Flash!

If you think about how lightning occurs, you could call it an enormous electrical circuit in the atmosphere. Lightning is the visible result of neutralization of positively and negatively charged areas of a cloud, the air, and the ground. Sounds complicated, doesnt it? The basic function is actually relatively simple.

Under normal conditions, the ground is negatively charged while the upper air is positively charged. When a cumulonimbus cloud (thunderhead) forms the charge distribution changes. The ground below the cloud becomes positively charged. A narrow region at the base of the cloud and the upper portion of the cloud also become positively charged.

Just above the base of the cloud, however, a negatively charged saucer-shaped region develops. This region is roughly 1,000 feet (300 meters) thick and several miles in diameter.

As the thunderhead reaches the mature development stage, the electrical charge differential is so great, it overcomes the natural insulating properties of the air. When that happens electricity, in the form of lightning, surges between differently charged areas in order to neutralize the opposing charges.

There are several theories in meteorology as to why different parts of a storm cloud develop different charges, but generally, precipitation, ice crystals, and hail circulated on up and downdrafts are considered mechanisms for moving positive and negative charges around inside a cloud. Without getting into the physics of why this occurs, ice crystals are positively charged, and they are thought to carry positive charges to the upper portions of the cloud because they are light enough to reach that level.

Hail, because it is heavier, is thought to carry negative charges to the lower portions of the cloud. Regardless of how it actually develops, the negatively charged area at the base of the cloud repels electrons on the ground level, which creates a positively charged area that tracks along the ground under the storm cloud. This is why thunderstorms repeatedly fire lightning along their path of travel.

There are many types of lightning, and their names pretty much define what they look like. Basically, there are four general categories: in-cloud, cloud-to-cloud, cloud-to air, and cloud-to-ground. In-cloud and cloud-to-cloud lightning are the most common types. Most lightning occurs within a single cloud and closes the circuit between the positively charged upper cloud and the negatively charged lower cloud. Cloud-to-cloud lightning connects the positive charge of one cloud to the negative charge of an adjacent cloud.

Cloud-to-air lightning is a weak form of lightning that tends to occur at the top of the cloud high in the atmosphere. It is almost always too distant to have an audible thunder component. Cloud-to-ground lightning is of the greatest interest to us, even though it only accounts for about 20 percent of all lightning, because it is the only type that endangers people and objects on the ground.

Lightning only lasts for two-tenths of a second, but the visible flash is the result of a complex process. It begins when an invisible flow of electrons, called a step leader, surges from the negatively charged cloud base toward the ground. It is called a step leader because it approaches the ground in a step-wise incremental fashion that creates a branched pattern as it goes.

When the step leader nears the ground, electrons surge up from the ground to meet it. This return stroke, as it is known, typically rises from a tall, pointed object since the tallest objects in the region have the greatest positive charges. A return stroke travels at one third the speed of light and takes a bare one ten thousandth of a second to reach the cloud.

Once the return stroke contacts the step leader, the two open an ionized channel between the cloud and the ground. The negative charge from the cloud pours into the ground and the positive charge from the ground shoots up into the cloud. It is this upward positively charged surge that produces the visible flash we see. In the next one tenth of a second, several more lightning strokes may occur along this ionized channel and they discharge the remaining buildup of negatively charged electrons in the base of the cloud.

Dart leaders in the base of the cloud initiate these strokes. Like step leaders, dart leaders are met near the ground by positive charges from the ground. The whole process happens so quickly, it seems like one brightly flickering bolt of lightning to a viewer.

Names for lightning are as varied as the people who see it, yet sheet, ribbon, bead, forked, and even fabled ball lightning, to name just a few, are all forms of one of the four basic types, as are St. Elmos Fire and heat lightning. St. Elmos Fire, most often seen in the rigging of ships at sea, is cloud-to-ground lightning that grounds to the ship because its the only game in town.

It can race along the spars and glow with an eerie flame. (No wonder sailors were such a superstitious lot!) Heat lightning is simply lightning that occurs too far away to be heard. It often occurs just over the horizon on hot summer nights when the sky above the viewer is clear.

We see lightning practically the instant it occurs because light travels so quickly. Thunder, the audible part of a lightning stroke, takes longer because sound travels considerably slower than light does. The old rule of thumb for judging the distance of a thunderstorm actually works pretty well. It takes roughly 5 seconds per mile for the sound of thunder to reach a listener once lightning has been observed. (Yes, counting one thousand one, one thousand two, etc, really works.)

The sound of thunder differs with distance as well. A storm that is very close releases a very sharp, very loud clap of thunder. A distant storm makes a rumbling thunder because the sound is re-echoing over the distance and over the whole height of the cloud. The longer and lower the rumble, the farther the storm is from the listener.

Lightning is a fascinating, dangerous natural phenomenon that is still not entirely understood. Mankind has observed and studied it for thousands of years, yet there is still much left to learn. For instance, did you know that lightning takes on a range of colors that vary with atmospheric conditions?

Blue lightning inside a cloud indicates the presence of hail and red lightning in a cloud indicates rain. Yellow or orange lightning indicates a heavy dust concentration in the atmosphere. White lightning indicates low humidity. Lightning ... its a very flashy subject!

Written by Melody L. Higdon, 14 WS/DOPA