The climate of the British Isles
Origin and nature of depressions. Weather changes associated with the passage of a depression
Depressions are otherwise
known as mid latitude low pressure systems and affect the
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
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.
The Weather in a Depression
Below you can see
the weather a depression brings to the
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
The famous Great Storm of 1987
has become better known for Michael Fish and his prediction that the
storm would not hit the
Warnings of severe weather had
been issued, however, to various agencies and emergency authorities,
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
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
highest recorded wind speed (gust) was 122 mph
(196 km/h) at Gorleston in
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.