Lightning

Lightning occurs in a thunderstorm when an electrical potential builds up that is strong enough to exceed the resistance of the atmosphere to a flow of electrons between the centers of opposite charge. Most cloud-to-ground discharges originate in the cloud and progress to the ground. They take place in two stages. First, a leader stroke works its way downward to the ground in a series of probing steps. Then a number of return strokes flash upward to the cloud so rapidly that they appear as a flickering discharge. The average number of return strokes in a lightning flash is four. Lightning discharges taking place within a cloud usually do not show return strokes.

The processes that generate the electrical potential are not fully understood, and a number of theories have been advanced. Regardless of the method or methods by which electrical potentials are generated, measurements with specialized electronic equipment have established where in the thunderstorm opposite charges tend to accumulate and how charges vary during storm development.

In fair weather, the atmosphere has a positive electrical charge with respect to the earth. This fair weather potential gradient has an average value of about 30 volts per foot. When a cumulus cloud grows into a cumulonimbus, the electric fields in and near the cloud are altered and intensified. The upper portion of the cloud becomes positively charged and the lower portion negatively charged, although other smaller positive and negative charges develop. The negative charge near the cloud base induces a positive charge on the ground--a reversal of the fair-weather pattern.

Cloud-to-ground lightning is usually a discharge between the negative lower portion of the cloud and the induced positive charge on the ground and accounts for about one-third of all discharges. Most lightning discharges, however, are within a cloud or cloud-to-cloud. Many of the within-cloud discharges take place between the negative charge in the lower portion of the cloud and a positive charge center carried downward from the upper portion of the cloud by the falling rain in the precipitation core. This positive charge center disappears when the heavy rain stops.

Lightning sometimes occurs in the cumulus stage, but reaches its greatest frequency at the time the cell reaches maturity and its greatest height. The start of rain beneath the cloud base at the beginning of the mature stage marks the onset of the greatest lightning danger. The most extensive horizontal flashes occur at altitudes extending from the freezing level upward to where the temperature is about 15° F. Although lightning may occur throughout a thunderstorm cell, the strongest flashes to the earth usually originate in the lower portion of the cell. Many cloud-to-ground lightning strikes reach out laterally for considerable distances from the cloud base. Once lightning has started, it may continue well into the dissipating stage of the cell. Apparently, less cloud height is needed to maintain continuing discharges than to initiate the first. But as the height of the cell decreases after reaching maturity, the frequency of lightning flashes decreases. However, individual flashes may remain strong.

The noise of thunder is due to compression waves resulting from the sudden heating and expansion of the air along the path of the lightning discharge. These compression waves are reflected from inversion layers, mountainsides, and the ground surface so that a rumbling sound is heard, instead of a sharp explosive clap, except when the discharge is very near. Since light travels so very much faster than sound, it is possible to estimate the distance of a lightning flash using the elapsed time between seeing the flash and hearing the thunder. The distance to a flash is about 1 mile for each 5 seconds of elapsed time.

Weather radar, in which portions of transmitted radio signals are reflected back from precipitation areas in clouds and displayed as radar echoes on an indicator, is helpful in locating, tracking, and revealing the intensity of thunderstorms and their associated lightning.