Faster cold fronts

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Why does the cold front catch up with the warm front, as the wave breaks and the depression starts to decay?

Conceptually, it’s easiest to think of what is going on at the two fronts. At the cold front, relatively cold, dense air is pushing at warmer, less dense air – a bit like a heavier person pushing a lighter person. On the warm front, it’s the less dense, warmer air which is pushing at more dense, colder air – a bit like a lighter person trying to push a heavier person out of the way. If you think of it like that, it’s easy to see why the cold front could move faster. But what is actually going on?

Firstly, let’s think about the rising air in the warm sector. It has to go somewhere, and so you end up with small circulations around the fronts. This is known as ‘ageostrophic flow’ – geostrophic flow is air flow where the pressure gradient and Coriolis forces are in balance (for example in the anticlockwise circulation of air around a depression). Ageostrophic flow is air flow where other forces are having an impact on where the air goes.

Figure 1: Westerly winds (green) and the small circulations around the cold and warm fronts (yellow) © David Schultz, University of Manchester

Figure 2: Rainfall on the cold and warm fronts, where evaporative cooling might be expected © David Schultz, University of Manchester

At the cold front, the rain is mainly falling in to the warm air ahead of the front, cooling it down and reducing the temperature gradient across the front. On the warm front, the rain is mainly falling in to the cold air ahead of the front, cooling it down even more. This also has an impact on the relative speed of the fronts.