Static electricity is the cause of lightning. Strong winds often rise to a great height. In so doing they displace water drops in a cloud so violently that some are torn apart. Due to friction between the wind and the clouds, the clouds get electrically charged. In general, the upper part of clouds are positively charged and the lower parts are negatively charged. Moist clouds are good conductors. Hence, charge flows with high speed from one cloud to the other through the air between them. Moist air is also a good conductor.
Conduction of electric charge through the air takes place. This is called electric discharge in air. The electric discharge appears in the form of a powerful spark called lightning. Sometimes, the charges in the cloud may be discharged into the earth. The electric discharge lasts for about 0.1 milliseconds (1/10,000th of a second). It releases a tremendous amount of heat and light. Due to this tremendous heat, the air gets heated up suddenly and this causes sudden expansion of air. Scientists believe that the thunder results from the sudden expansion of air during the hot lightning flash. Such a large volume of air present within clouds probably produce the loud sound which we hear as thunder. Thus, lightning and thunder are analogous to the electric spark (in electric switches) and the accompanying noise, which we hear.
Benjamin Franklin (1752) performed the famous kite experiment. He showed that clouds and earth store static electricity. The high luminous electric discharge was the cause of lightning
The electric discharge in air can pass:
Between a charged cloud and a point on the surface of the earth.
Between two charged clouds.
Between oppositely charged layers of the same cloud.
Lightning can cause temperatures as high as 30,000°C. The average current in a lightning stroke is about 10,000 amperes.
Trees are good conductors of electricity. By electric discharge we mean that the charge that has built up in the body suddenly leaves the body, i.e. the body gets discharged. The electric discharge can take place through a tree or even through a human body. Remember never to seek shelter under a tall tree during a thunderstorm. Lightning usually seeks out and strikes the tallest object (or building) in any particular region. It can set a tree on fire and shatter buildings. Lightning can kill men and animals.
Lightning rods protect tall buildings from damage during lightning. Lightning rods are also called lightning conductors. A lightning rod consists of a pointed thick metal rod erected at the top and going through the building, and is then connected to the ground. It extends above the highest point on the building. The lower part of the rod ends in a large metallic plate buried deep under the earth.
In the event that a building with a lightning rod is struck by lightning, the rod will conduct the electrical discharge safely into the ground without causing damage to the building. The lightning rod was invented by Benjamin Franklin.