Thunderstorms

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Thunderstorms are caused in hot places, where hot air moves up and cold air comes down very suddenly.

On very hot days, the Earth’s surface gets heated by the sun. The air that is in touch with the Earth also heats up.

Think of a hot air balloon. Because it is full of warm air, it starts floating up into the sky. Similarly, the air heated by the Earth expands, and immediately rises up. Now there cannot be gaps in our atmosphere – so as soon as hot air rises, cool air rushes down to take its place. This causes strong winds to blow – leaves are shaken from the trees, your hair whips around your face, and your clothes billow out.

In some places where there are lots of water bodies like lakes, rivers and the oceans, the hot air that goes up carries with it lots of water vapour. Once the hot air moves up, it cools more and more and the water vapour forms clouds.

Clouds forming just before a thunderstorm. © Alan Cressler

Clouds forming just before a thunderstorm. © Alan Cressler

The wind blows clouds that form in different places together. Now the cloud is full of water, and bits of ice, that bump into each other, and get charged up. The whole cloud fills with electric charges, like the power socket you have at home. But, your power sockets are insulated to protect you from getting a shock. There is no such insulation in the clouds – so when they bump into each other, and they give out electricity, making lightning flash across the sky.

A flash of lightning. © Simon Colferai

A flash of lightning. © Simon Colferai 

Lightning is followed by thunder. When lightning strikes, it literally slices across the air, like knife going through butter. After the strike, air rolls back to fill the gap left by the slice, making the sound we call thunder.

Why do you think you first see lightning, before hearing thunder? This is because light travels much, much faster than sound.

When does it rain?

When there is enough water vapour in the air, and when it’s cool enough.

If you take a glass of water, what do you see? You see a liquid, which is colourless and odourless. It looks like it’s standing still, but actually, water is made of molecules. A molecule is too small for you to see, but it’s there in everything.

In ice, the molecules are very tightly packed. When you warm ice, the molecules get restless, start moving around, and break free from each other to form water. When you warm water still more, the molecules move further apart, and form water vapour.

This can still happen when it is cold, if the water is left open for long enough. This is why water disappears when you leave a glass uncovered. The water molecules at the surface, which are in touch with the air above, try to break free from the liquid water below. This is called evaporation.

All around the world, water evaporates from all the water bodies – lakes, ponds, rivers, the seas and oceans. This releases water vapour into the atmosphere. The amount of water vapour in the atmosphere is called humidity.

When the humidity increases, and the temperature drops, water molecules in the water vapour in the atmosphere start getting together to form liquid drops of water. This is called condensation. Once there enough water drops fuse together, they get too heavy to be floating around in the air, and they fall down, usually as rain. This is called precipitation.

Evaporation, condensation and precipitation together form the water cycle. This is how water on the earth is moved around from place to place, and we get enough to drink and use.

A simple figure showing the water cycle

A simple figure showing the water cycle

Whether precipitation occurs as rain, snow, sleet or hail depends on the temperature of the place. Sometimes, changes in the weather can be very sudden, causing thunderstorms. At other times, the sun can shine and it can rain at the same time, causing a rainbow.