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The climate of the British Isles
Origin and nature of depressions. Weather changes associated with the passage of a depression
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Depressions are otherwise known as mid latitude low pressure systems and affect the British Isles throughout the year at varying, frequencies, magnitudes and locations.  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.

The Polar Front is the contact point between Tropical air migrating North from the Tropics, and cold polar air migrating south from the Arctic Regions, and in effect is where the Polar cell meets the Ferrel cell on the Tri cellular model.  This contact point is most likely to be over the British Isles from Autumn to early spring, and this is when we experience most of our depressions in the British Isles.  However, it is possible for the British Isles to get depressions outside of this time frame, and North West Scotland has a much larger period of time during the year when it is hit by these storms.  These storms are characterized by large scale bands of precipitation, several hundreds of kilometers long and up to 160km wide.

 

The sequence of formation for mid latitude depressions is as follows;

 

Warm air (Tropical maritime) migrating north from the tropics meets cold dense air (Polar Maritime) migrating south from the Polar region. This is as a result of the global pattern of atmospheric circulation set up by imbalances in the global heat budget.

Where these 2 air masses meet an EMBRYO depression is formed, which is recognized as a wave in the polar front shown below.  This area tends to be a broad zone rather than the abrupt line shown in the diagram, and marks the first contact point between warm and cold air.

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 centre of the storm. 

The whole storm is dragged from South West to North East across the Atlantic, because of the West to East motion of the upper Polar Front Jet Stream and because of the prevailing wind direction.

At both fronts air is rising, so it cools initially at the Dry Adiabatic Lapse rate (9.8°C per 1000m ascent) and then at the MORE VARIABLE Saturated Adiabatic Lapse rate as latent heat is released during condensation. This eventually results in cloud formation and eventually rain (once the droplets have collided enough to be big enough to fall) AT BOTH FRONTS.    

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.  Behind the warm front, Cumulonimbus and stratus clouds form at the trailing cold front, as the uplift of warm here is more rapid.  This means that the second band of cloud brings shorter but more intense bouts of precipitation.   

Air rushes in from higher pressure areas around the depression giving the high winds we often associate with depressions. Due to the Coriolis Effect of the Earth’s rotation these anticlockwise winds come from the South West and travel into the centre of the low pressure. 

Within a mature depression, the amplitude of the initial wave is amplified, the cold front starts to catch up with the warm front and the warm is squeezed between the 2 fronts.  The pressure gradient steepens as more and more air is uplifted, and this creates stronger and stronger winds. 

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.  This occluded front may bring one rain event, but gradually pressure rises, cloud covers diminishes and rainfall peters out, and the storm is over.

 View an animated sequence of the life cycle of  depression from the Met Office (at the bottom of the page)

Animation of mid latitude depressions

 

The Weather in a Depression

Below you can see the weather a depression brings to the UK.  Note that this is a cross section through the depression. As this storm passes from the West to the East it drastically changes the weather based upon the processes of formation outlined above.  This weather changes over time, as the storm progresses on its regular journey from South West to North East.  The Warm front is associated with the more gentle uplift of warm tropical air above the colder polar air.  Therefore as the warm front passes over the British Isles places will tend to have falling pressures, winds that are picking up in strength, high and thinner clouds such as Cirrus and Cirrostratus giving way to lower altostratus and nimbo stratus, and a slow rise in humidity and rainfall levels. Temperatures tend to be cold or cool in this area.  As the depression passes overhead, the warm front will move away and the warm sector will take over.  Pressure will be low but steady in this zone, winds from the South West and Strong, with milder temperatures, high relative humidity but low amounts of cloud, rain will stop or be reduced to drizzle.  As the depression finishes is journey over the British Isles the cold front will pass over.  This will cause pressures to slowly rise, winds will veer round to North Westerlies and often be of gale force, temperatures will drop suddenly, clouds will become very thick, either cumulonimbus or cumulus, and these will bring the heaviest precipitation.

Depressions cross section

Case study – the great storm of 1987

This  case study offers an example of an EXTREME depression or storm event, however, don’t forget that most depressions do not have such drastic impacts nor do they cover as much of the British Isles.  Most depressions bring difficult conditions for the people living in the areas affected, such as high winds, lots of rainfall that can cause flooding or variable temperatures, but these regular storms can be adapted to and coped with. 

The famous Great Storm of 1987 has become better known for Michael Fish and his prediction that the storm would not hit the UK.  The storm occurred during the night of 15/16 October 1987, when  a depression system caused winds to hit much of southern England and northern France. It hit England and was responsible for the deaths of 18 people in England.  Michael Fish was correct that the storm was not a hurricane, as these only occur where sea temperatures are above 27°C, so this excludes the British Isles even though this storm had winds of hurricane force. The storm was a rare event, expected to happen only once every several hundred years. It developed progressively as it moved across the Atlantic Ocean.  The weather forecasters had predicted bad weather 4 to 5 days earlier, but by the middle of the week weather models predicted that only the South coast of the British Isles would be affected.  During the afternoon of 15 October, winds were very light over most parts of the UK. The pressure gradient was slack. A depression was drifting slowly northwards over the North Sea off eastern Scotland. The first gale warnings for sea areas in the English Channel were issued at 0630 on 15 October and were followed, four hours later, by warnings of severe gales. At 2235, winds of Force 10 were forecast.  By midnight, the depression was over the western English Channel, and its central pressure was 953 mb. At 0140 on 16 October, warnings of Force 11 were issued. The depression now moved rapidly north-east, filling a little as it did, reaching the Humber estuary at about 0530 UTC, by which time its central pressure was 959 mb. This is a very low pressure. Dramatic increases in temperature were associated with the passage of the storm's warm front.

Warnings of severe weather had been issued, however, to various agencies and emergency authorities, including the London Fire Brigade. Perhaps the most important warning was issued by the Met Office to the Ministry of Defence at 0135 UTC, 16 October. It warned that the anticipated consequences of the storm were such that civil authorities might need to call on assistance from the military.

In south-east England, where the greatest damage occurred, gusts of 70 knots or more were recorded continually for three or four consecutive hours. During this time, the wind veered from southerly to south-westerly. To the north-west of this region, there were two maxima in gust speeds, separated by a period of lower wind speeds. During the first period, the wind direction was southerly. During the latter, it was south-westerly. Damage patterns in south-east England suggested that whirlwinds accompanied the storm.

Effects

Cornwall and Devon where hit first and suffered gusts of up to 100knots or 190kph

The storm downed an estimated 15 million trees blocking roads and railways

Trees also caused structural damage to buildings.

Several hundred thousand people were left without power, which was not fully restored until more than two weeks later.

At sea, as well as many small boats being wrecked, a ship capsized at Dover and a Sealink cross-channel ferry, the MV Hengist,was driven ashore at Folkstone.

In London, many of the plane trees lining the streets were blown down overnight, blocking roads and crushing parked cars.

Building construction scaffolding and billboards had collapsed in many places, and many buildings had been damaged

Much of the public transport in the capital was not functioning, and people were advised against trying to go to work.

The storm cost the insurance industry £2bn, making it the second most expensive UK weather event on record, to insurers.

 The highest recorded wind speed (gust) was 122 mph (196 km/h) at Gorleston in Norfolk

The MetOffice were critised including Michael Fish, but the lack of a weather ship in the Southwest Approaches, due to Met Office cutbacks meant the only manner of tracking the storm was by using satellite data, as automatic buoys had not been deployed at the time.