Lightning and thunder must have been among the first things about nature that mystified and frightened primitive man. When he saw the jagged tones of lightning in the sky and heard the claps and rumbles of thunder, he believed the gods were angry and that the lightning and thunder were a way of punishing man.
To understand what lightning and thunder actually are, we must recall a fact we know about electricity. We know that things become electrically charged-either positively or negatively. A positive charge has a great attraction for a negative one.
As the charges become greater, this attraction becomes stronger. A point is finally reached where the strain of being kept apart becomes too great for the charges Whatever resistance holds them apart, such as air, glass, or other insulating substance, is overcome or broken down.
A discharge takes place to relieve the strain and make the two bodies electrically equal. This is just what happens in the case of lightning. A cloud containing countless drops of moisture may become oppositely charged with respect to another cloud or the earth.
When the electrical pressure between the two becomes great enough to break down the insulation of air between them, a lightning flash occurs. The discharge follows the path which offers the least resistance. That’s why lightning often zigzags.
The ability of air to conduct electricity varies with its temperature, density, and moisture. Dry air is a pretty good insulator, but very moist air is a fair conductor of electricity. That’s why lightning often stops when the rain begins falling.
The moist air forms a conductor along which a charge of electricity may travel quietly and unseen. What about thunder? When there is a discharge of electricity, it causes the air around it to expand rapidly and then to contract.
Currents of air rush about as this expansion and contraction take place. The violent collisions of these currents of air are what we hear as thunder. The reason thunder rolls and rumbles when it is far away is that the sound waves are reflected back and forth from cloud to cloud.
Since light travels at about 186.284 miles per second and sound at about 1.100 feet per second through the air, we always see the flash first and then hear the thunder later.
Read: What Is a Storm?