The movement of hot and cold air is very important for moving heat and energy around our planet Earth. Hot air is less ‘dense’ than cold air. In case you do not know the word ‘dense’ yet, I essentially mean that hot air is lighter and cold air is heavier. Therefore, hot air rises and cold air sinks. Now, what happens? Perhaps you know that when you go up mountains or up in an air balloon or air plane it gets cold outside. For every 1000 meters in elevation, the temperature typically falls by 6.5°C. So, as warm air rises, it gets colder and colder. Therefore, the warm air cools. Similarly, when cold air falls it gets warmer. Of course, wherever air moves away, there has to be some ‘other’ air that moves into place where the air was before. Otherwise, we would have places without air.

So, air moves around in a BIIIIG circle: Hot air rises, and is replaced by cold air. The warm air gets cold, and moves, and eventually falls again when it gets cold enough. This is what makes wind on the earth.

Big things happen when the air holds water. Yes, air can ‘hold’ water. For example, sometimes before a thunderstorm you feel that the air is really “sticky” or “humid”, that is because there is a lot of water in the air. Or, when you boil water, there is steam (very small water drops) that come out from the water and that disappear in the air. The water is not gone. It is just ‘in’ the air. Now, the important thing is that warm air can hold more water than cold air. Therefore, when warm air rises, the air may not be able anymore to hold all the water that it has. Then the water will come “out of the air” and form very tiny drops– this is a cloud. If there is a lot of water, the drops can get bigger and bigger until they are heavy enough that they fall from the sky – it rains. So, the movement of cold and hot air brings all kinds of weather to the planet. It makes wind, and rain, and snow.