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July/August 2008

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Monsoon Myssteries in America's Southwest

Aplume of moisture vapor flows northeast from Mexico, bearing water from the Gulf of California and the streams and rivers of the Sierra Madre. It collects more water along its route, liberating moisture from swimming pools, sprinklers, and irrigation canals. After a day’s march, it stumbles into the 7,000-foot (2,133 meter) Mogollon Rim, the great rock escarpment that separates central Arizona from the high Colorado Plateau. Diverted by air currents, the cloud turns south. As it travels, it grows from cumulus to cumulus congestus, then cumulonimbus—the towering anvil-head formation that marks a midsummer’s skyscape in the desert.

Powerful things happen within that mixture of rising air and moisture. At the top of the cloud, 30,000 feet (9,144 meters) in the air or even higher, the temperature is -60°F (-51°C). There, ice crystals carrying a positive electrical charge float and collide. At the much warmer bottom, particles of water, also positively charged, swirl about.

Somewhere in the middle, at about 10°F (-12°C), lies a zone made up of hail and graupel that rises and falls with the air currents. This graupel has acquired a negative electrical charge somewhere along the way. Between positively charged ice crystals and negatively charged graupel, Mother Nature creates an impressive show. First there comes a flicker, a low-powered leader firing down in an infinitesimally quick burst, perhaps 20 millionths of a second. It stairsteps down, forking and streaking several times to the ground at a speed of 460 million feet (140 million meters) per second. Then a “return stroke” of lightning—the flash that we earthbound observers see and that has yielded the erroneous belief that lightning travels from the ground up—meets it.

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