Occasionally, when the lower air is warm and muggy, this column of air surges upward at speeds of 100 miles an hour or more and the cloud piles up mile after mile. The fog droplets combine to form large rain drops which start to fall but are torn into mist and whirled upward to reform again and again as rain or hail in the blast of this invisible chimney. In this churning cloud, enormous charges of static electricity are built up between its different parts or between the cloud and the earth. When great enough they discharge as giant sparks sometimes miles long. Instantly, the main charge of electricity flows back along this path with a burst of light and a clap of thunder.
Thunder is merely the explosive expansion of the air along that channel.
Alive tree struck by lightning may be blown to pieces or only a narrow strip of bark may be ripped off. Such damage is caused by the heating of the tree's sap into steam causing the wood to explode or be cooked to death. Regardless of species, it has been found that the taller trees in a forest and trees standing alone or in small groves are most likely to be struck. Lightning, overall, is a major cause of fires. Each year in the United States it takes the lives of about 400 people and cripples another thousand.