We usually think of air as being weightless, but in fact air is quite heavy. The air in a large classroom has the same weight as a small car! The air of the Earth’s atmosphere reaches upwards for several hundred kilometres. The effect of this is that the air at ground level presses on everything it surrounds.
The exact air pressure changes from day to day. Studying air pressure, and the way it is changing, helps to tell us how the weather will change in the next few hours and days. Usually, high pressure brings good weather whereas low pressure brings bad weather. A device called a ‘barometer’ measures air pressure and is used to predict the weather.
On this map you can see areas of high pressure (H) and low pressure (L).
The effects of air pressure
You can see an effect of air pressure with a washing-up liquid bottle. If you remove most of the air from inside the bottle, by sucking it, the bottle collapses. This is because the air around it pushes inwards. Normally the air inside balances this force. Simple barometers measure changes in pressure in a similar way.
Large swirls of cloud like this one often indicate areas of low air pressure.