Lightning
Although only about 20 percent of lightning reaches the ground, strikes occur on earth over 100 times every second. In North America alone, about 400 people are struck by lightning each year, and about one in every four of these strikes are fatal.
Lightning , which accompanies thunderstorms, is caused by electrical currents that pass between the cloud and the ground. The top of a cloud is positively charged. The bottom of the cloud has a negative charge. This negative charge begins following a zigzag pattern toward the earth, which has a positive charge.
As it nears the ground, the positive charges and the negative charges meet. The positive charges from the ground travel up to the cloud at a speed of 60,000 miles per second. The charges jump back and forth between the cloud and the ground until there is a balance of electrical charge between the two.
Lightning can repeat along the same path in less than a second making the lightning flicker.
There are three basic types of lightning: cloud-to-air, cloud-to- cloud, and cloud-to-ground.
Cloud-to-air is the least powerful lightning and is caused by too much positive charge in the air surrounding a cloud. After the strike, the charges are usually balanced and further lightning isn't generated.
Cloud-to-cloud is the most common form of lightning. In this type, lightning travels within a cloud from the negative bottom to the positive top. It may also occur from one cloud to another, although it is not very common. This lightning occurs in high clouds, so it is too far away to hear thunder from the electrical charges. Thunder is only audible for about twenty miles. It is called "silent lightning".
Cloud-to-ground lightning, as stated above, is caused by the negative charges in the bottom of the cloud and the positive charges from the ground.
Lightning may be many different colors. Red lightning indicates that there is rain in the cloud. Blue lightning indicates hail is forming in the cloud. Yellow lightning occurs because of dust in the air. And white lightning is an indication of low humidity. This is the type of lightning responsible for many of the forest fires that are started during storms.
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Thunder is heard because some of the tremendous energy created by the lightning flashes is turned into heat and then into sound waves.
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