Why do cold weather fronts move faster than warm fronts?

Chris Smith put this to climate scientist Doug Crawford-Brown from the University of Cambridge...

Chris - Now: Watching the weather forecast has made Martha ponder the following:

How do cold fronts move faster than warm fronts, if cold air molecules moves slower than warm air molecules? Doug - Right. So continuing in the same theme of there being a lot more going on here than one would normally think. Molecules move for a lot of different reasons; they spin around, they rotate, they move laterally meaning that they're being pushed by air currents for example. They have diffusion so they're moving in all sorts of directions. Cold air molecules do move more slowly by diffusion than do warm air molecules.

But cold fronts also bring in closer isobars, in other words you get a greater drop in pressure per unit distance when you're dealing with cold weather fronts. And what drives the wind is really what that pressure difference is, as Darcy's Law shows, which is why the weather person always shows you the isobars. Take a look at the map sometimes and you'll see a cold weather front have isobars that are lines of equal pressure that are really close together, whereas the warm weather fronts have them far apart.

Chris - What causes the changes in pressure across the Earth's surface?

Doug - A lot of different things but, ultimately, it's all driven by sunlight hitting the surface of the Earth and being absorbed up in the atmosphere too, so ultimately it's driven by the Sun.

Chris - And that gives energy to the atmosphere which makes it expand?

Doug - It does, because it's non-homogeneous, it's going to have different pressures at different places.

Chris - Doug, thank you very much.