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Introduction to depressions

Mid-latitude depressions, extra-tropical storms, cyclones (at mid-latitudes) or low pressure systems are all different names for the same thing. They all refer to the storms which bring the UK most of its weather, particularly in the autumn and winter. But what are they, how do they form in mid-latitudes, and why do they cause weather?

These storms typically last several days, are a few hundred kilometres in size and travel approximately eastwards across the north Atlantic. They are characterised by a swathe of cloud the same scale as the UK, and when shown on a weather map (also known as a synoptic chart), are distinguished by low pressure in the middle and distinctive fronts. Bringing us both our ‘normal’ and our ‘extreme’ weather.

A black white weather chart showing an example of low pressure and weather fronts

Figure 1: A weather map or synoptic chart showing a typical mid-latitude depression approaching the UK. © Crown Copyright, Met Office

Before we explore how these storms are formed, let’s start with a bit of history. The first scientific study and classification of these weather systems was performed by a group of Norwegian meteorologists: the Bergen School. The location was no coincidence, Bergen experiences a lot of weather systems.

Working without the benefit of satellite and radar pictures, the tools we use to tell us about the atmosphere today, this group did a remarkable job of describing the nature of weather systems. Using only ground based observations and information from weather balloons, they realised that depressions occur as a consequence of the pole-to-equator temperature gradient.

The Pole-to-equator temperature gradient

As we live on a spherical planet, you can see in Figure 2, somewhere on the Earth’s surface will always be at right angles to the Sun’s rays, such that the Sun’s light is fairly concentrated (a). In contrast, in other areas on the Earth’s surface which slope away from the Sun’s light, the Sun’s light will be more spread out (b). As the Earth’s surface is warmed by the Sun, the more concentrated light falling on (a) will warm the surface more than in area (b). This is why the Tropics are always warmer than the Poles.

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Figure 2: © Crown Copyright, Met Office

You can see this in further detail in Figure 3. In this graph, the latitude is recorded along the bottom (the x axis) and the amount of energy is recorded on the left (the y axis). The blue line shows that, over the course of the year, the Tropics receive more energy than the North and South Poles. The red line shows the amount of energy the Earth is losing to space.

As you can see, the Tropics lose more energy than the Polar regions, but the two lines are not the same. The Tropics are absorbing more energy than they are losing, whilst the North and South Poles are losing more energy than they are absorbing. If energy absorption and loss are the only factors at play this would mean that the Tropics would be always warming, and the Polar regions always cooling. However this is not the case, something else must be transporting heat from the Tropics to the poles.

This is where storms, such as mid-latitude depressions (and hurricanes in the Tropics) come in to play.

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Figure 3: A graph showing how the amount of energy the Earth receives from the Sun (blue line) and the amount of energy the Earth loses to space (red line) varies with latitude. If the blue line is above the red line, the Earth is getting more energy than it is losing at that latitude. If the red line is higher, the Earth is losing more energy than it is getting.

The map of the world in Figure 4, shows changes in temperature from the Polar regions to the Tropics, using different colour bands. The temperature cools most rapidly where the colour bands on the map are narrowest, in the mid-latitudes. As you can see, the rate at which the Earth’s temperature falls, is not steady as you travel from the Tropics to the Polar regions.

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Figure 4: A map of average December temperature of the atmosphere about 1.5km above the ground. © NOAA-ESRL Physical Sciences Division, Boulder Colorado. Data source NCEP/NCAR Reanalysis 1: Summary

The Bergen meteorologists realised this temperature gradient was a key part of the formation of weather systems. They saw the mid-latitudes as being where the warm, tropical air met and ‘fought’ the cold, Polar air. They were working at a time when fronts were very much part of people’s vocabulary, they referred to the temperature gradient as ‘the Polar front’; a boundary separating competing warm tropical air from colder air near the pole.

All depressions form on this Polar front, because it is ‘unstable’. But what do meteorologists mean by this?

The ‘unstable’ Polar front

Consider a ball in a valley (figure 5, 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 5: © 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 6: © 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 7: © 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 8: © 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 9: © 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 and where the storms originate that cause most of our weather in the UK.

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 10: © Dr Peter Inness

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