Storms, thunderstorms and lightning

When the land is heated, the warm, moist air above it becomes lighter and rises, leaving low pressure behind. Cooler air from around rushes in to take its place, like moving up into an empty seat. That air warms up, rises in turn, and the cycle keeps going: a constant circulation of wind.

As the rising air spreads out, it cools, and its moisture condenses into water droplets, which is how clouds form. The droplets join into bigger, heavier drops and fall as rain, hail or snow. Strong winds together with rain make a storm. In hot, humid places like India, storms are common.

Sometimes the warm air rises so high that the cold up there turns the droplets into ice particles. Strong winds blowing up and down make the ice particles rub against the water droplets. Rubbing charges things (remember the balloons in Exploring Forces), so static charges build up inside the cloud.

The charges separate:

When the negative lower part comes near the ground, it makes the ground and things on it, such as trees and buildings, positively charged.

Air is normally an insulator: it keeps the opposite charges apart. But when the charge gets very large, the air can't hold it back any more, and the charges suddenly rush across in a bright flash: lightning. It can happen inside one cloud, between clouds, or between a cloud and the ground. The flash heats the air so quickly that it expands with a bang: thunder. A storm with lightning and thunder is a thunderstorm.

Some local thunderstorms have their own names: Kalboishakhi in West Bengal, Bihar and Jharkhand, and Bordoisila in Assam. They come before the monsoon and help kharif crops grow. In Kerala, Karnataka and Tamil Nadu they are called mango showers, because they help mangoes ripen; in Karnataka they help coffee plants too.

Lightning is dangerous: it can start fires, damage buildings and cause bad burns or death. Stay safe:

• keep away from tall things such as trees, poles and towers;
• in the open, go to a low-lying place, crouch down with your head low, and touch the ground as little as possible; don't lie flat;
• don't use an umbrella with a metal rod;
• get out of water;
• inside a car or bus you are fairly safe, so stay in.

Tall buildings have a lightning conductor: a metal rod whose pointed top sticks up above the highest point and whose bottom is buried deep in the ground. It gives the charge an easy path into the earth.