Lightning is the deadliest phenomenon associated with a thunderstorm. Although verification of lightning-related deaths is difficult, it is estimated that as many as 200 Americans die as a result of lightning with the number of injuries much higher.
Death can be caused by the direct stroke or by nearby lightning as it strikes other objects and induces a spark or a voltage difference in the ground.
More than 10,000 fires in forests and grasslands are caused by lightning each year accounting for 35% of forest lost to fire.
Forms of Lightning
Intracloud (60%): The lightning is contained completely within the cloud between regions of positive and negative charge.
Forked or Cloud to Ground: Formed between the cloud base and the ground. Can travel from ground to cloud.
Ribbon: Formed when wind disrupts the ionized column.
Sheet lightning: The apparent flash of sky light is a result of the lightning bolt not being visible to the observer. Heat lightning is a result of a distant thunderstorm.
St. Elmo's Fire: A bright glow around an object as a result of positive charges that are attracted by an overhead thunderstorm. Thought to protect ships from lightning damage.
Ball: A controversial form of lightning that is a moving ball of electricity. Claimed to enter houses until it explodes.
Causes of Lightning
Thunderstorm can generate lightning within 20 minutes. Therefore, there must be an effective mechanism to separate charge. Because of the dipolar structure of water, the electrification is thought to be from processes pertaining to the freezing of water, either in ice crystals or hailstones.
Thermoelectric effect allows for the positively ionized molecules of water to travel from warm to cool temperatures.
Process begins with preliminary breakdown. It is here that the stepped leader is initiated and propagation downward ensues. Usually, the stepped leader lowers negative charge from the base of the cloud. The direction of the forked branches indicates the direction the stepped leader is taking. The air is being ionized so that a conducting channel is formed. It will form along a path of least resistance to the best conducting location on the ground.
As the stepped leader approaches the ground, the electric field at the ground becomes very large. A propagating discharge forms and starts the attachment process until it connects with the conducting channel. Now the return stroke begins. A very strong electrical current propagates upward at about 1/3 the speed of light. It is this that is the initial visible "lightning".
After about 40 to 80 milliseconds (1/1000 of a second), a dart leader is formed that does not step but usually travels down the same channel. The channel is ionized because of the intense heat which breaks up oxygen molecules into its ionic components. Several strokes may occur within fractions of a second thereby giving the lightning the appearance of flickering.
The channel can be heated to 30,000°C in a microsecond (10-6) or less, which creates explosive expansion setting up a shock wave. The shock wave weakens into an acoustic wave which we call thunder.
Lightning kills more people per year than tornadoes or hurricanes. At the first sign of lightning, people should seek shelter immediately.
Stay indoors and away from electrical appliances. If caught outdoors, stay away from high or conductive objects. If you feel your hair stand on end or your skin tingle, the air is ionizing and lightning is probable. Crouch down to make the smallest possible target, but do not lay flat on the ground. Keep the least amount of contact with the ground.
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