Most lightning occurs within or between clouds. The destructive cloud-to-ground lightning bolt occurs much less frequently and can carry either a positive or a negative charge. Of the two, negative lightning is the most common type (about 90 percent). The process involved in generating a lightning stroke explains why lightning usually seeks out and strikes the highest point on the surface.  

First, a negatively charged stepped leader from the cloud approaches the ground. During the approach, the leader’s tip causes electric fields on the ground to increase in strength. Positive ions gather around pointed objects as small as pine needles and grass blades and then flow in streams towards the leader. When they get close enough, closure of the cloud-ground circuit takes place and the leader is neutralized. Now a much more powerful return stroke flows through the ionized channel from the ground to the cloud. The grounded object serves as the focal point of the positive ion flow. That object, such as from tree to golfer with an upraised club, is considered "struck" by lightning. The whole process, from leader approach to discharge, takes place in less than a second.

The return stroke is easily visible to the human eye, with the brightness of more than 100 million light bulbs. Actually, this bolt may have traveled back and forth between the cloud and the ground more than a dozen times — all in less than a second. The entire event is called a lightning flash.

Positive lightning carries a positive charge to the ground. It makes up less than 10 percent of a storm’s lightning strikes and typically takes place at the end of a storm. However, the positive lightning strike has the potential to cause more damage. It generates current levels up to twice as high and of longer duration than those produced by a negative bolt. It’s the long-duration, or "continuing current" components, of lightning that causes heating, burning and metal punctures. For that reason, scientists are especially interested in developing ways to detect the areas of a thunderstorm that develop positive bolts.