Air masses and their sources

Air masses bring variety of weather. Six basic types of air masses affect the weather of the British Isles. They can bring anything from tropical warm and humid days to arctic cold depending on the type of air mass. Fronts form the boundaries of air masses with differing properties. The most severe weather usually occurs when dry-cold continental polar air clashes with warm-humid maritime tropical air.

picture The term 'air mass' was introduced some 70 years ago by Norwegian meteorologists from Bergen, Norway. Air mass is a large body of air, whose properties - temperature, humidity and lapse rate - are largely homogeneous over an area several hundred kilometres across.

The nature of air masses is determined by three factors: the source region, the age and the modifications that may occur as they move away from their source region across the earth's surface.

The primary classification of air masses is based on the characteristics of the source region, giving Arctic (A), Polar (P) or Tropical air (T), and on the nature of the surface in the source region: continental (c) or maritime (m). In addition, a large variety of secondary types of air masses are defined. For example, equatorial air (E) or Mediterranean air. Sometimes there is a letter (k) or (w) attached to the two-letter initials indicating whether the air is wa rmer or colder than the surface. The former becomes more stable, and the latter more unstable.

Some older works use the term of an 'returning air mass'. This usually refers to maritime polar air that has been altered moving across the relatively mild Atlantic and is returning polewards eventually.

The air masses prevalent for the British Isles are:

Arctic Air (A):

Continental arctic (cA): Extremely cold temperatures and very little moisture. Originating over the Arctic ocean in winter, when high pressure dominates and differs only slightly from the continental polar (cP) air masses that develop over Siberia and northern Canada.
Maritime arctic (mA): From the same source region, but less dry and less cold - in a way less extreme.

Polar Air (P):

Continental polar (cP): Cold and dry, originating from high latitudes, typically as air flowing out of the polar highs. This air mass often brings the rattleing cold, dry and clear weather on a perfect winter day and also dry and warm weather on a pleasant day in summer.
Maritime polar (mP): Cool and moist, often originating as continental polar air over the North American and Asian land masses and is modified as it moves out over the Atlantic and Pacific Oceans. Heated by the relatively warm water bodies this air mass becomes rather unstable resulting in blustery showers over the sea and windward coasts.

Tropical Air (T):

Continental tropical (cT): Hot and very dry, orginating from the arid and desert regions during summer. The least common air mass to affect the British weather. However, approaching from the south and south-east in summer it can bring record heat to southeast Britain, particulary in late-summer.
Maritime tropical (mT). Mild and damp in winter, very warm and muggy during summer. Originating from the Azores this air mass approaches the British Isles from the west, leading to overcast skies with prolonged rain for the western half of the country and to many clouds, sometimes broken with drizzle for the eastern half. In summer tropical maritime air often produces warm weather with abundant sunshine in the southeast.

Add to social bookmarking:  | more |
A is for Air
Accessory clouds
Air masses and their sources
Air-mass Thunderstorm
Alpine Glow
Atmosphere - Diagram
Aurorae - Northern Lights
Average rainfall over England and Wales
Azores High
Banner Cloud - the peak's flag
Beaufort Scale
British Weather Terms
Brocken Spectre
Bubble High
Burning Time
CAPE - Convective Available Potential Energy
Cap cloud
Cc floccus
Cc lacunosus
Cc stratiformis
Cc undulatus
Central England Temperature
Centres of action
Ci fibratus
Ci radiatus
Ci spissatus
Ci uncinus
Ci vertebratus
Clocks go Back from BST to GMT
Cloud classification
Cloud seeding
Cloud species
Cloud streets
Cloud types (genera)
Cloud variety
Clouds - sentry of the sky
Cold low
Comma Cloud
Comma Cloud
Coriolis effect
Crepuscular rays
Cut-off low
Dew Point
Discovery of the Jet Stream
Doppler radar
Drifting snow - blowing snow
Earth's Atmosphere
Easterly wave - the Hurricane's cradle
El Nino
Föhn (foehn) wind
Föhn wall
Flash Flood
Fog and Mist
Forecasting weather
Frost hollow
Fujita Scale Statistics
Fujita Tornado Scale
Funnel cloud
Genoa Low
Geostationary Satellites
Geostrophic Wind
Glaze and Black Ice
Grass Minimum Temperature
Hailstorms in Britain
Highs and Lows and Winds
History of Hurricane Names
Hoar Frost
Isobars on surface maps
Jack Frost
Jet stream cirrus from space
Katabatic winds
Key to our weather symbols
Lake-effect snow
Latent Heat
Levanter cloud
Millibar and hectopascal
NOAA satellites
North Atlantic Drift (Gulf Stream)
Polar Orbiting Satellites
Polar low - the arctic hurricane
Precipitation Map
Rain gauge
Roll cloud
Rotor cloud
Saffir-Simpson scales
Sc duplicatus
Sc perlucidus
Sc undulatus
Shelf Cloud
Sometimes a bit fishy
Southern Oscillation
St. Swithun's Day
Standard Reference Period
Stevenson Screen
Sun pillar
Supercooled clouds
Surface wind
Thunderstorm Probability
Tornado Alley
Troposphere - Diagram
UV Index
Ultraviolet radiation
Virga or Fallstreak
What Makes Northern Lights Happen?
What does it mean?
Why Skies are Blue
Why Thunder Rumbles
World Meteorological Organisation (WMO)