The friction created by the movement of various sized rain drops and ice crystals results in the larger droplets being negatively charged while the smaller are positively charged. The location of electrical centers within a thunderstorm is determined by the weight and size of the droplets and the strength of the vertical currents. Lightning is dangerous to both man and the environment. It is the greatest single cause of forest fires in the western United States. It is estimated that lightning ignites more than 7,500 forest fires annually, with the greatest number of occurrences in the Rocky Mountains.
Humans take hits too.
Nearly 200 people die annually from lightning strikes. The greatest numbers of casualties are males engaged in outdoor recreation or work. The sudden heating and expansion of air along the lightning channel immediately after a lightning stroke, followed by the instantaneous cooling of the air, creates the major vibration known as thunder. It is sharpest when there are very abrupt changes in the electrical field. Thunder is seldom heard at distances greater than 15 miles from the point of discharge.