Maritime Arctic Air (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.
When air is brought from the Arctic, it is called an Arctic outburst. Associated weather resulting from this type of air along with its track (the path that the air moves), is that of thunderstorms, snow showers and hail. Showers produced by this type of airmass follow so closely upon one another that at times, they develop into long periods of continuous precipitation, and are often accompanied by periods of very intense/rough weather (squalls). During the squalls, strong turbulence, poor visibility (how far one can see), and blinding sea spray can cause problems.
Provided that there are still large portions of the Baltic Sea that are unfrozen during the winter, similar conditions are produced on its southern shores by direct Arctic outbursts, and winds may reach storm force (48 kts or greater) in places affected by funneling, such as the channel between Malmohus, at the southern tip of Sweden and Rugen.
The worst effects of the Arctic outbursts are during the early stages, particularly in the north and east of the region. The air often reaching the Baltic having crossed the Norwegian Mountains (a part of the Kjolen Mountain Range), and on such occasions foehn effects produce a broad belt of clear skies which sometimes extends into Northern Germany.
During the summer, Arctic air is brought into the North Sea and the Baltic as a result of strong low pressure systems located to the east of the region. This is not uncommon during early summer, but become less frequent from June onwards. During the summer, this type of air (airmass) brings clear nights and cool mornings to the coast of the Baltic Sea. Strong daily heating can bring strong thunderstorms with gusty winds during daylight hours.
Maritime Polar Air (mP):
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.
Maritime Tropical Air (mT):
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.
Continental Arctic Air (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.
Continental Polar Air (cP):
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.
Mediterranean Air (Med):
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.