WHAT IS LIGHTNING? Lightning is a form of static electricity. We experience static electricity every time we drag our feet on the carpet and then touch a conducting surface, like a metal doorknob. The shuffling causes our bodies to pick up extra electrons. Touching something with a positive charge, like metal, causes the electrons to "jump" across the small gap from our fingers to the object, and we experience a tiny electric shock. Similarly, lightning occurs because clouds become negatively charged as the water droplets inside rub up against each other during the natural process of evaporation and condensation, when moisture accumulates in the clouds. This charge seeks out something with a positive charge -- the ground, ideally -- and the lightning is the "spark" closing the gap between the two.
As more and more water droplets collide inside a cloud, the friction between them produces enough extra energy to knock off electrons. The ousted electrons gather at the lower portion of the cloud, giving it a negative charge. Eventually the charge becomes so intense that electrons on the Earth's surface are repelled by the growing negative charge and burrow deeper into the Earth. The Earth's surface becomes positively charged, and hence very attractive to the negative charge accumulating in the bottom of the cloud. All that is needed is a conductive path between cloud and Earth, in the form of ionized air -- another byproduct of the collision process. When the two charges finally meet, a current jumps between the earth and the cloud, producing lightning.
HOW STORMS DEVELOP: Storm clouds form as moisture evaporates from the earth into the atmosphere, where the droplets congregate and jostle against each other. The air cools off rapidly with altitude. Sometimes a cold front -- the boundary between where the cold air from one thunderstorm meets the air outside the storm for example -- will force the moist air upward into the colder air. This moist air cools off and the water vapor "condenses" into liquid drops, forming clouds. The process -- called the convective process by meteorologists -- continues: more and more water vapor turns into liquid, and the moist air warms up even more and rises higher and higher. A thunderstorm results.