BACKGROUND: Most people know that thunderstorms tend to form on warm days, but new satellite observations indicate that in order for lightning to form, thunderstorm clouds need to have a high content of ice.
THE RESEARCH: Walter Petersen, a meteorologist at the University of Alabama, Huntsville, analyzed observations of lightning and precipitation from 1998 to 2000 taken from the Tropical Rainfall Measuring Mission (TRIMM), launched in 1997. The relationship between the number of lightning strikes and how much ice crystals are present in clouds is the same regardless of different atmospheric environments over oceans, coasts, and continents. The relationship between rain and lightning, in comparison, does not show this same level of consistency. The results support previous assumptions about the basic physics of lightning and ice. As a result, the density of lightning in a storm could be used in the future to predict the amount of ice that is present.
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 with as it reaches higher altitude. Sometimes a cold front – the boundary between where the cold air from one thunderstorm meets the air outside the storm for example – will force the moist air upward into the colder air. This moist air cools off and the water vapor "condenses" into liquid drops, 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.
The American Meteorological Society contributed to the information contained in the TV portion of this report.