We cannot see air but it is all around us. Gravity pulls the atmosphere down onto the earth. This is air pressure. You do not normally feel this pressure because there is an equal pressure inside your body pushing outward. At ground level, the pressure is greatest because there is a large weight of air pushing down. The higher you go, the less air there is, so it exerts less pressure.

If the air is cold, it sinks, pushing down to create a higher pressure on earth. As the air is squashed together, it warms up, bringing fine weather. If the air is warm, it rises, so there is a lower pressure on earth. The warm air may also evaporate water from the sea and take it up to form clouds. This is why low pressure can bring rain.

Air pressure is measured with barometers. An aneroid barometer looks a bit like a clock. It contains a sealed metal box with no air inside. The pointer is joined to the box. When air pressure rises, the box is squashed inward and the pointer shows that change on a dial. Changing air pressure is a good indicator of weather to come.