What Causes the Loud CRACK of Thunder?

Lightning and thunder-mysterious in themselves-are nevertheless the result of a number of well-understood processes. As the air travels across the earth's surface features, the molecules of our atmosphere undergo friction and collisions. Cosmic rays also generate ions as they collide with atoms in earth's atmosphere. Neutral atoms lose electrons to become charged "ions." This accumulates as electricity in clouds.

Lightning and thunder eventually result, during storm events. Negative charges at the bottom of clouds induce a positive charge to earth's surface. When conditions are ripe, a series of small charged "leader" lightning strokes initiates channels of electrical transfer toward earth.

The downward-thrusting leaders intensify earth's positive charge by induction as they approach. Eventually one of the leaders establishes a relatively resistance-free path which allows a large upward stroke, the main bolt of lightning, to occur with its intense luminosity.

Lightning and Thunder - a Unified Phenomenon

Lightning and thunder are simply different aspects of a single phenomenon. They are not separate events. Both involve tremendous heat-super-heating. Both require gas. Yet, to the observer, there does appear to be a kind of natural separation into the audio and the visual. Light travels at the speed of-well light! That is far more rapid than the transmission of sound. This explains why there is a time lapse between the visual of lightning and the audio of thunder (unless one is at the actual location of the event). In fact, the time between the two can be used to determine the distance from the lightning bolt with reasonable accuracy.

Gas and Plasma

Gas consists of particles not closely associated. Although they may exhibit a degree of interaction, gas particles maintain freedom of movement. Plasma is similar to gas, except plasma contains ionized gas atoms. Plasma is formed by intensely heating gas. How? Since heating gas speeds up particle movement, more particle collisions occur. If heating is sufficient, electrons are knocked from their orbitals in large numbers producing plasma. For a simple yet fascinating read about what plasma is, enjoy the Gettysburg College - Plasma Physics Introduction.

Dielectric Property of Air

The lightning and thunder phenomenon requires the medium of plasma, yet plasma requires the heat provided by thunder and lightning. This sounds like a quandary. In fact, charge must build up first, if lightening is to occur. To illustrate, in the summer, one is at no particular risk on opening a car door. Do that in the winter, however, and the individual is likely to receive a static electric shock. This is because the moist air of summer does not allow electric charge to build much, whereas in the dry air of winter it does, resulting in static shock-a kind of mini-lightning. In the dark of night one might even see the spark. This feature is due in part to the "dielectric" property of air.

Resistance of Air

In a gathering storm, charge and moisture increase to the point where the dielectric property of air breaks down, and the leaders form. Then is experienced the full force of a lightning bolt passes from the air to the ground. Even though the air, in the form of plasma, is now more of a conductor-it is nowhere near to being a perfect conductor-it possesses a measure of resistance. Resistors allow electrical current to flow, yet they convert some of it into heat. Lightning bolts generate a great deal of heat-tens of thousands of degrees-very suddenly.

Charles' Law

Charles' Law states that the volume of a gas is directly proportional to its temperature. The intense heat produced by lightning causes the gas to expand tremendously rapidly-violently-producing the deafening CRACK of thunder that scares us if we are standing too close.

Nova Science Now - How Lightning Works - by Joe Dwyer

Glenn Elert's Hypertextbook - Dielectric Strength of Air

Davidson College - Charles' Law