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UK Weather hazards

UK Weather Hazards - Depressions and Anticyclones
It is hard to believe at times but the British Isles are home to nearly every hazard on planet Earth.  Despite being far from a plate margin, Britain gets small earthquakes from old fault lines that run through the country.  Britain also has extinct volcanoes - Edinburgh castle is built on top of an ancient volcanic rock outcrop! Our most common hazards include floods, which seem to be increasing in intensity and frequency, cold weather - the winter of 2010-11 being a great example, and storms, brought to us by mid latitude depressions from the Atlantic. 

We also get less obvious hazards, Britain gets over 30 Tornadoes a year, suffers from extreme coastal erosion, has had many heat waves and droughts (especially in the last 30 years).

These hazards have a DISTINCT GEOGRAPHIC PATTERN of distribution.  The storms or depressions that bring windy and wet weather to Britain occur mainly in the WEST and the North, and the North West of Scotland gets these storms most often and at the greatest intensity.  Flooding occurs in low lying areas around rives and at the coast, and more people than ever are living in flood risk areas.  Extreme cold weather can affect all areas of the British Isles but is most likely in the North and at altitude, whereas heat waves are most likely in the South East.  Tornadoes are most likely in the south in inland areas.

A depression is an area of low atmospheric pressure and the British Isles experience them throughout the year but the most severe and frequent occur in autumn & winter. They bring strong winds and lots of rainfall.

The formation of a depression

The formation of a depression
1. Depressions are low pressure storm systems and affect the British Isles throughout the year. The often occur at the Polar Front, which is also associated with a jet stream above that front, and bring wet and windy weather to the British Isles.
2. Warm Tropical air migrates north from the Tropics, and meets cold polar air migrating south from the Arctic Regions over the Atlantic Ocean.  They meet at a point called the Polar Front. These storms are characterized by large scale bands of precipitation, several hundreds of kilometers long and up to 160km wide.
3. Where these 2 air masses meet an EMBRYO depression is formed, which is recognized as a wave in the polar front.
4. The warm air is undercut by the advancing cold air at the fronts and because it has more energy and is less dense is forced to rise upwards at a COLD FRONT. Ahead of this, warm air advances into cold air and is also forced to rise above this denser cold air at a WARM FRONT. The air rises in a spiral motion, and this creates low pressure at the earth’s surface at the center of the storm.
5. At both fronts air is rising, so it cools down and creates water droplets (cloud formation) and eventually rain (once the droplets have collided enough to be big enough to fall) AT BOTH FRONTS.
6. The cloud types at the 2 fronts are different however. Cirrus, cumulus and Nimbo stratus are common on the warm front where warm air is slowly lifted over the cold air in front of it. This gives prolonged but lighter rainfall. Cumulonimbus and stratus clouds form at the trailing cold front, as the uplift of warm here is more rapid.
7. Air rushes in from higher pressure areas around the depression giving the high winds we often associate with depressions.
8. The final stage of the depression life cycle model is where the cold front catches up with the warm front and an OCCLUDED FRONT is created. This is the decay stage of the storm where there is no warm Tropical Maritime air in contact with the ground, it has all been uplifted.
These storms are hazardous because they bring extreme rainfall at both fronts, very high wind speeds and these winds also create huge waves which batter our coastline (and could cause coastal flooding).

Anticyclones are the OPPOSITE of depressions.  They are areas of high pressure where air sinks to the earth’s surface. As the air sinks it warms so condensation does not occur and there are very few clouds. This gives clear skies. Anticyclones have low wind speeds, widely spaced isobars and stable conditions. Anticyclones only involve one type of air mass which usually cover large areas and do not have any fronts. Anticyclones can occur in both winter and summer. Anticyclones can be very large, at least 3,000 km wide which is much larger than depressions. They can give several days of settled weather.

In winter
In winter the longer nights combined with clear skies leads to intense cooling of the land. There is an increased risk of dew, frost and thicker, more extensive fog patches which may be slow to clear or even persist.
Under very calm conditions, both frost and fog may persist for several days. An anticyclone's very stable conditions and little air movement means that pollution is trapped at low levels, resulting in very poor air quality such as smogs.
Winter anticyclone

© Copyright Richard Webb and licensed for reuse under this Creative Commons Licence

In summer
In Britain in summer an anticyclone will mean heat waves during the day. At night, however, as there are no clouds, heat will be quickly lost.  The ground will cool sufficiently to cause condensation of water vapour in the descending warm air and mist or heavy dew may form. This will clear quickly in the morning sun. After a few days, a layer of hot air builds up at ground level, which eventually will give rise to thunderstorms, ending the anticyclone. Indeed, summer anticyclones can result in “Heat wave” conditions with temperatures significantly above average.  One such event occurred in the summer of 2003 affecting continental Europe and the UK, it proved to be particularly hazardous to humans.

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