Reported September 2009

HOW STORMS DEVELOP: Storm clouds form as moisture evaporates from the earth into the atmosphere, where the droplets jostle against each other. The air cools off rapidly as it reaches higher altitudes. Sometimes a cold front -- the boundary between where the cold air from one air mass meets the surrounding air -- will force warm, moist air upward into the colder air. This moist air cools off and the water vapor condenses onto tiny particles in the air, called condensation nuclei, collectively forming clouds. The process continues: more and more water vapor turns into liquid and the moist air warms up even more and rises higher and higher. A thunderstorm results.

WHAT CAUSES LIGHTNING? As more and more water droplets collide inside a cloud, their atoms bounce off each other more forcefully. This knocks off electrons. The ousted electrons gather at the lower portion of the cloud, giving it a negative charge, while the upper part of the cloud becomes positively charged. Eventually the growing negative charge becomes so intense that electrons on the Earth's surface are repelled and burrow deeper into the Earth. The Earth's surface becomes positively charged, and hence very attractive to the negative charge accumulating in the bottom of the cloud. All that is needed is a conductive path between cloud and Earth, in the form of ionized air.

ABOUT GLOBAL WARMING: Global warming refers to an average increase in the earth's temperature, which has risen about 1 degree Fahrenheit over the past 100 years, and to changes in climate. A warmer Earth may lead to changes in rainfall patterns and a rise in sea level due to factors like melting glaciers. Some of this rise is due to the greenhouse effect: certain gases in the atmosphere trap energy from the sun so that heat can't escape back into space. Without the greenhouse effect, the earth would be too cold for humans to survive, but if it becomes too strong, the earth could become much warmer, causing problems for humans, plants and animals.

The American Meteorological Society and the American Geophysical Union contributed to the information contained in the TV portion of this report.

If you would like more information, please contact:

Dr. Robert J. Trapp
Purdue University
Department of Earth and Atmospheric Sciences
West Lafayette, IN 47906-2051
(765) 496-6661

American Meteorological Society
Boston, MA 02108-3693
(617) 227-2425
http://www.ametsoc.org

Peter Weiss
American Geophysical Union
Washington, DC 20009-1277
1-800-966-2481
http://www.agu.org
pweiss@agu.org

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