Lightning has fascinated and excited humans for as long as they have watched the skies. Although meteorologists understand the cloud conditions necessary to produce it, lightning cannot be forecasted. At any moment, there are as many as 1,800 thunderstorms in progress somewhere on earth and each is producing deadly lightning. Lightning detection systems in the U.S. see an average of 25 million strokes of lightning each year, from some 100,000 storms. It is estimated that the earth is struck by 100 lightning bolts every second.
The clouds at the high levels of the thunderstorm are made of ice crystals. The formation of ice in a cloud is an important element in the development of lightning. Those storms that fail to produce large numbers of ice crystals may also fail to produce a lot of lightning. Strong rising and sinking motions within the cloud are important too, as they enhance collisions among cloud particles causing a separation of electrical charges. Positively charged ice crystals rise to the top of the thunderstorm and negatively charged ice particles and hailstones drop to the middle and lower parts of the storm.
As the differences in charges continue to increase, positive charges rise up taller objects such as trees, houses and telephone poles. The charge can also move up you, causing your hair to stand on end! This is natures final way of warning you that lightning can strike near you very soon.
If the negatively charged area at the bottom of the storm gets large enough, it sends out a channel toward the ground called a step leader. It is invisible to the human eye and moves in steps toward the ground. When the step leader nears the ground, or a target such as a radio tower, it repels all the negative charges in the surrounding area and attracts all the positive charge. As the positive charges collect in high enough concentration, they send out small bolts of ground to air lightning called streamers. If the streamers can make contact with the step leader, an electric current wave propagates up the channel as a bright pulse -- lightning!
And of course, with lightning comes thunder. Lightning heats the surrounding air to as much as 50,000 degrees Fahrenheit, which is five times the temperature of the surface of the sun. When air is heated, it expands, and this expansion is what's causing the sound of thunder. The expansion is happening faster than the speed of sound, which creates a sonic boom.