Important information illuminating lightningâ??s flash crash

The following web story is a direct transcription from the National Weather Service regarding lightning awareness week. This is the second of a five part series on lightning safety which will run each day during New York's lightning safety awareness week. Todayâ??s topic will focus on the science of lightning.

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 storms 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 rise. When this air reaches a certain level, cumulus clouds form. These clouds grow vertically with continued heating and are one of the first signs of a developing storm.

In the final stages of development, the top of the cloud becomes anvil shaped. As the storm grows, precipitation begins to form with mostly ice crystals in the upper level, a mix of ice crystals and hail in the middle of the cloud and a mix of rain and melting hail below. Due to air movement and collisions between these particles, the various precipitation particles become charged. The lighter ice crystals gather a positive charge and are carried upward. The heavier hail gathers a negative charge and falls toward the lower part of the storm. The top of the storm becomes positively charged, with the lower part negatively charged.

Normally, the earthâ??s surface has a negative charge. However, as the negative charge builds up in the lower part of the storm, the ground and air immediately around the base becomes positively charged. As the clouds move, these 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. When the electrical potential between the positive and negative charge becomes too great, a discharge of electricity occurs.

Lightning can occur completely within the cloud, between the cloud and ground, or between clouds. Cloud to ground lightning can be categorized as negative or positive flashes. Negative flashes usually occur between negative charges in the lower part of the storm and positive charges in the ground under and near the cloud base. Prior to the flash, an almost invisible negatively charged channel of air forms near the cloud base and surges downward toward the ground, called a step leader. As it approaches the ground, streamers of positive charge shoot up from 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 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 the negative, except that the positive channel originates from the anvil 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 occurs. A much larger electrical potential is needed to initiate a positive flash due the distance between the anvil and the ground. Thus, positive flashes are more infrequent and widely scattered around the storm but generally involve a much greater charge and are usually more destructive.

The greatest danger with a positive charge is that they strike in areas where most people feel safe from the storm. They can strike well beyond the area where rain is falling and beyond where lightning and thunder are occurring. Consequently, many victims are caught off guard. Do not become a victim. Get to a safe place sooner and stay there longer. If you can hear thunder, you are within striking distance of lightning.

The topics for the remainder of the week are:

Wednesday: Lightning safety outdoors.

Thursday: Lightning safety indoors.

Friday: The medical aspects of lightning.

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