The smallest hailstone is about the size of a pea. Lightning usually does not occur in a thunderstorm until its top spreads laterally into an ice crystal anvil-shaped cloud. The vigorous upward and downward motion of the currents and the friction created by the collision and jostling of various sized raindrops appears to be the mechanism that alters the normal electrical field of the atmosphere. 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.