An area of air has properties that depend on where the air has come from and where the air has travelled over. An area of polar air will therefore be cold. It cannot mix with warm tropical air, and the area of transition between the two different air masses is called a 'weather front'.

A 'front' can either be cold or warm. If cold air is approaching and replacing warmer air then it is a cold front, and similarly if warm air is replacing colder air, then it is called a 'warm front'. A front is not only seen on the ground, but extends up into the atmosphere too. A warm front has warm air approaching behind some colder air.

Cold air is denser and therefore heavier than warm air, so the lighter, warmer air has a tendency to rise over the top. As the air rises it cools and the moisture contained within it condenses into clouds and eventually it rains. This is why it often rains as a warm front passes.

On a weather map, a warm front is denoted by a line with red semicircles pointing in the direction of travel. In a cold front, as the cold, heavier air approaches, it pushes under the warm air, causing the warmer air to rise, this causes condensation and hence rain.

A cold front is a line with blue triangles pointing in the direction the front is going. In the UK, often a cold front will be following a warm front. Cold fronts travel faster, so can catch up with the warm front. The warm area of air is forced up above the cold air it is approaching, and the cold air behind it will also be pushing under it. This means the warm air can be forced totally off the ground and this is called an occluded front. Fronts though, are only models. They are the perfect scenario, and in nature of course, things are rarely so cut and dried.

Sometimes it is difficult to tell exactly where the front is. Simply watching the temperature can be misleading. If the sun comes out after a cold front has passed, the temperature may actually rise. The clouds and rain are a better indication, as is a usual change in wind direction. However, mother nature works in mysterious ways and if she doesn't want to follow a model, she won't!

Other features in the Weather Basics series
Air Pressure
Clouds
Day and Night
Fog and Mist
Frost
High Pressure
Humidity
Jet Streams
Lightning
Low Pressure
Mirages
Northern Lights
Rain or Showers?
Thunder
Watercycle

Other Features: Air Pollution Air Pressure Clouds Clouds, Nature Of Clouds, Types Of Day in the life of Helen Young Day & night Driving E-met: your weather Flooding Fog Fronts Frost Frost and Snow Frost Fairs Gardening Weather Global Warming - Overview Global Warming - What We Can Do Jet streams Hibernation High Pressure History of Weather Broadcasting Humidity Ice Age - part 1 Ice Age - part 2 Low Pressure Migration Mirages Motorbike Weather Napoleon Russia Northern Lights Our Graphics Our Indices Our Symbols Pollen Forecast Pressure Rain or Showers? Rain, Drizzle and Fog Royal Met. Society, The Scuba Diving Snowmen Sun Index Solar Energy Snowflakes Thunder Water cycle Winter on the Roads Worst Winters Water Energy Wind energy Year so far