Why is it Dry?

March 16, 2013
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Why is it Dry?

If you live in the southwest United States, you may be in an area of infrequent precipitation. For example, Phoenix, Arizona gets less than 8 inches of rain per year. Compare that with New York or Chicago, each of which gets over 30 inches. So you may ask yourself “Why is it so dry? And why is it so rainy on the other side of the mountain?” So I will answer your questions. I hope you’ll follow along, because the answers will take us all over the globe.The Driest Place on Earth

The Atacama desert of Chile is located on the west slope of the Andes Mountains, with the Pacific Ocean to the west. The average annual rainfall is about four hundredths of an inch, and some stations have never recorded rain since instruments were introduced. Why is it so dry? The towering Andes to the east protect the Atacama from rain heading westward; and the cold Humboldt Current of the Pacific Ocean creates a stable layer of air offshore which prevents rainfall from encroaching from the west. These factors are unique to Atacama, but one thing is not: this desert is located at 25 degrees south latitude. Remember that number as we travel to other deserts.

The World’s Biggest Deserts

The common definition of a desert is a place that receives less than ten inches of rainfall per year. By this definition, the largest deserts in the world are the Arctic and Antarctic, each stretching for over five million square miles. But the factor that creates these deserts is the temperature of the air; the air is so cold it simply can’t hold much moisture. Though we think of these regions as perpetually covered with snow, the snow has very little water content. We’re more interested in the hot deserts of the lower middle latitudes, the largest of which, by far, is the African Sahara at over three million square miles. It is more than three times as extensive as any other non-polar desert. So how did it get that way? Well here’s one story; if you’ve heard it, just skip to the next paragraph.

A scrawny fellow walks into a Canadian lumber camp and asks if he can have a job as a lumberjack. The foreman tells him he’s too skinny, but the youngster persists and asks for a chance to show his skill. So they go out in the woods and the foreman asks him to chop down a small sapling, which the kid quickly dispatches. The foreman takes him to a medium sized tree, which goes down with a couple of chops. So they go to one of the biggest trees in the forest and the skinny kid knocks it down with just a few strokes. “Where’d you learn to do that?” asks the foreman. “Sahara Forest,” replies the youth. “But the Sahara is a desert,” says the foreman. And the skinny kid says: Yeah, now it’s a desert.

Well it wasn’t quite that way, although if you follow the tectonic plate on which the Sahara sits in its wanderings though millions of years, there undoubtedly was a time when it was forested. Today, The Sahara receives an average of about an inch of rainfall per year — somewhat more on the fringes. But why so little rain? Well, for one thing, the desert stretches inland across Africa and most of it is far from any source of moisture. Also, it’s centered at about 26 degrees latitude.

The Arabian Desert

The Arabian Desert comprises nearly a million square miles in the Middle East; it is the second largest non-polar desert. The average latitude is 27 degrees. There are no jokes about how it came to be a desert, possibly because it isĀ  considered the cradle of modern civilization and three of the world’s great religions began there. So now let’s see if the deserts we’ve visited have anything in common — and anything in common with the deserts of the southwest United States.

The General Circulation of the Atmosphere

The circulation of air in the atmosphere seems complicated: windy today, calm tomorrow; rainy today, dry tomorrow; chile today, hot tomale. OK, go ahead and groan. But underneath the unpredictable weather are basic patterns: east winds in the tropics; west winds in the mid-latitudes; and east winds near the poles. That’s about all there is to it, except — and it’s an important ‘except’ — the earth is not flat, nor is it uniformly covered with water.

By now you’ve probably noticed that the deserts we visited are all located within a stone’s throw of thirty degrees latitude — well, yes, you’d have to be a very good stone thrower — and on a global basis that’s significant. Between the easterlies of the tropics and the westerlies of mid-latitude lies a zone of quiet weather and sinking air. These conditions are associated with fair weather, so, other things being equal, two belts around the globe at thirty degrees latitude contain most of the world’s deserts. The other major factor is, of course, moisture. I live in Florida at 27 degrees north latitude and we get plenty of rain. That is because we are surrounded by water. The Sahara is far from any source of moisture, and so are other major deserts.

Deserts of the United States

The 30 degree latitude line hits the U.S. east coast near Jacksonville, Florida, heads west along the Gulf Coast, through southern Texas, and into northern Mexico and Baja California. Places east of central Texas are too close to the copious moisture of the Gulf of Mexico to become arid. In fact, Miami, at 26 degrees, is the wettest major city (population over 100,000) in the continental U.S. Over five feet of rain falls in Miami in an average year. The driest major city is Las Vegas (36 degrees latitude), where less than five inches of rain falls in an average year.

 

 

The southwestern United States, far from the moisture of the Gulf of Mexico and out of the path of Pacific storms, is not uniformly dry because the land is not flat. Mountains create upslope winds on the windward side and these can produce a lot of rain and snow. The lee side of a mountain is considered a “rain shadow” and can be drier than surrounding areas — all the moisture was squeezed out on the rainy side.

Driest City in the Continental U.S.

As we said, Las Vegas is the driest major city in the lower 48; but the driest city of more than population five thousand is Yuma, Arizona, with an annual rainfall of under three inches. Still, Yuma does have a rainy season, or at least a rainy month, August, when over a half inch of rain normally falls. This can be attributed to a monsoonal flow from the far off Gulf of Mexico — a tiny monsoon, but a monsoon nonetheless.

 

 

 

 

 

 

 

 

 

 

 

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