Air pressure is the force of the air pressing down on the Earth's surface. As air is all around us, there is air pressure all around us too. Near sea level there is more air above you than there would be on the top of Mount Everest, therefore the air pressure is higher near the sea than it is on the top of a mountain.

Air pressure does not only vary with height, there are many other factors, one main influence is temperature. The sun does not heat the earth uniformly, so different parts of the air are at different temperatures. If air is given more heat, this is extra energy for its molecules. If they have more energy, they can move further apart, so there will be fewer molecules occupying the same space and the air pressure will decrease.

Similarly, if the temperature is lower, the air pressure will increase. Warm air near the surface will tend to rise. A mass of low pressure is an area of air that is rising. As is rises it expands and cools. Cooler air cannot hold as much water as warmer air, so as the air rises the water will condense and form clouds. This is why an area of low pressure will often be accompanied by clouds and rain.

Conversely, an area of high pressure is a section of air which is sinking. As the air sinks it warms, so is able to hold more water, and therefore areas of high pressure are often accompanied by fair weather. Points of the same pressure can be joined up to form lines called 'isobars'. These form rings around high and low pressure centres.

Winds blow in an attempt to combat the differences in air pressure. They try to flow directly from a high to a low pressure, but due to the spinning of the earth and friction of the surface, actually flow around the pressure centres, following the isobars. The larger the difference in pressure the stronger the winds will blow.

When isobars on a weather chart are close together, it will be a blustery day, and when this is accompanied by an area of low pressure, it will also be wet. A wet and windy day is not exactly unusual for the British Isles nor indeed are areas of low pressure and closely packed isobars.

Other features in the Weather Basics series:

- Clouds
- Day and Night
- Fog and Mist
- Fronts
- Frost
- High Pressure
- Humidity
- Jet Streams
- Lightning
- Low Pressure
- Mirages
- Northern Lights
- Rain or Showers?
- Thunder
- Watercycle