Air masses are huge masses of air which are warm, cold, dry or moist depending on the nature of the land or sea they pass over. They cover vast areas, some of them are about 1,000,000 square km, nearly the same size as Egypt. Air masses move over the Earth's surface and help spread the Sun's heat around the world.

The main source regions are the high pressure belts in the subtropics, which produce tropical air masses, and around the poles, that are the source of polar air masses.

They are classified according to the area they originally came from, called their source.

Generalized Map of Global Air Masses

Continental Arctic (cA)- very cold; very dry

Continental Antarctic (cAA) - very cold; very dry

Continental Polar (cP) - cold & dry

Continental Tropical (cT) - warm & dry

Maritime Tropical (mT) - warm & moist

Maritime Equatorial (mE) - very warm; very moist

Maritime Polar (mP) - cool & moist

MARITIME (m) AIR MASSES

Air masses which form over seas and oceans are called maritime (m) air masses.

Maritime Tropical (mT) air mass  develops over warm seas. It is warm and moist. During the summer this air is brought up to the Baltic region by the semi-permanent high pressure system located near the Azores. The high is often referred to as the Azores high. This air is very moist as it originates over water and its track remains over water. As very little vertical lifting is required to form clouds due to the high moisture content, lower clouds called stratus clouds are common products over water and land. Sea fog and hazy conditions are also typical. Thunderstorms may develop if the air is pushed up mountain ranges. When this air mass is pushed upwards by a cold front heavy and frequent thunderstorms are the result.

Maritime Polar (mP) air mass forms over the sea near the poles. It is cold and moist. Air originating from the Polar region which has a long Atlantic path modifies or changes somewhat due to the time spent over water. When this air reaches Ireland it replaces the old continental air there rising the surface temperature sufficiently enough in the winter to thaw the ground. When this air hits the Baltic land areas and an ice cover sea convective activity or the chance of showers is very doubtful as the air is cooled by the land and ice on the sea making the air more stable (air doesn't rise). When this air moves over an non-ice covered Baltic Sea in the winter the relatively warmer waters of the sea make the air rise and give way to an increased chance of showers.

Maritime Polar Air is the most frequently received cold air mass to reach the Baltic region in the summer, and is the coldest when it arrives from a strong low over Scandinavia. Over the North Sea this air mass produces vigorous shower activity as it moves through. When the air moves over sun heated land heavy showers, sometimes thunderstorms are produced.

CONTINENTAL ( c ) AIR MASSES

Air mases form over land.

Continental Tropical (cT) air mass develops over hot, dry land. It is warm and dry.  Tropical continental air usually comes with south-easterly or southerly airstreams. It originates in North Africa and often travels over the Mediterranean Sea, Spain and France.

Continental Polar (cP) air mass develops over land near the poles. It is cold and dry. Air modified or changed by its long path across Central Europe is called Continental Polar. This is because the content of the air has changed so much that it does not show the properties of its originating area. Thus the air mass is renamed. During the summer this air mass is responsible for clear skies and good visibility along with cool nights and mornings. Although the temperatures feel cool, by midday it feels relatively warm.

ARCTIC AIR MASSES

 Arctic air masses are exceedingly cold, with the Arctic Ocean as their source region. Such air masses should not be confused with polar air masses.

Maritime Arctic  (mA): This air mass originates over the Arctic ice and is brought across the British Isles and surrounding waters by the air currents behind strong low pressure system over Northern Europe. This happens frequently during the winter and often result in cold air being brought as far south as Spain and the Western Mediterranean.

Continental Arctic  (cA): This air mass rarely moves into the Baltic sea, but during the winter when it does it tends to last for a long time. The air is brought southward from north of the Barent Sea near 71 degrees north, 50 degrees east by a strong high pressure which are located in the vicinity of Scandinavia. As the air travels southward, it moves over snow covered ground and continues to cool through the winter darkness of Northern Siberia with temperatures falling to minus 70 degrees Celsius. The moving does stir the air around so when this easterly breeze moves over the Baltic region the temperatures do not commonly fall this low.

Tropical air is moved out of the Mediterranean, usually by a low straddling the Atlas Mountains and Spain. As the air is lifted and cooled, most of the moisture is lost. When the air reaches the Baltic Sea, on occasion it is dry, warm and cloudless apart from some patches of mid level cloudiness.

Tropical continental air usually comes with south-easterly or southerly airstreams. It originates in North Africa and often travels over the Mediterranean Sea, Spain and France before reaching the British Isles. In summer, even easterly winds from central Europe or the Ukraine could be included in this category, as the continent becomes so hot at this time of year. The air picks up some moisture over the Mediterranean (and perhaps the Bay of Biscay), but overall the air tends to be quite dry and the skies are typically cloudless.

Strictly speaking, an air mass cooled from below on its northward journey should be stable. Sometimes, however, moisture may have found its way to medium levels in the atmosphere. Then, if there is a layer of unstable air and a trigger to set off convection, altocumulus castellanus clouds can develop, looking like turrets. These are often the forerunner to tremendous thunderstorms, which can occur by day or night.