Why do storm clouds have such a clean, flat top?

Sarah - We actually had some great responses to this on our forum from Paul.fr but basically the story is that thunderstorms are formed of what are known as cumulo-nimbus clouds. These grow from the billowy cumulus clouds like the ones you see in cartoons and these grow upwards. The reason they do this is because if the sun heats up the ground you get thermals which is like warm air and it rises, pushing up the clouds. Ben - Are these the same thermals that birds use to lift themselves really high up in the sky, and also in hang-gliders?

Sarah - Yes. Big heavy birds like vultures, ones that live in quite hot areas, will wait for the thermals to rise before they can get up in the air because they're quite heavy so they find it hard to take off. Ben - Once these thermals have lifted the clouds how do they get the flat top?

Sarah - They keep growing upwards and then reach what is known as the tropopause which is the part of the atmosphere between the troposphere which is the bit nearest to the ground and the stratosphere which is the next layer up. Ben - What happens there?

Sarah - What you get here is known as an inversion which is where you get cold air sitting over warm air. What everyone's used to is the warm air rising and therefore being above the cold air. This doesn't happen in the stratosphere because it gets colder and colder as you go up. This means that the clouds can't go any farther as it hits the wall of warm air. It spreads out along the top and it's very flat. Obviously because the climate is very complicated there are other factors involved but that's basically the story.