An air mass is an extensive body of air that has a relatively homogeneous temperature and moisture content over a significant altitude. Air masses typically cover areas of a few hundred, thousand, or million square kilometers. A front is the boundary at which two air masses of different temperature and moisture content meet. The role of air masses and fronts in the development of weather systems was first appreciated by the Norwegian father and son team of Vilhelm and Jacob Bjerknes in the 1920s. Today, these two phenomena are still studied intensively as predictors of future weather patterns.
Air masses form when a body of air comes to rest over an area large enough for it to take on the temperature and humidity of the land or water below it. Certain locations on the earth's surface possess the topographical characteristics that favor the development of air masses. The two most important of these characteristics are topographic regularity and atmospheric stability. Deserts, plains, and oceans typically cover very wide areas with relatively few topographical irregularities. In such regions, large masses of air can accumulate without being broken apart by mountains, land/water interfaces, and other features that would break up the air mass.
The absence of consistent wind movements also favors the development of an air mass. In regions where cyclonic or anticyclonic storms are common, air masses obviously cannot develop easily.
The system by which air masses are classified reflects the fact that certain locations on the planet possess the topographic and atmospheric conditions that favor air mass development. That system uses two letters to designate an air mass. One letter, written in upper case, indicates the approximate latitude (and, therefore, temperature) of the region: A for arctic; P for polar; E for equatorial; T for tropical. The distinctions between arctic and polar on the one hand, and equatorial and tropical on the other are relatively modest. The first two terms (arctic and polar) refer to cold air masses, and the second two (equatorial and tropical) to warm air masses.
A second letter, written in lower case, indicates whether the air mass forms over land or sea and, hence, the relative amount of moisture in the mass. The two designations are c for continental (land) air mass and m for maritime (water) air mass.
The two letters are then combined to designate both temperature and humidity of an air mass. One source region of arctic air masses, for example, is the northern-most latitudes of Alaska, upper Canada, and Greenland. Thus, air masses developing in this source region are designated as cA (cold, land) air masses. Similarly, air masses developing over the Gulf of Mexico, a source region for maritime tropical air masses, are designated as mT (warm, water) air masses.
The movement of air masses across the earth's surface is an important component of the weather that develops in an area. For example, weather patterns in North America are largely dominated by the movement of about a half dozen air masses that travel across the continent on a regular basis. Two of these air masses are the cP and cA systems that originate in Alaska and central Canada and sweep down over the northern United States during the winter months. These air masses bring with them cold temperatures, strong winds, and heavy precipitation, such as the snowstorms commonly experienced in the Great Lakes states and New England. The name "Siberian Express" is sometimes used to describe some of the most severe storms originating from these cP and cA air masses.
From the south, mT air masses based in the Gulf of Mexico, the Caribbean, and western Atlantic Ocean move northward across the southern states, bringing hot, humid weather that is often accompanied by thunderstorms in the summer.
Weather along the western coast of North America is strongly influenced by mP air masses that flow across the region from the north Pacific Ocean. These masses actually originate as cP air over Siberia, but are modified to mP masses as they move over the broad expanse of the Pacific, where they often pick up moisture. When an mP mass strikes the west coast of North America, it releases its moisture in the form of showers and, in northern regions, snow.
The term front was suggested by the Bjerkneses because the collisions of two air masses reminded them of a battlefront during a military operation. That collision often results in warlike weather phenomena between the two air masses.
Fronts develop when two air masses with different temperatures and, usually, different moisture content come into contact with each other. When that happens, the two bodies of air act almost as if they are made of two different materials, such as oil and water. Imagine what happens, for example, when oil is dribbled into a glass of water. The oil seems to push the water out of its way and, in return, the water pushes back on the oil. A similar shoving match takes place between warm and cold air masses along a front. The exact nature of that shoving match depends on the relative temperature and moisture content of the two air masses and the relative movement of the two masses.
One possible situation is that in which a mass of cold air moving across the earth's surface comes into contact with a warm air mass. When that happens, the cold air mass may force its way under the warm air mass like a snow shovel wedging its way under a pile of snow. The cold air moves under the warm air because the former is denser. The boundary formed between these two air masses is a cold front.
Cold fronts are usually accompanied by a falling barometer and the development of large cumulonimbus clouds that bring rain showers and thunderstorms. During the warmer seasons, the clouds form as moisture-rich air inside the warm air mass, which is cooled as it rises; water subsequently condenses out as precipitation. Cold fronts are represented on weather maps by means of solid lines that contain solid triangles at regular distances along them. The direction in which the triangles point shows the direction in which the cold front is moving.
A situation opposite to the preceding is one in which a warm air mass approaches and then slides up and over a cold air mass. The boundary formed in this case is a warm front. As the warm air mass meets the cold air mass, it is cooled and some of the moisture held within it condenses to form clouds. In most cases, the first clouds to appear are high cirrus clouds, followed sometime later by stratus and nimbostratus clouds.
Warm fronts are designated on weather maps by means of solid lines to which are attached solid half circles. The direction in which the half circles point shows the direction in which the warm front is moving.
A more complex type of front is one in which a cold front overtakes a slower-moving warm front. When that happens, the cold air mass behind the cold front eventually catches up and comes into contact with the cold air mass underneath the warm front. The boundary between these two cold air masses is an occluded front. A distinction can be made depending on whether the approaching cold air mass is colder or warmer than the second air mass beneath the warm front. The former is called a cold-type occluded front, while the latter is a warm-type occluded front. Once again, the development of an occluded front is accompanied by the formation of clouds and, in most cases, by steady and moderate precipitation. An occluded front is represented on a weather map by means of a solid line that contains, alternatively, both triangles and half circles on the same side of the line.
In some instances, the collision of two air masses results in a stand-off. Neither mass is strong enough to displace the other, and essentially no movement occurs. The boundary between the air masses in this case is known as a stationary air mass and is designated on a weather map by a solid line with triangles and half circles on opposite sides of the line. Stationary fronts are often accompanied by fair, clear weather, although some light precipitation may occur.
See also Atmospheric circulation; Atmospheric composition and structure; Atmospheric pressure; Clouds and cloud types; Weather forecasting methods; Weather forecasting
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