What is an air mass?
The air mass is a large volume of air, the horizontal dimensions of which are often up to several thousand, and vertical to several kilometers, characterized by a relatively high uniformity in terms of physical characteristics. By adopting the thermal criterion, there is a distinction between the warm air mass and the cold (cold) air mass. From the point of view of the geographical location, Arctic (Antarctic), polar, tropical and equatorial air are separated, which separate the so-called main fronts (climatological fronts). In terms of humidity content, we divide air mass into marine and continental ones.
Types of air masses
The following basic types of air masses are distinguished:
Equatorial air – the mass of air that forms above the equatorial zone due to strong heating from the bottom. It is in a state of unstable equilibrium, after further heating it is easily convective, which causes heavy rainfall. It is moist air. In this zone, land air is not much different from the sea.
Tropical air masses (T) – the mass of air that is formed in the tropical zone or – in summer – in the subtropical zone. It usually has a dry adiabatic layer, and over the seas, especially cold sea currents, there is a thermal inversion due to the strong cooling from the ground. On the other hand, the tropical land air has a low relative humidity in all its mass. The oceanic sea air is moist in the lower layer, but above it is as dry as the continental one. After a long stay at the sea, the height of the moist and at the same time inversion layer increases, the air becomes convective enough to create precipitation.
Polar air masses (P) – the mass of air generated in the temperate zone (despite the name) from the mixing of tropical air with the Antarctic or Arctic. The polar air of the sea is generally unstable and humid, in the summer relatively cold, in winter – warm. In the winter, continental polar air exhibits a constant equilibrium due to the strong cooling from the ground. In the spring, when the entire air column is quite cool and the bottom is strongly heated, it creates a shaky balance. In the summer the state of constant or relative equilibrium again, but then the air is quite dry in all its mass.
Arctic air (A) – air mass formed in polar zones. It is air in steady state, constantly chilled from the bottom, cold. Due to the covering of sea waters in this zone with an ice cover, the Arctic sea air is not much different from the terrestrial one. In the southern hemisphere, there are no sea areas in this zone.
Narrow transition zones separating air masses with different thermal and moisture properties are weather fronts.