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Winter storms

From The Great Storm of October 1987 to the 'St Jude's Day' storm in October 2013, winter storms are amongst the most severe weather events experienced in the UK bringing heavy precipitation, low temperatures and gale-force winds.

What are winter storms?

Winter storms that hit the UK are events in which regions of low atmospheric pressure are accompanied by severe weather, such as heavy precipitation (and the potential for flooding), lower temperatures and gale-force winds. Different forms of precipitation, such as heavy snow, freezing rain or sleet can also occur if the conditions are cold enough.

Winter storms are synonymous with European windstorms which are most frequent in autumn and winter. They are some of the strongest weather systems which occur across the European continent and are associated with strong wind and heavy precipitation which can cause widespread damage and disruption.

European windstorms or winter storms are also described by meteorologists as 'depressions', 'winter lows', 'autumnal lows', 'North Atlantic lows' or 'European cyclonic systems' - all refer to European storms that have a significant wind or gale component.

A winter storm can also be referred to as an extra-tropical cyclone; this means the circulation pattern is anti-clockwise when looking down on the storm from space and it is 'extra-tropical' as it forms outside the Tropics, making it distinct from hurricanes and tropical cyclones. They can also be simply called 'low pressure systems'.

Storm damage in Aberystwyth 4 January 2014. Photo: Ian Capper

How do winter storms form?

At mid-latitudes, cold polar air meets warmer tropical air, these cross at what is commonly referred to as the polar front. Above the interface of the polar and tropical air masses a stream of strong wind is formed, known as the jet stream.

Atmospheric jet streams consist of bands of strong winds, reaching up to 200 mph. These bands can be found above the Earth's surface between 9 and 16 km (at about the cruising altitude of a commercial aeroplane) and can move weather systems around the globe. In winter, the jet stream separates cold, Arctic air masses from warmer large-scale tropical air masses of the mid-Atlantic. The location of a jet stream varies within the inherent fluctuations in the environment.

The storms mostly form in the winter months when the temperature differentiations between the polar and tropical air masses are at their greatest. This brings about a fortification of the jet and causes the polar front to become unbalanced, which in turns allows large disturbances in the form of vortices or cyclones to form.

These instabilities can give rise to waves or disruptions along the jet stream, which can cause the formation of Atlantic depressions to further deepen at the surface as they are steered towards the UK, so they are significant to the UK's weather and to meteorologists.

The strong winds of the jet stream within the upper atmosphere, remove and replace rising air from the Atlantic more rapidly than the air is replaced at lower levels, and therefore reduces the pressure in the centre of the cyclone. This larger-than-normal pressure gradient at the Earth's surface produces the strong wind component of the winter storm. In turn, heavy precipitation is created by the clashing of the two differing air masses. On a more localised scale strong gusts and even thunderstorms can be produced, especially in the areas where the updrafts are particularly strong.

The deep low pressure systems, that are comparatively widespread over the North Atlantic, frequently track past the north coast of Britain and Ireland and on into the North and Norwegian Seas. However, they can sometimes track at lower latitudes where they can affect almost any country in Europe.

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