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
The Polar front is unstable. This means that if a small disturbance is introduced to the gradient, it will grow with time. The blue line below shows the Polar front, which has westerly (west to east) winds blowing along it. What happens if something nudges those winds? Imagine, for example, if there is a hill in the way and the wind blows around it.
Figure 2: © Dr Peter Inness
Because the Polar front is unstable, instead of returning to a westerly flow, the ‘nudge’ gets bigger, growing into a wave over a day or so. This is similar to the way an ocean wave, approaching the coast over shallower water, gets bigger.
Figure 3: © Dr Peter Inness
Cold air is displaced towards the Tropics and warm air towards the Poles.
Figure 4: © Dr Peter Inness
Then, in the same way that an ocean wave approaching the coast eventually gets so steep that it can’t hold together any longer and breaks, the atmospheric wave also breaks.
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.