In an area of low pressure the air has a tendency to rise. This general upward motion means that there is less pressure from the air pushing down on the earth, in other words there is low pressure. As air rises, it cools and if there is enough water vapour it may condense to form clouds and rain. This is why a low pressure is generally associated with wet weather.

Low pressures often form on the boundary of warm and cold air. A small disturbance may cause the pressure to fall along this boundary, causing a circulation of air to develop. This is the beginning of a low pressure, and the boundaries of warm and cold air become weather fronts. The UK is currently seeing a succession of these weather systems. This is common in autumn when there is a larger temperature difference between the cold arctic air and the warmer air over the sea.

Other common types of lows are 'thermal lows'. One thermal low, a heat low, recurs over Spain and Portugal. During a summer day, the Iberian Peninsula gets much hotter than the surrounding sea. This in turn heats the air nearer the ground, which rises. This rising air lowers the pressure and a low forms. As the air rises, if there is enough moisture present, intense thunderstorms can develop.

Polar lows are another type of thermal low, but as the name suggests, this low forms in the polar regions. When air from the arctic moves south across the sea, it is heated from below and the rising air gives a region of low pressure. Polar lows are heated throughout the day and night, so can become very intense. They are quite common in the UK, and often bring heavy snow to the north-facing coasts. They can cause problems for weather forecasters, as they are often rather small, so difficult to locate and track.

Old hurricanes from the Caribbean are also types of low pressure. Normally hurricanes die away after drifting north into cooler waters, but sometimes an old depression can intensify again on the way to the UK. This will bring us very wet windy weather, as ex-hurricane Iris did in September 1995. Fortunately this only happens every few years, but as we are seeing at the moment, it is not only ex-hurricanes that bring us heavy rain and strong winds.

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