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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.

Left picture: An orange ball in a valley.
Right picture: An orange ball on a blue hill

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.

A horizontal line with a small bump in the middle, with the words cold air above the line and the words warm air underneath the line.

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.

A horizontal line that has 2 large waves, one dipping down and the other rising up.

Figure 3: © Dr Peter Inness

Cold air is displaced towards the Tropics and warm air towards the Poles.

A horizontal line that has 2 deeper waves, one wave dipping with a blue arrow pointing down inside it which represents cold air. The other rising up, with a red arrow pointing up inside it, representing warm air.

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.

A horizontal line that has two large waves from above, that have developed into deep U shapes: one dipping down while the other rising up.

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.

A horizontal line that now has a slight wave. The U shapes have broken off and developed into ovals Undeneath the line there is a blue oval with cold written inside, and above the line there is a red oval with warm written inside.

Figure 6: © Dr Peter Inness

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This article is from the free online course:

Come Rain or Shine: Understanding the Weather

University of Reading