What is air pressure? – Solids Liquids and Gases for Kids

Look out of your window.
Is the air moving?
It is probably moving enough to make the branches on the trees sway slightly.
The weight of this air pressing down all around the earth produces air pressure.
If you live high up above sea level, or even if you go up to the top of a tall skyscraper, the air pressure is less. This is because there is less air above you, and therefore less air pressure.
On the surface of the earth, or near it, air always moves from areas of high pressure to areas of low pressure. Temperature has a great effect on air pressure.
As warm air rises, it produces an area of low pressure near the ground.
As the air pressure changes from place to place, and from day to day, it helps move air across the earth’s surface all the time.
As cooler air moves down, it leaves behind an area of high pressure.

Cool air is heavier than warm air. 
Cool air sinks down from an area of high pressure, while warm air rises from an area of low pressure.