Air Masses Affecting the UK

If air remains stationary in an area for several days, it assumes the temperature and humidity properties of that area. Stationary air is mainly fouund in the high pressure belts of the subtropics (The Sahara and Azores) and high latitudes (Siberia and Northern Canada). THe areas in which homogenous air masses develop are called Source Regions.

They are classified according to:

• The latitude in which they originate, which determines their temperature - Arctic, Polar or Tropical
• The nature of the surface over which they deveop, which affects heir moisture content - Maritime or Continental

Each air mass is uniue and dependent on:

• climatic conditions in the source region
• the pathway it follows
• season in which it occurs
• the vertical characteristics of the atmosphere at the time

When two air masses meet, they od not mix readily due to differences in temperature and density. The point at which they meet is called a front. A warm front is fouund where warm air is advancing and being forced to override cold air. A cold front occurs when advancing cold air undercuts a body of warm air.

In both cases, the rising air cools and usually produces clouds. These clouds often generate precipiation. Fronts maybe several hundred kilometeres up into the atmosphere. The most notable type of front, the polar front, occurs when warm, moist Tropical maritime air meets colder, drier Polar maritime air. It is at the polar front that many depressions form. they form most readily over oceans in mid-latitudes , and track eatwards brigning cloud and rain to western margins of continents

Arctic Maritime - frrom the Artic Ocean.

• Very cold conditions in winter, cold in spring and rare in summer.
• IT slowly heats up as it crosses the sea, pcking up some moisture and becoming unstable in its lower layers.
• Snow in winter in Soctland, hail in sring and often brings heavy showers
• Usually good visibility and often lasts several days.
• Winds are often strong in the North

Polar Maritime - From Northern Canada and the Arctic Ocean

• It gives cool conditions througghout the year
• Warms slightly as it crosses the Atlantic to become unstable in its lower layers
• Gives heavy showers as it crosses the highlands, separated by bright intervals
• Associated with Cumulus clouds
• Good visibility with otfen strong winds and gales after the passing of a cold front

Tropical Maritime:- From the Azores high pressure area (very common in the UK)

• Occurs during the warm sector of a depression
• Very mild and wet in winter, with thick cloud cover. Often stratus cloud giving hill and coastal fog.
• Poor visibility but not frost
• Warm in summer, thougght not hot. Lower air is stable but it forced to rise over hills, the upper layers can become unstable to give thundery showers
• Winds usuually moderate to fresh

Polar Continental:- From Siberian High Pressure area

• Very cold temperatures in winter
• stable but warms slighly crossing the North Sea to become unstable in lower layers - giving heavy snow in Eastern Britain
• Often lasts seeral days if blocking anticyclones interrupt prevailing westerlies
• Wind chill factor is high
• In the Summer, it brings warm conditions and is more stable

Tropical Continental:- From the Saharan sub-tropical high pressure area

• Occurs in summer when sub-tropical high pressure moves north. Heatwave conditions
• very stable in the lower layers, thouggh the upper layers may become unstable to give thunderstorms
• Gentle winds with a dusty haze. NW Scotland can be cloudy and wet