Depressions and Anticyclones

Depressions

Depressions are areas of low pressure,in other words the air is rising. They bring clouds and rain to the United Kingdom. They originate over the Atlantic and then travel with the prevailing winds in a North East direction across the country.
Depressions occur when two air masses meet, one is a warm, moist mass from the tropics, whilst the other is a cool, dense mass from the Pole.
The two air masses meet at a front, and the warm air is forced to rise over the cold air. This is called the warm front. At the same time the cold air circles around behind the warm air and undercuts, lifting the warm air off the ground. This is called the cold front. The two air masses spiral around each other and slowly move across the country.

As a depression moves overhead you can recognise 5 stages in order:

• Initially as the depression approaches you will get a build up of high cirrus and stratus clouds. This is because the warm air that has been forced to rise has begun to condensate.

• The cloud will continue to accumulate overhead as the warm front approaches. The clouds will be lower nimbostratus ones and there will be steady rainfall. The temperature will also fall.

• Once the warm front has passed, you are in the section between the two fronts where the warm air still is touching the ground. This causes the temperature to increase, often from approximately 6 to 12C. There may be some light drizzle, but mostly it is dry during this warm sector.

• The next part is the arrival of the cold front. You would be able to see this clearly coming towards you as it is a steep front with huge high cumulonimbus clouds, bringing with them very heavy rain. The temperatures also fall asthe cold front moves overhead.

• Finally, once the cold front has passed, you get a mixture of sunshine and heavy showers, caused by pockets of rising air in the air that follows the depression.

• The whole process takes between 12 and 24 hours.
  

Anticyclones

Anticyclones are areas of high pressure,where the air is falling towards the ground exerting pressure below it. Although not as common as depressions, anticyclones can remain for days, even weeks.They bring warm, stable conditions, with clear skies and lots of sunshine.
Winds are very light in an anticyclone, as the air moves outwards from the centre in a clockwise motion (in the Northern Hemisphere).
Anticyclones can occur throughout the year and cause different effects when they happen in the winter rather than the summer.
Summer Effects:

• Anticyclones bring hot and sunny weather due to there being clear skies (because of the descending air). The days are long and the sun high in the sky, which gives ample opportunity for the land to be heated up. These conditions could lead to convectional rainfall occurring.

Winter Effects:

• Anticyclones bring clear days in the winter as well. However the sun is low in the sky and the days are short, meaning you get cool, crisp days.

• Clear skies on a winter's night will allow frost to form. The land quickly loses heat during the night, as there is no cloud cover to act as insulation. The rapidly cooling ground cools and condenses any moisture in the air above it, forming droplets of ice when the temperature falls below freezing. This is frost.

• Fogs are also caused by clear winter nights. The ground loses heat. This cools the air above it causing moisture to condensate around dust particles in the layer of air closest to the ground surface. This is fog.

All Year Round Effects:

• The descending air creates dry conditions, as air will warm as it descends, meaning that it can actually hold more water, and so will not need to drop it.( link to this material)

TASK:

a) Describe the type of weather associated with anticyclones.

b) Describe the type of weather associated with depressions.

c) Explain why these types of weather occurs.

d) What is pressure measured in?

e) What type of pressure would you expect to find during an anticyclone.


Answer