The polar front and the birth of a depression

Overview

What the polar front is, where the jet stream sits above it, how a kink in the front becomes a depression, and why depressions arrive in families.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is the polar front?

   Model answer

   The polar front is the sloping boundary in the mid-latitudes, at roughly 50 to 60 degrees, where warm moist tropical maritime air moving polewards meets cold polar maritime air moving equatorwards. The two do not mix, so the warm lighter air is forced to rise over the cold denser air, giving the sub-polar belt of low pressure. It is not a fixed line: it shifts north and south and buckles, and it is along this front that mid-latitude depressions are born.

2. 2

   Where does the Polar Front Jet Stream sit in relation to the polar front, and why is it there?

   Model answer

   It sits at the tropopause, about 9 to 12 kilometres directly above the surface polar front, and flows from west to east. It is there because the polar front is where the horizontal temperature contrast between tropical and polar air is at its sharpest, which produces the steepest pressure gradient aloft. The pressure gradient force accelerates the air polewards, the Coriolis effect deflects it to the right in the Northern Hemisphere until it flows almost parallel to the front, and the result is a narrow core of very fast westerly wind.

3. 3

   Why is the Polar Front Jet Stream strongest and furthest south in winter?

   Model answer

   Its strength depends on the temperature contrast across the polar front. In winter the Arctic cools far more than the subtropics, so the contrast is at its greatest, the pressure gradient aloft is steepest and jet speeds are highest, sometimes exceeding 400 kilometres per hour. The whole polar air mass also expands equatorwards in winter, pushing the front and the jet above it to a lower latitude. This is why the deepest and most frequent Atlantic depressions reach the British Isles in the winter half of the year.

4. 4

   How does the jet stream turn a small kink in the polar front into a depression?

   Model answer

   A small wave forms where warm air pushes polewards into cold air. If a section of the jet stream above that wave is diverging, with air spreading apart and leaving faster than it arrives, air is removed from the top of the column more quickly than it can be replaced from below. The weight of the air column falls, so surface pressure drops and a centre of low pressure appears beneath the divergence. Surface air spirals inwards to replace what has been drawn away, is forced to rise, and the wave amplifies into a full depression.

5. 5

   What is upper-air divergence and why is it the key to a deepening depression?

   Model answer

   Upper-air divergence is the spreading apart of air near the tropopause, so that more air leaves a region aloft than enters it. It matters because surface pressure is simply the weight of the whole column of air above. As long as divergence aloft removes air faster than convergence at the surface supplies it, the column keeps losing mass and the central pressure keeps falling, so the depression deepens and its winds strengthen. When surface convergence catches up with the divergence aloft, deepening stops.

6. 6

   How do the Rossby waves in the jet stream decide the weather type the British Isles gets?

   Model answer

   The jet follows long meanders around the hemisphere. When a meander places the jet straight over or just north of the islands, depressions are generated and driven across them one after another, giving a mild, wet, windy and unsettled spell. When a ridge in the wave lifts the jet well to the north of the islands, the Azores High extends over them and the weather turns settled and anticyclonic. When a trough in the wave carries the jet south of the islands, cold air spills down from higher latitudes and depressions track across France and Iberia, so the islands turn cold, and in winter easterly outbreaks and snow become likely.

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