Severe Weather

Lightning and Thunder

Lightning is simply a discharge of electricity, a giant spark, that occurs in mature thunderstorms. The lightning stroke can heat the air through which it travels to an incredible 30,000°C (54,000°F), which is 5 times hotter than the surface of the sun. This extreme heating causes the air to expand explosively, thus initiating a shock wave -- called thunder -- that travels outwards in all directions from the flash.

In most years it is the thunderstorm's greatest killer. A possible contributing reason for this is that lightning victims frequently are struck before or just after the occurrence of precipitation at their location. Many people apparently feel safe from lightning when not experiencing rain.

Lightning is an electrical discharge produced to balance the differences between positive and negative charges within a cloud, between two clouds, or between the cloud and the ground.

A close lightning flash striking a tree at a distance of 60 meters

A negative charge at the base of the cloud is attracted to a positive charge at the earth's surface. This charge is densest on isolated or protruding objects, like trees and tall buildings. When the difference between these charges becomes sufficiently strong, the normal insulating qualities of the air break down and a lightning bolt strikes.

What occurs as an instantaneous flash to the human observer actually is a complex progression of events. First, a usually invisible discharge of electrons races down from the cloud. This downward moving bolt of negatively charged electricity (called a stepped leader) incites a discharge of positively charged electricity from the ground.

"Triggered lightning", the discharge is triggered by the presence of the tall tower atop Mount San Salvatore, near Lugano, Switz.

This second bolt races upward to meet the descending charge, and it is when they connect that the luminous flash of electricity we know as lightning races up from the ground to the cloud in perhaps 1/10,000th of a second. This final step in the lightning process is called a "return stroke."

During a thunderstorm, not only is the ground positively charged, so is everything on it. The tallest object in the area is most likely to be struck, whether it is a building, a tree, or a person standing alone in a field.

Lightning rods are placed on buildings to protect them from lightning damage. The rod is made of metal and has a pointed tip, which extends well above the structure. The positive charge concentration will be maximum on the tip of the rod, thus increasing the probability that the lightning will strike the tip and follow the metal rod harmless down into the ground.

When lightning strikes a car, lightning normally leaves the passengers unharmed because it usually takes the quickest path to the ground along the outside metal casing of the vehicle.

Different Types

Although cloud-to-ground lightning strikes pose the most danger to people on the ground, they make up only about 20% of all lightning strikes. The most common type of lightning in a thunderstorm is in-cloud lightning, which occurs within the cloud itself.

Cloud-to-cloud lightning is a common occurrence in which opposite electrical charges in one cloud attract those in another.

Lightning detection and safety

Cloud-to-ground lightning is located by means of an instrument called a lightning direction-finder, which works by detecting the radio waves produced by lightning. A web of these magnetic devices is a valuable tool in pinpointing lightning strokes. Lightning detection devices allow scientists to examine in detail the lightning activity inside a storm as it intensifies and moves (see link on the right).

Learn to reduce your lightning risk through outdoor and home lightning safety education. A very good source of information is provided by the Lightning Safety Group.

History

Benjamin Franklin, with his son William, conducts his famous lighting experiment -- flying a kite during a thunderstorm with a key attached to the string -- in 1752. The experiment proved Franklin's theory that lightning is electricity and led to his invention of the lightning rod. (Britannica Student Encyclopedia)

Benjamin Franklin performed the first systematic, scientific study of lightning during the second half of the 18th century. Prior to that time, electrical science had developed to the point where positive and negative charges could be separated. Electrical machines could, by rubbing together two different materials, store the charges in primitive capacitors called Leyden Jars from which sparks could be generated and observed.

While others had previously noted the similarity between laboratory sparks and lightning, Franklin was the first to design an experiment which conclusively proved the electrical nature of lightning. In his experiment, he theorized that clouds are electrically charged, from which it follows that lightning must also be electrical. The experiment involved Franklin standing on an electrical stand, holding an iron rod with one hand to obtain an electrical discharge between the other hand and the ground. If the clouds were electrically charged then sparks would jump between the iron rod and a grounded wire, in this case, held by an insulating wax candle.

This experiment was successfully performed by Thomas Francois D'Alibard of France in May 1752 when sparks were observed to jump from the iron rod during a thunderstorm. G. W. Richmann, a Swedish physicist working in Russia during July 1753, proved that thunderclouds contain electrical charge, and was killed when lightning struck him.

Before Franklin accomplished his original experiment, he thought of a better way to prove his hypothesis through the use of a kite. The kite took the place of the iron rod, since it could reach a greater elevation and could be flown anywhere. During a Pennsylvania thunderstorm in 1752 the most famous kite in history flew with sparks jumping from a key tied to the bottom of damp kite string to an insulating silk ribbon tied to the knuckles of Franklin's hand. Franklin's grounded body provided a conducting path for the electrical currents responding to the strong electric field buildup in the storm clouds.

In addition to showing that thunderstorms contain electricity, by measuring the sign of the charge delivered through the kite apparatus, Franklin was able to infer that while the clouds were overhead, the lower part of the thunderstorm was generally negatively charged.