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Introducing weather fronts

Weather fronts are the boundaries between air masses. Watch Met Office meteorologist Alex Deakin demonstrate a front using a tank of coloured water.

A front is the boundary between air masses. They were named in 1919 by Bjerknes, a Norwegian meteorologist, who thought of a front as a battle ground between two opposing air masses, probably because the First World War had just finished.

There are three main types of weather front.

Cold fronts

Graphic of a cold front: a blue line running horizontally, with blue triangles pointing up

A cold front is represented on a synoptic chart as a blue line with triangles. When synoptic charts were first drawn up, they were printed in black and white, so the triangles were used to symbolise icicles. The presence of a cold front means that cold air is advancing and pushing underneath warmer air. This is because the cold air is ‘heavier’, or more dense, than the warm air. Cold air is thus replacing warm air at the surface. The tips of the ‘icicles’ indicate the direction of movement of the cold air.

Warm fronts

Graphic of a warm front: a red line running horizontally, with red semicircles pointing up

A warm front is symbolised as a red line with semicircles. The semicircles can be thought of as half suns. The presence of a warm front means that warm air is advancing and rising up over cold air. This is because warm air is ‘lighter’ or less dense, than cold air. Warm air is replacing cooler air at the surface. The edges of the ‘suns’ indicate the direction of movement of the warm air.

Occluded fronts

Graphic of an occluded front: a purple line running horizontally, with alternate purple triangles and semicircles pointing up

An occluded front is symbolised on a weather chart as a purple line with both semicircles and triangles. These are slightly more complex than cold or warm fronts. The word ‘occluded’ means ‘hidden’ and an occlusion occurs when the cold front ‘catches up’ with the warm front as it moves more quickly than a warm front due to the weight of the dense, cold air behind it. The warm air is then lifted up from the surface, and therefore ‘hidden’. An occlusion can be thought of as having the characteristics of both warm and cold fronts.

Because warm and cold air meet at a front, the less dense warm air is forced to rise, this air then cools, water vapour condenses into water droplets, and cloud is formed. So we associate fronts with lots of cloud. The strength of a front, and therefore the amount of rain we get on a front, depends on the temperature difference between the air masses that they separate. In a similar way to jet stream being strengthened by temperature gradients, so too are fronts. Fronts slope through the atmosphere, a forward slope (from left to right) in the case of warm fronts and backwards (right to left) in the case of cold fronts.

Diagram showing a warm and cold front in cross-section through the atmosphere: On the left, a warm front slopes from bottom left, to top right, with cold air on the forward side and warm air to the rear. On the right, a cold front slopes from bottom right, to top left, with warm air on the forward side and cold air to the rear Forward sloping warm front (on the left) Rearward sloping cold front (on the right)

These slopes mean that we can often see high cloud spilling way ahead of a warm front.

Cloud associated with a warm front and cold front. (More about clouds and their names and types next week)

So, on the passage of a warm front we see a change from cool, dry air to warmer and moister air. And as a cold front moves through, we see a change from warm and moist to cooler, drier air, and then we often see showers after a cold front has passed.

If you’d like to have a go at representing fronts yourself, there’s a worksheet to accompany the video in the files section below.