An air mass is a huge, identifiable body of air possessing a relative homogeneity of temperature and moisture characteristics compared to the air surrounding it. Most air masses have dimensions of hundreds of thousands of square kilometers and are sometimes pushed thousands of kilometers from their source regions. Boundaries between air masses are known as fronts and the various types of fronts are described elsewhere.
Although air is a continuous fluid, its properties are such that it frequently organizes into air masses having differences substantial enough to cause significant weather changes as they pass. When the air of the lower troposphere passes over Earth’s surface, it exchanges energy and moisture characteristics with the surface. Air slowly moving large horizontal distances starts to take on the characteristics of the surface, be it tropical ocean or polar tundra. Some areas are well known as air mass source regions. Source regions occur in all latitudes and are characterized by non-mountainous terrain and frequent dominance of high pressure. High pressure is associated with light winds and this allows air to take on the surface temperature and moisture characteristics over the course of several days. As air masses move out of their source regions, they are modified. It is not rare for Arctic air to leave Siberia and travel over central North America to the Gulf Coast. While its original temperature might be −40°C, it might moderate temperatures slightly below freezing. This is a cold shock to the Gulf Coast but a testimony to air mass modification.
One well-appreciated instance of air mass modification is lake effect snow. In early winter the Great Lakes of the United States and Canada are not yet frozen over. Polar and Arctic air masses stream over the lakes. These cold air masses do not contain much moisture but have high relative humidities. As the air passes over a few tens of kilometers of the lake surface, the air mass gains water vapor and saturates. Downwind of the lakes, the arrival of the air over land initiates a small bit of lifting that cools the air lower than its dewpoint temperature. Condensation and the precipitation processes begin and make copious amounts of snow without the presence of a winter storm.
A simple air mass classification scheme considers the surface and latitude over which air passes. For instance, consider the differences in air types that can be generated over the great Antarctic ice sheets versus the tropical reaches of the Indian Ocean. Air masses having their origins in these places will provide vastly different weather as they progress over a location.
Maritime Tropical air (MT) is a product of tropical and subtropical oceans. It is warm at any time of the year and is associated with a high amount of humidity and latent heat. Its air is a prolific bringer of precipitation into the middle latitudes as it is transported away from its tropical origins. This air mass is usually unstable and it has numerous summer thunderstorms. Ironically, it is this air mass that provides most of the moisture falling as snow in the middle latitudes. The maritime Tropical air is drawn into middle-latitude cyclones and lifted and mixed with polar air.
Continental tropical air (cT) is a hot, dry air mass associated with the subtropical deserts of the world. It produces the hottest temperatures on the planet while not containing enough water vapor to produce significant precipitation. Record summertime temperatures are associated with the circulation of this air into the middle latitudes. Although this air mass is unstable in the first few hundred meters above the surface because of extreme heating of the surface, thought of as a whole, continental Tropical air has great stability because of the subsidence of air into the subtropical high.
Continental Polar air (cP) has source regions on the continents of the upper middle latitudes and, with the maritime Polar air mass, continental Polar is behind most of the cold fronts passing through the middle latitudes. This air is cold, dry, and usually stable. Because of these characteristics, continental Polar air is not associated with significant precipitation.
Maritime polar air (mP) has its source regions over the oceans of the upper middle latitudes. It is moister and milder than continental Polar air. Landfalling middle latitude cyclones fueled by maritime Polar air can bring prolific precipitation to mountainous coasts of the upper middle latitudes.
Arctic air (A) is the coldest air in the Northern Hemisphere. It is very cold and stable in the winter and not associated with significant precipitation. Because of the land/sea configuration around the Arctic Ocean, Arctic air originates in northern Canada, Alaska, and Siberia in the winter.
Antarctic air (AA) is the coldest air mass on Earth. In fact, it is much colder than the Arctic air of the Northern Hemisphere. It is generated on the Antarctic continent, which is covered by ice sheets having surfaces frequently in excess of two kilometers above sea level. Besides being cold, the air is very stable so organized low-pressure systems and their consequent precipitation are rare. Most middle latitude locations have considerably greater snowfall totals than Antarctica because of the inability of Antarctic air masses to hold much moisture.
Equatorial air (E) is an extremely humid air mass found in the lowest latitudes. Maritime and continental varieties are not usually designated because both types of sources produce similar temperatures and humidities. The temperatures tend not to be as hot as continental Tropical and maritime Tropical air masses because clouds and high humidities inhibit surface heating. Equatorial air masses are present year-round near the Equator with incursions of other air masses being extremely rare. Equatorial air becomes unstable by the middle of the afternoon and produces daily air mass thunderstorms for which the deep tropics are so well known.
The classification scheme can also include a third letter, a lowercase k, or w. A k in the air mass designation indicates the air mass is colder than the surface over which it passes while a w denotes the air mass is warmer than the surface. Examples of this would be cPk air as it passes over North America in the summer. The continental surface is warm, and the air is cool, and this sets up instability in the lower atmosphere. Conversely, a winter mPw air mass can be warmer than snow-covered North America and tend to be stable.
Climatologically, air masses have preferred areas of occurrence that enlarge, contract, and shift latitudes with seasons. Seasonally, the zones of dominance shift. For instance, the maritime Tropical source region strengthens, enlarges, and edges into the lower middle latitudes during the summer season. Also, there are geographic variations associated with the positioning of continents. Continental Tropical air vanishes from North America in winter because of the small size of the continental surface at those latitudes. In North Africa, this air mass persists year-round.
Some locations are associated with the monotony of weather caused by the firm entrenchment of air masses. If one alights in Bele´m, Brazil in the heart of tropical Amazonia, she/he is struck by the sameness of weather day after day as dictated by the presence of Equatorial air. In the middle latitudes, in particular, there are places that are neither air mass origin zones nor dominated by one air mass throughout the year. In these places, there is a hearty daily variation of the weather, especially in winter as the polar front jet stream forces the clash of tropical and polar air masses.
