Weather & Climate- British Isles Climate

Air Masses Affecting the British Isles

• Air masses are large parcels of air with homogenous climate characteristics of temperature and humidity. 
• They form in high pressure regions where air spends a lot of time standing still and where atmospheric conditions are calm. In time though this air begins to move away from these high pressure zones and as it does it interacts with the surface characteristics that it moves over. 
• In Britain there are six air masses that at different times influence the climate and weather. 
• The most dominant air masses influencing the British Isles is the Polar Maritime. The British Isles are quite unique in their location as they stand on the Polar Front. The Polar Front is the line of latitude that marks the path of the Polar Jet Stream. It is along the Polar Front that different air masses collide and the result is a complex and variable climate. 

Depressions

Characteristics:

• Low pressure weather systems- low atmospheric pressure, below 1,000mb
• Represented by closed isobars decreasing towards the centre
• move west to east across the British Isles
• isobars close together producing steep pressure gradient from out to in
• winds strong and blow towards the centre anticlockwise
• Wind 'veer' in south UK- change direction from s/sw to w/nw
• Wind 'back' in north UK- change direction from se/e to ne/e

Formation:

A depression affecting the BI originate in the North Atlantic where two different air masses meet along the polar front.

• Polar maritime- dense, moist, cold
• Tropical maritime- light, moist, warm

The warmer less dense air rises and is removed by strong upper atmosphere winds (jet stream). This rising twisting vortex of air produces a wave to form at sea level in the polar front which becomes more exaggerated as the wave form develops eventually becoming a depression.

1. A wave forms on the polar front. Cloud and rain occur. Pressure falls.
2. Winds blow around depression. Pressure falls. Cold front moves faster than warm front.
3. Cold front catches up with warm front. Pressure rises. Depression starts to die as no warm air to lift near the centre of the low.

Synoptic Charts