Atmospheric Processes - Depressions

The cold front follows the warm front with marked changes in conditions. At the head the pressure still falls and there is light rain and drizzle from cumulo-stratus clouds, however the south westerly wind picks up in speed. At the passage the conditions drastically deteriorate. As the cold air pushes the warm front it undercuts it and forces the warm air to rise quickly. This creates instability and the pressure significantly rises. This causes heavy cumulonimbus clouds to form bringing with them heavy hail rain and thunderstorms. The wind direction also veers from south westerly to westerly and increases in speed, with squalls. As the rear approaches, there continues to be a steady rise in pressure, although conditions tend to stabilise. As visibility improves greatly and there are bright intervals appearing through the scattered showers from the cumulus clouds. The depression itself continuous to follow Buys Ballet Law and moves to the right with the wind direction changing once more from westerly to north westerly.

 

During a depression the atmosphere is not always the same, this can have an effect on the scale of the depression. Examples of this are Catofronts and Anofronts. A Catofront occur when the troposphere is low and it presses against the depression. This can act as a lid; this stops air from rising and increases stability. This increase in stability results in the creation of more stratiform clouds and there is not as much rain. The opposite can occurs during an Anofront, as the troposphere is higher and weather is more pronounced, with cumuli formed clouds more evident, resulting in higher winds and more rain.

After the depression has fully developed both a cold and a warm front, the cold front moves more rapidly than the warm front. The cold front eventually catches up with it and forces the warm air to rise above the ground, in some cases causing conditionally instability, and more uniform air fills the gap left, resulting in the decay of the system

Mid-latitude depression can affect the British Isles, in some cases, much more extremely. An example of this is ‘the Great Storm’ which occurred on the 16th October 1987. The storm originated from a cold front in the  that met with a warm front headed north from Africa. When the two systems collided, a severe low pressure front developed with a central pressure of 958mb, which is comparable to a Category 3 hurricane. How such a low pressure system developed on this particular occasion is argued over, however one theory is that it may have been a result of the jet stream in the wake of the 1987 hurricane Floyd, coinciding with exceptionally warm weather from the Bay of Biscay.

This depression had severe effects on Britain with 18 people being killed and 15 million trees being destroyed, Peak wind velocities were in the early hours of the morning, and this probably reduced the death toll. Many transport links were affected by the fallen trees, which lay on roads and train lines. Electricity was also cut off for many people with many power lines being destroyed. The storm cost the insurance industry £2 billion; this makes it the second most expensive UK weather event.

The impacts experienced on the British Isles from depression are varied by a number of different factors. The depressions themselves bring a number of different conditions in both the cold and warm fronts. The state of the atmosphere at the time can also have some impact on the depression itself, with the example being Catofronts and Anofronts, therefore it makes the depressions themselves much harder to predict, and therefore the weather itself much more volatile.

Sources

Advanced Geography - Edexcel

Weather and the Climate – John Mason

Wikipedia.com/The_Great_Storm_1987

Scool.com/Alevel/Geography