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 water droplets and ice particles collide inside a cloud, they exchange positive and negative charges in a way that is not completely understood. Bigger ice particles acquire a negative charge and fall lower in the cloud, leaving smaller, positively charged ice crystals above. The negative charge lower in the cloud attracts a positive charge from the Earth’s surface below the storm. Eventually, the combined effect of these two charges is large enough to create a spark between them. The spark then develops into a lightning flash that descends to the Earth. Sparks also occur above the negative charge, and these develop into flashes that move upward into the cloud’s positive charge. Meteorologists continue to study and learn more about what causes lightning.
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, for example, as the polar glaciers melt. 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.