The unstable Polar front

We now know that a Polar front is the boundary which separates warm tropical air from the colder air near the pole. All depressions form on this Polar front, because it is ‘unstable’. But what do meteorologists mean by this?

Consider a ball in a valley (Figure 1, left). However much you nudge the ball, it will end up back where it started. This is a ‘stable’ situation. Now, consider a ball on the top of a hill, however small a nudge it gets, the ball will roll down the hill and end up a long distance away from where it started. This is an unstable situation.

Figure 1: © Dr Peter Inness

Figure 2: © Dr Peter Inness

Figure 3: © Dr Peter Inness

Figure 4: © Dr Peter Inness

Figure 5: © Dr Peter Inness

As the wave breaks, it leaves warm air stranded on the poleward side of the Polar front, and cold air on the tropical side. This is how depressions move heat from the Tropics to the poles.

Figure 6: © Dr Peter Inness

You may also like to take a look at the links in the below ‘see also’ area, which provide additional information on weather systems and atmospheric pressure.