By Danny Keating, Director of the Louisiana/Mississippi HVAC Insider
As we sit here in Louisiana and Mississippi drenched and soggy from all the rainfall during the month of May, I got to thinking that we might happily want to share some of our bounty with the people who really need it.
The first places that came to my mind were the Sahara Desert, the interior of Australia, the southwest United States, and believe it or not Antarctica. Well, Antarctica didn’t actually pop into my mind, but the continent is a desert. It’s a very cold desert, but nonetheless it’s still a desert.
Most of the world’s surface is covered in water, 71%. The remaining landmass of the earth amounts to approximately 29 percent of the surface. Of this remaining 29 percent, deserts of all types constitute an estimated 33 percent, or one-third, of the earth’s total landmass.
In the United States all the deserts are in the western portion of the country. Deserts are defined to be areas that receive less than ten inches of precipitation (rain, snow, etc.) a year. The Mojave Desert receives less than 2 inches of precipitation every year, which makes this desert the driest in North America. The hottest temperature recorded there is 134° Fahrenheit.
The Great Basin Desert is generally considered the largest of the four U.S. deserts. It is considered a temperate desert that experiences hot and dry summers with cold winters. This desert is home to the oldest known living organism in the world, the Bristlecone Pine tree. Some of these trees are estimated to be over 5,000 years old.
In general, the U.S. deserts are deserts because a mountain range stands between them and the ocean. The mountain range doesn’t allow any moisture-laden air to cross over its top. All the rain or snow falls on the windward slopes and very little on the leeward side. These deserts are said to be in the “rain shadow” of the mountains.
The largest (non-polar) desert in the world is the Sahara, in North Africa, which spans an area measured at roughly 3.5 million square miles. The whole of the United States’ contiguous 48 states could easily fit in this desert. Since the Sahara isn’t in a “rain shadow”, something else must be responsible for it being so dry. The culprit appears to be the Hadley circulation.
This is a tropical atmospheric circulation that rises near the equator. It is linked to the subtropical trade winds, tropical rain belts, and the jet stream. Where it descends in the subtropics, it can create desert-like conditions. Most of the earth’s arid regions are in areas beneath the descending parts of the Hadley circulation. Its low relative humidity coupled with an already dry mass of air results in extremely dry conditions at the edge of Hadley cells. This leads to very little precipitation to these regions and, consequently, atmospheric conditions that provide us with most of the world’s hot deserts. If you look at a map of the earth, most of the planet’s hot deserts, such as the Saharan desert, Arabian Desert, Australian deserts, and Kalahari Desert, are located at similar latitudes. The reason for this isn’t simply because these regions are hot, but because they are all located at the edge of Hadley cells.
In the Hadley circulation air rises about ten miles into the atmosphere at or near the equator, flows toward the North or South Pole far above the surface of the earth. It returns to the earth’s surface in the subtropics (about 30 degrees latitude) and flows back towards the equator to begin the process again. It has become cool and dry, and as it is heated up its relative humidity decreases, and it begins to absorb moisture from the land.
There is not much anyone can do about this, and these deserts will continue to get drier until there is some minor change in the earth’s orbit or some other dramatic climate change.
In Antarctica, the very cold air is not able to hold much moisture, and therefore it doesn’t rain or snow very often and that makes it a desert. It’s so cold that the snow doesn’t melt and piles up, but technically it is still a desert. The Arctic desert is almost as large as the Antarctic desert. Either one of them is about 1 ½ times the size of the Sahara Desert.
Upon further review, I think I like my drenched and soggy home.