Humidity is a measure of the amount of water vapour in the air. A high humidity means that there is a lot of water vapour, and a low humidity means a little. In high humidity it can feel 'sticky' or 'close'. It often feels like this before a large thunderstorm. A low humidity feels more pleasant even on the hottest days. If the air is too dry however, this can irritate asthma or other respiratory diseases.

The humidity depends mainly on where the air has come from. If it has come across the Atlantic, then it is likely to contain a lot of water vapour and therefore be quite humid. If the air came from across the continent then it is usually dry with a low humidity.

When air contains a certain amount of moisture it becomes 'saturated', and cannot hold any more water. The amount of water that air can hold is dependent on its temperature. The hotter the air, the more moisture it can hold. This makes water vapour an interesting greenhouse gas. A greenhouse gas is one that absorbs energy that would otherwise escape into space, and so causes the earth's atmosphere to warm up. Water vapour does this, but the fascinating thing is that as it warms up the atmosphere, it makes the air able to hold more water, so more heating can take place, so it can hold more water and so on.

This temperature dependence on the saturation of air is fundamental in the development of clouds. When air rises, it expands and the temperature drops. This means that it can no longer hold as much water. When the air reaches its saturation point, and droplets begin to condense to form clouds. If there is enough water then it will rain too.

This explains why the western side of the British Isles receives so much more rain than the east. Plymouth in the west has an average annual rainfall of 950mm, whereas London in the east is much drier, with just 593mm. Humid air from the Atlantic reaches the western side of the country first. It has to rise to travel over the land, and as it rises it cools. The water vapour condenses to form clouds and it rains.

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