T H U N D E R   A N D   L I G H T N I N G

Thunder and lightning have impressed humans with their spectacle and violent results from the earliest times. The Greeks thought it was their high god Zeus who threw lightning bolts. The Vikings said their main deity, Thor, struck lightning by hammering an anvil while traveling by chariot across the sky. Native Americans believed that the "Thunderbird" flapping its wings created the sound of thunder and that its bright feathers were the source of lightning flashes. For young children, thunder and lightning are still some of the most frightening weather phenomena. But nearly 2 centuries of research on thunder and lightning have helped us understand a lot about what causes them.

Thunderstorms bring several different kinds of dangerous weather: tornadoes, hail, flash floods, and, of course, lightning. Flash floods (also caused by the drenching rain of hurricanes) are the #1 cause of weather fatalities in the United States, and lightning is #2, with more than 100 deaths per year. Lightning injures hundreds more, and causes millions of dollars worth of damage. Hail can wreck whole fields of crops in minutes.

At any one time some 1,000 to 2,000 thunderstorms are going on somewhere around Earth. Researches using satellites to get a global picture now estimate there may be some 100 lightning flashes a second!

To grow, thunderstorms need unstable air, with warm air near the ground and cold air up above. While these kinds of storms can occur at any time of year, they're more frequent in spring and summer. They're "convective storms", meaning that in them warm air rises, releases heat energy and cools, and then falls down once again, like soup boiling on a stove.

Getting a thunderstorm started requires one of a number of triggering events to set the warm air moving upwards. It can be the arrival of a cold front—heavier, denser cold air running into lighter, warm air, and forcing it upwards. Or it can be a local change in elevation, or some other cause of excessive heating close to the surface.

There are places across the continent where such conditions are more common, as in the Great Plains, where there's the so-called "dryline", a frequently-found boundary between dry air moving east from the deserts of the Southwest and humid air from the Gulf of Mexico.

To understand a thunderstorm it helps to think of the basic building block as being a "single cell" storm, but in the real world most thunderstorms are in fact "multi-cell", made up of multiple cells. Once a thunderstorm has formed, it can trigger other storms as much as a hundred miles farther away, in a kind of chain reaction, by creating a gust of cold wind that bumps into this new region of warm air and starts it moving upwards. At lesser distances, this pulse of cooler air can also trigger mult-icell thunderstorms.

The most severe thunderstorms, known as "supercells", require special weather conditions, and so appear less frequently. But the United States is also the place where such conditions can be found most often. "Supercells" can last for hours, triggering tornadoes.