For many people who live in the middle latitudes, which include much of the United States, summer heat waves and winter cold spells are familiar experiences. In the first instance, several days of high temperatures and oppressive humidity may finally end when a series of thunderstorms pass through the area, followed by a few days of relatively cool relief. By contrast, the clear skies that often accompany a span of frigid subzero days may be replaced by thick gray clouds and a period of snow as temperatures rise to levels that seem mild compared to those that existed just a day earlier. In both examples, a period of generally constant weather conditions followed by a relatively short period of change is experienced and then a new set of weather conditions is established that remains for perhaps several days before changing again.

What Is an Air Mass?

The weather patterns just described result from movements of large bodies of air, called air masses. An air mass, as the term implies, is an immense body of air, usually 1600 kilometers (1000 miles) or more across and perhaps several kilometers thick, characterized by a similarity of temperature and moisture across a given altitude. When an air mass moves out of its region of origin, it will carry its temperatures and moisture conditions with it, eventually affecting a large portion of a continent (▼).

An excellent example of the influence of an air mass is illustrated in ▼, which demonstrates a cold, dry mass from northern Canada moving southward. With a beginning temperature of -46°C (-51°F), the air mass warms to -33°C (-27°F) by the time it reaches Winnipeg. It continues to warm as it moves southward through the Great Plains and into Mexico. Throughout its southward journey, the air mass becomes warmer, but it also brings some of the coldest weather of the winter to the places in its path. Thus, the air mass is modified, but it also modifies the weather in the areas over which it moves.

The horizontal uniformity of an air mass is not perfect, of course. Because air masses extend over large areas, small differences occur in temperature and humidity from place to place. Still, the differences observed within an air mass are small compared to the rapid changes experienced across air-mass boundaries.

Because it may take several days for an air mass to move across an area and be modified, the region under its influence will likely experience fairly constant weather, a situation called air-mass weather. Certainly, there are some day-to-day variations, but the weather will be relatively consistent until an air-mass boundary comes through and a different air mass replaces it. The air-mass concept is an important one because it is closely related to the study of atmospheric disturbances, such as thunderstorms and tornadoes. Most atmospheric disturbances in the middle latitudes originate along the boundary zones that separate different air masses.

Source Regions

When a portion of the lower atmosphere moves slowly or stagnates over a relatively uniform surface, the air assumes the distinguishing features of that surface, particularly with regard to temperature and moisture conditions.

The area where an air mass acquires its characteristic properties of temperature and moisture is called its source region. The source regions that produce air masses influencing North America are shown in ▼.

Air masses are classified according to their source region. Polar air masses (P) and arctic air masses (A) originate in high latitudes toward Earth’s poles, whereas those that form in low latitudes are called tropical air masses (T). The designation polar, arctic, or tropical gives an indication of the temperature characteristics of an air mass. Polar and arctic indicate cold, and tropical indicates warm.

In addition, air masses are classified according to the nature of the surface in the source region. Continental air masses (c) form over land, and maritime air masses (m) originate over water. The designation continental or maritime thus suggests the moisture characteristics of the air mass. Continental air is likely to be dry, and maritime air is likely to be humid.

The basic types of air masses, according to this scheme of classification, are continental polar (cP) continental arctic (cA), continental tropical (cT), maritime polar (mP), and maritime tropical (mT).

Weather Associated with Air Masses

Continental polar and maritime tropical air masses influence the weather of North America the most, especially east of the Rocky Mountains. Continental polar air masses originate in northern Canada, interior Alaska, and the Arctic—areas that are uniformly cold and dry in winter and cool and dry in summer. In winter, an invasion of continental polar air brings the clear skies and cold temperatures we associate with a cold wave as it moves southward from Canada into the United States. In summer, this air mass may bring a few days of cooling relief.

Although cP air masses are not, as a rule, associated with heavy precipitation, those that cross the Great Lakes during late autumn and winter sometimes bring snow to the leeward shores. These localized storms often form when the surface weather map indicates no apparent cause for a snowstorm. These are known as lake-effect snows, and they make Buffalo and Rochester, New York, among the snowiest cities in the United States (▼).

What causes lake-effect snow? During late autumn and early winter, the temperature contrast between the lakes and adjacent land areas can be large. The temperature contrast can be especially great when a very cold cP air mass pushes southward across a lake that is not yet frozen over. The satellite image near the beginning of this page illustrates the process. Notice that as the cloud-free air moves across Lake Superior, clouds develop because the air acquires large quantities of heat and moisture from the relatively warm lake surface. By the time the cP air reaches the opposite shore, the air mass is humid and unstable, and heavy snow showers are occurring.

Maritime tropical air masses affecting North America most often originate over the warm waters of the Gulf of America (Gulf of Mexico), the Caribbean Sea, or the adjacent Atlantic Ocean. As you might expect, these air masses are warm, moisture-laden, and usually unstable. Maritime tropical air is the source of much, if not most, of the precipitation in the eastern two-thirds of the United States. In summer, when an mT air mass invades the central and eastern United States, and occasionally southern Canada, it brings the high temperatures and oppressive humidity typically associated with its source region.

Of the two remaining air masses, maritime polar and continental tropical, the latter has the least influence on the weather of North America. Hot, dry continental tropical air, originating in the Southwest and Mexico during the summer, only occasionally affects the weather outside its source region.

During the winter, maritime polar air masses coming from the North Pacific often originate as continental polar air masses in Siberia. The cold, dry cP air is transformed into relatively mild, humid, unstable mP air during its long journey across the North Pacific (▼). 

As this mP air arrives at the western shore of North America, it is often accompanied by low clouds and shower activity. When this air advances inland against the western mountains, orographic uplift produces heavy rain or snow on the windward slopes of the mountains. Maritime polar air also originates in the North Atlantic, off the coast of eastern Canada, and occasionally influences the weather of the northeastern United States. In winter, when New England is on the northern or northwestern side of a passing low-pressure center, the counterclockwise cyclonic winds draw in moist, maritime polar air. The result is a storm characterized by snow and cold temperatures, known locally as a nor’easter (▼).

• An air mass is a large body of air, usually 1600 kilometers (1000 miles) or more across, that is characterized by a sameness of temperature and moisture at any given altitude. When this air moves out of its region of origin, called the source region, it carries these temperatures and moisture conditions elsewhere, perhaps eventually affecting a large portion of a continent.

• Air masses are classified according to the nature of the surface in the source region and the latitude of the source region. Continental (c) designates an air mass of land origin, with the air likely to be dry; a maritime (m) air mass originates over water and, therefore, will be relatively humid. Polar (P) and arctic (A) air masses originate in high latitudes and are cold. Tropical (T) air masses form in low latitudes and are warm. According to this classification scheme, the four main types of air masses are continental polar (cP), continental tropical (cT), maritime polar (mP), and maritime tropical (mT).

• Continental polar (cP) and maritime tropical (mT) air masses influence the weather of North America the most, especially east of the Rocky Mountains. Maritime tropical air is the source of much, if not most, of the precipitation received in the eastern two-thirds of the United States.

• air mass: A large body of air that is characterized by relatively homogeneous temperature and humidity.

• air-mass weather: The conditions experienced in an area as an air mass passes over it. Because air masses are large and fairly homogenous, air-mass weather is fairly constant and may last for several days.

• arctic air mass (A): Designated by the letter “A,” a bitterly cold air mass that forms over the frozen Arctic Ocean.

• continental air mass (c): Designated by the letter “c,” an air mass that forms over land; it is normally relatively dry.

• lake-effect snow: Snow associated with a cP air mass to which moisture and heat are added from below as the air mass traverses a large and relatively warm lake (such as one of the Great Lakes), rendering the air mass humid and unstable.

• maritime air mass (m): Designated by the letter “m,” an air mass that originates over the ocean. These air masses are relatively humid.

• nor'easter: The term used to describe the weather associated with an incursion of mP air from the North Atlantic into the Northeast and Mid-Atlantic regions. These events include strong northeast winds, freezing or near-freezing temperatures, and precipitation in the form of rain or snow.

• polar air mass (P): Designated by the letter “P,” a cold air mass that forms in a high-latitude source region.

• source region: The area where an air mass acquires its characteristic properties of temperature and moisture.

• tropical air mass (T): Designated by the letter “T,” a warm-to-hot air mass that forms in the subtropics.