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Medicine for the Outdoors

Dr. Paul Auerbach is the world's leading outdoor health expert. His blog offers tips on outdoor safety and advice on how to handle wilderness emergencies.

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The Desert and Desert Survival 1

This is the third post based upon educational sessions and syllabus material presented at the Wilderness Medical Society Annual Meeting & 25th Anniversary held in Snowmass, Colorado from July 25-30, 2008. This post about the desert and the next two about desert survival are based upon the extensive knowledge of Dr. Edward J. (“Mel”) Otten, who is Professor of Emergency Medicine and Pediatrics, and Director of the Division of Toxinology at the University of Cincinnati. Mel was resplendent in his bee costume at the Wilderness Medical Society ball - it takes someone with supreme self confidence and poise to generate so much genuine and figurative "buzz." Mel is a master!

Deserts are land areas that receive less than 10 inches (25 centimeters) of rain per year. The most influential climatic processes that produce desert areas are the six cells of cold air currents that descend at the poles and near the Tropic of Cancer and Tropic of Capricorn. These air currents, driven by the sun and rotation of the earth, create areas of relatively warm, dry conditions. Many of the world’s deserts are located in “rain shadows,” areas to the leeward side of mountain ranges that prevents the small amount of moisture that is present in the air to move over the mountains. As the air rises, the moisture cools and precipitates in the higher elevations. Therefore, the area in the “shadow” of the mountain range receives little moisture. The air that does descend is quite dry and adds to the evaporative effect. The Atlas Mountains shadow the Sahara, the Andes the Patagonian, the Great Dividing Range the Australian, and the Sierra Nevada and Cascades the Great Basin deserts. The amount of rainfall is not an absolute indicator of “dryness,” because the rate of evaporation and timing of the rainfall must also be taken into consideration. The amount and type of vegetation, soil composition, altitude, average temperature, wind speed, and solar radiation all contribute to “dryness” and desert formation. Antarctica would be the world’s largest desert by the definition of less than 10 inches of rainfall annually. Some areas of that continent have had no recorded rain in 30 years. In contrast to Antarctica is the northern coast of Alaska, which receives less than 4 inches of rain annually, but evaporation is so low that the area is quite wet.

Deserts are increasing in size, likely due to human as well as geological factors. Over-grazing, destruction of forests, global warming, and other aspects of increased human population contribute to desertification. Currently about 15% of the land area of the earth is desert ( 30% if Antarctica is included). Most of the earth’s deserts can be found between 30 degrees South and 30 degrees North latitude, making them hot as well as dry. These deserts include the Sahara, Arabian, Kalahari, Australian, Atacama, Thar, Namib, and southwest United States. Approximately 50% of Africa is desert; the Sahara by itself almost as large as the United States. Approximately 8% of the United States, or 300,000 square miles, is desert. Most of the U.S. desert areas are adjacent to National Parks and Forests and are frequently visited, for example Grand Canyon, Big Bend, Arches, Zion, Organ Pipe, Joshua Tree, Great Basin, Saguaro, and Capital Reef. Beyond 40 degrees South and North latitude and at elevations over 10,000 feet are the “cold” deserts, which have wide swings in temperature. Examples include the Patagonian, Turkestan, Gobi, and Takla Makan.

The large temperature variations in desert regions are greater at higher elevations and latitudes, but are present in all deserts. Lack of vegetation, cloud cover and ground water surface allows 90% of the solar radiation to reach the desert surface. By contrast, a forest may reflect 50 to 60% of the solar radiation and its vegetation disperses the rest. At night, lack of cloud cover and vegetation allows almost 100% of the accumulated heat to escape, as opposed to only 50% from a humid climate. This explains why the desert temperature may reach 120° F (49° C) during the day and drop to 40° F (5° C) at night. Tropical rainforests may only reach 95° F (35° C) during the day but at night the temperature only drops to 85° F (30° C).

It would seem that the extreme desert climate would only allow for sparse life, but this is not the case. Death Valley, one of the harshest environments in North America, where air temperatures have been recorded at 134° F (57° C), is home to 600 species of plants, 30 species of mammals, 25 species of reptiles and 2 species of fishes.

Oases are found in most deserts. These are isolated depressions usually fed by a constant source of water. Underground springs and wells often supply moisture for plants and animals. If water is not visible, one must dig to find it at the lowest point of the depression. Many named oases have supported camel caravans, allowing them to move from oasis to oasis, and thus cross an otherwise impenetrable desert. Many ancient oases have wells hundreds of feet deep that are drying up because of overuse. When the water is used up, the oasis will disappear and desert life along with it.

The next post based on presentations from the WMS summer meeting will continue with the theme of desert survival.

photo of Namib Desert courtesty of
photo of Mel Otten by Luanne Freer, M.D.

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About the Author

Dr. Paul S. Auerbach is the world’s leading authority on wilderness medicine.