a good point Alan, most of the non-desert area of Australia is on the coast. I will eventually go in depth to answer your question, but I'll try to give a simple and direct answer first.

Australian deserts formed from many factors, but mainly because of location on the planet and the fact that the desert areas are so far from any water. For most deserts on earth location is a big factor. This is because certain weather phenomena occur to create a dry climate at certain latitudes on the globe. The details surrounding this are explained below. But just think of it this way... why do glaciers form? Their location is the main factor, as parts of the globe are better suited, weather and climate wise, to support glacier formation. The same thing is true with deserts, but may be less obvious.

As far as deserts increasing in size, yes this is true. According to one site I found, it said that 20,000 square miles per year are becoming deserts (http://www.didyouknow.cd/deserts.htm). Desertification is mainly thought to be a problem of people. In particular, too many people. Overgrazing of lands, over-cultivating land, and destruction of forest land for development are all human related activities that impact the increase in desertification.

Now For The Nitty Gritty Details...

How Are Deserts Formed?

Desertification is the process by which an area of land becomes a desert. A desert forms because the amount of evaporation exceeds the amount of rain received. Some deserts get a surprising amount of rain, but all that water is not soaked up by the land and plants, and evaporates quickly. How and why a desert forms has a lot to do with location.

Global Location and Subtropical High Pressure:

http://pubs.usgs.gov/gip/deserts/atmosphere/


Almost all major deserts across the globe are found in the trade winds region (colored in yellow at left)(1). Major deserts including the Great Australian Deserts are found in a location on the planet near or under Subtropical high pressure, or the "Horse Latitudes".

Let's break down this term "Subtropical High Pressure" and explain why it helps in the formation of a desert.

Subtropical just relates to the location of the high pressure, it is outside of the tropics. The tropics are characterized by hot weather and wet weather. What happens to the air after it dumps it's moisture over the rain forests in Brazil?

That air rides high above the surface of the earth, eventually sinking over the "Subtropical" regions. So it is in this area of the globe we find the dried out air, as it has left it's moisture over the tropics. But wait! There's More...this is where High Pressure comes in. High pressure is characterized by sinking air. When the air sinks from above it warms and increases evaporation. The atmosphere underneath high pressure tends to be dry & cloud free.

So let's add this all up...air has become dry by pouring rain over the tropics, then sinks over the subtropics (roughly 30 degrees North/South of the equator), and as it sinks becomes even more dry (or less moist. Tomayto...Tomotto). The result is atmospheric conditions that support the formation of a desert. All this is happening in an area called "The Horse Latitudes" where we find our subtropical high pressure.

Examples:

A dominant high pressure (meaning it pretty much stays there for either a season or all year long) exists over Australia. It does meander a little bit from about the center of the continent to the Western Coast, but all this sinking air over Australia is helping to evaporate water and create a dry climate. You'll notice the moisture rich areas of Australia are on the East Coast, farthest from the general center of the subtropical high pressure.

Another dominant high pressure area exists off the coast of California. That is not only why Southern California is so dry, but also why a Hurricane has never hit the Western Coast of the US. As air sinks it spreads outward at the surface and tends to push any storm systems away.

Rain Shadow Effect

Most coastlines are not deserts, and Allan mentioned this about Australia in his email. Areas inland, not near any oceans or seas, are more likely to support a desert climate. Not only that, but inland areas hiding behind mountains tend to help support a desert climate...the rain shadow effect.

What happens is that winds flow in off the moisture rich oceans hit the coastal mountains and have no where to go but up. This is called orogrpahic lifting, and "lift" is essential in the formation of rain. As the air rises, it cools, forms clouds, then rain. Then the air flows down the mountain. Not only does it not have any more water (as it dumped it on the coastal mountain side), but the air is flowing downhill or sinking. As discussed above, air that sinks increases evaporation. As air sinks it also heats up. So, the result is one side of the mountains near the coast with plenty of rain, lush green plant life...and the inland side of the mountain (leeward side) is dry, hot, and has a drastically different ecosystem.

All deserts in North America are influenced by the rain shadow effect.

There are Polar deserts, Monsoon Deserts, and even Coastal Deserts. But instead of writing the book on this stuff, I decided to explain the steak and potatoes of it and leave the rest of it for you to research on your own.

Thanks for the question Allan.
I am proud to know that the saviness is spreading to other continents, you gotta love the internet.

If you have a weather question, email me...HeySteve@WeatherSavvy.com

(1)http://pubs.usgs.gov/gip/deserts/types/