Fronts
A front is an imaginary line drawn by the meteorologists on a weather map to show where two air masses with different basic characteristics meet. There are different types of fronts.
A warm front is where a mass of warm air overtakes cold air – when tropical air meets cold polar air. This causes precipitation over large areas – for example, rain over northern England.
On a synoptic chart warm fronts are shown by a solid line with semicircles pointing towards the colder air and in the direction of movement. On coloured weather maps, a warm front is drawn with a solid red line with red semicircles.
A cold front is where a mass of cold air overtakes warmer air – when cold polar air meets warm tropical air. This causes the temperature to drop suddenly and there showers and thunderstorms. The rain is very localised: it may rain in one place but a few miles away it may be dry.
Cold fronts are shown on synoptic charts by a solid line with triangles along the front pointing towards the warmer air and in the direction of movement. On coloured weather maps, a cold front is drawn with a solid blue line with blue triangles.
An occluded front is where a warm and cold front merge. Occluded means “trapped”. The weather usually becomes much colder and cloudier and there is widespread precipitation. In winter an occluded front often brings snow.
On a synoptic chart occluded fronts are represented by semi-circles and triangles positioned next to each other, the triangles in blue, the semi-circles in red, or both are purple (mixing both blue and red colours together).
There are very few places that have the same type of weather all the time. Usually there are differences between winter and summer. For example, some places have a cold, snowy winter and a hot, wet summer. Other regions have less variation – for example, a lot of sunshine and little rainfall throughout the year (desserts), but different temperatures between day and night.