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.
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.
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.