Friday, February 15, 2008


Lightning is characterized by the discharge of electricity between rain clouds or between a rain cloud and the earth. It is usually seen as an arc of extremely bright light which can be many kilometers in length, however, there are other forms as well. Accompanying the lightning is the giant roar of thunder. The thunder is caused by the expansion of air that has been heated by the lightning which then collides with cooler air, creating the sound of an explosion.

Thunderstorms are the most common types of storms, and thunder itself, although frightening, is not dangerous. It is the lightning that causes the problems. Lightning and thunder occur together, however we hear the thunder after we see the lightning. Sound waves travel about one mile in five seconds, while light travels at more than 186,000 miles per second. Therefore, lightning is seen immediately when it occurs, and thunder is heard a little later.

The different sounds of thunder (deep roar/loud crash) is caused by the different types of lightning, for example, the thunder that has the sharp crackle like sound is caused when the large trunk of lightning forks into many branches. (See types of lightning)

How Lightning is Formed

Before lightning is formed, the cumulonimbus cloud of thunderstorms must become electrically charged. In most rain clouds, the bottom of the cloud is negatively charged and the top is positively charged. It is not known how the cloud becomes charged, but scientists have formed numerous theories to try to explain this phenomenon. These theories have been divided into two main categories: those that require ice and those that do not. However, meteorologists are leaning toward the theory that requires ice because lightning is not often seen unless ice has formed in the upper layers of the rain cloud.

Theories

The first theory describes how ice gains a negative charge while frozen and unfrozen water keeps its positive charge. Another theory shows how when large droplets fall swiftly, they gain a negative charge, while slowly falling water gains a positive charge.

Theories that do not require ice gives the explanation that the cloud gets its charge by attracting negative charges from the ionosphere. These negative charges are pushed to the base of the cloud by strong downdrafts, while the positive particles are pushed upwards by warm air within the rain cloud.

Strokes

Eventually the negative charge of the base of the cloud gives the earth a positive charge. When the electrical potential reaches approximately ten thousand volts per centimeter, ionization occurs along a narrow path and the result is a flash of lightning. The negative particles descend from the base of the cloud to the ground.

However, most lightning flashes are not a single event, but rather numerous strokes followed by a leader stroke. There can be up to 42 strokes to a lightning bolt. The time between successive strokes is 0.02 seconds. The average bolt lasts only one fourth of a second.

Safety Tips

Since lightning can be very dangerous and it has killed and maimed many people, it is important for us to follow safety measures and protect ourselves and property.

During a storm stay indoors, however if you are caught outside in a severe storm, never stand under any tall objects such as trees. It is better to lay down in an open field away from any surrounding tall objects. Avoid any large bodies of water and if you are swimming, leave the water immediately.



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