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Desert, a term for regions of the earth

Photo: Albert P. Bekker

categorized as receiving less than 10" of annual rainfall where evaporation exceeds said precipitation. High temperatures also contribute to the description of deserts, and because of the lack of soil moisture and low atmospheric humidity, temperatures can reach over 130� F in the shade. Night time temperatures can fall to near freezing due to heat radiating back into the atmosphere. Deserts are usually caused by a combination of geologic features of the terrain and climatic conditions.

Most desert regions of the Earth have been formed by air movement. As cool air rises it loses it's moisture, and as it descends it warms and picks up moisture drying the Earth's top soils. Two subtropical zones are formed as hot air flows north and south over the equator and then cools in the upper

Photo: Gerald and Buff Corsi

atmosphere descending as high pressure areas. Ascending low pressure systems are created farther north and south. There are two additional, even farther north and south, regions of descending air in the polar regions. Two belts of deserts have been created by this air movement. Along the tropic of Cancer, the northern deserts include the Gobi desert in China, the Sahara, the Arabian deserts and the deserts of southwestern North America. The southern hemisphere deserts run along the tropic of Capricorn and include, the Kalahari Desert in southern Africa, Patagonia and the Great Victoria and Great Sandy deserts of Australia. Some deserts are caused by the cooling of air as it moves across cold ocean waters moving from the Arctic and Antarctic regions toward the equator. Arid coastal regions of southwest Africa, Chile and Baja California are examples of the effect of these air flows.

Plant Life
All but a few of the most arid deserts support life in abundance. A large number of desert plants are succulents, cacti and the like, storing water in stems,

Photo: Brother Alfred Brousseau

leaves and roots. The cactus thorns, which are modified leaves, serve to protect the plants precious water from animals. To prevent evaporation, the pores (stomata) of these plants close during the day, subsequently, some desert plants take in and store carbon dioxide only at night. Woody desert plants usually have one of two root systems, either deep reaching or shallow and far spreading. These root systems serve to reach deep water tables or to soak up as much surface water as possible, from dew or the occasional rain.

Small leaves also serve desert plants to contain water, reducing the area in which transpiration can occur. Still, other plants simply drop their leaves during the hottest summer months, photosynthesis is then taken over by the plant stems. Some species of flowering desert plants are ephemeral. Sometimes for years their seeds will lie dormant on the desert floor, waiting for the soaking rain that will cause germination. The plant germinates, grows, blooms and dies in the course of a few days.