Science of Lightning

Hello, I'm meteorologist John Jensenius from the National Weather Service. By definition, all thunderstorms contain lightning. Lightning is a giant spark of electricity that occurs within the atmosphere or between the atmosphere and the ground. As lightning passes through the air, it rapidly heats the air to 50,000 degrees Fahrenheit, considerably hotter than the surface of the sun. During a lightning discharge, the sudden heating of the air causes it to rapidly expand. Afterward, the air contracts quickly as it cools back to a normal temperature. This rapid expansion and contraction of the air causes a shock wave that we hear as thunder.

All thunderstorms go through various stages of growth, development, electrification and dissipation. The process of thunderstorm development often begins early in the day when the sun heats the ground and pockets of warmer air start to rise in the atmosphere. When this air reaches a certain level in the atmosphere, cumulus clouds start to form. Continued heating can cause these clouds to grow vertically upward into the atmosphere. These towering cumulus clouds can be one of the first signs of a developing thunderstorm.

In the final stages of development, the top of the cloud becomes anvil shaped. As the thunderstorm grows, precipitation begins to form, with mostly ice crystals in the upper level, a mixture of ice crystals and hail in the middle of the cloud and a mixture of rain and melting hail in the lower level of the cloud. Due to air movement and collisions between the precipitation particles in the middle level of the clouds, the various precipitation particles become charged. The lighter ice crystals gather a positive charge and are carried upward into the upward part of the storm. The heavier hail gathers a negative charge and falls toward the lower part of the storm. The end result is that the top of the storm becomes positively charged and the lower part becomes negatively charged. Normally, the earth's surface has a negative charge. However, as the negative charge builds in the lower part of the storm, the ground beneath the base of the clouds and immediately around the base becomes positively charged. As the clouds move, these induced positive charges on the ground follow the cloud like a shadow. Farther away from the cloud base, but under the positively charged anvil, the negative charge can be further induced. In the initial stages of development, air acts as an insulator between the positive and negative charges. However, when the electrical potential between the positive and negative charges becomes too great, there is a discharge of electricity that we know as lightning.

Lightning can occur completely within the thunderstorm cloud or between the cloud and the ground. In-cloud lightning generally occurs between positive charges at the top of the clouds and negative charges at the bottom of the cloud. Cloud to ground lightning occurs between a cloud and the ground. Lightning can also occur between clouds. Cloud to ground lightning can be categorized as negative flashes and positive flashes. Negative flashes usually occur between negative charges in the lower part of the storm and the positive charges in the ground under and near the cloud base. Prior to a flash, an almost invisible negatively charged channel of air forms near the cloud base and surges downward near the ground. As the step leader approaches the ground, streamers of positive charges shoot up from trees, buildings and other high objects on the ground. When one of these streamers meets the step leader, the connection is complete and a surge of electrical current moves from the ground to the cloud causing the visible return stroke that we call lightning.

Positive flashes usually occur between the positively charged upper level of the storm and the negatively charged areas surrounding the storm. The process of the positive flash is similar to that of the negative flash except that in this case, the positive channel originates from the anvil of the storm and surges downward. Streamers of negative charges shoot up to meet the positively charged channel as it approaches the ground. When a connection is made, a positive flash of lightning occurs. Because the distance between the ground and the anvil is much greater than the distance between the ground and the cloud base, a much larger electric potential is needed to initiate a positive flash of lightning. For this reason, positive flashes are infrequent and widely scattered around the storm, but they generally involve the exchange of a much greater charge and are usually much more destructive

The greatest danger associated with positive charges, however, is that they strike in areas where most people think they are safe from storm. They can strike well beyond the area where rain is falling and well beyond the area where lightning and thunder are occurring. Consequently, many victims are caught completely off guard. Don't become a lightning victim: get to a safe place sooner and to stay there longer. Remember, if you can hear thunder, you are within striking distance of lightning. For more information on lightning safety, go to www.lightningsafety.noaa.gov .