Air masses and fronts

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Air masses and fronts

An air mass is a body of air with a relatively constant temperature and moisture content over a significant altitude. Air masses typically cover hundreds, thousands, or millions of 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.

Source regions

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 Earth’s surface possess the topographical characteristics that favor development of air masses. The two most important of these characteristics are topographic regularity and atmospheric stability. Deserts, plains, and oceans typically cover large areas and contain 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 disturb the air mass. The absence of strong wind also favors the development of an air mass; hence, air masses cannot develop easily in regions where cyclonic or anticyclonic storms are common.

Classification

The system by which air masses are classified reflects the fact that certain locations 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, and 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 whereas the second two (equatorial and tropical) refer 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 the temperature and the humidity of an air mass.

One source region of arctic air masses is the northern-most portions of Alaska, Canada, and Greenland. Thus, air masses developing in this source region are designated as cA (continental arctic) air masses. Similarly, air masses developing over the Gulf of Mexico, a source region for maritime tropical air masses, are designated as mT (marine tropical) air masses.

Properties of air masses

The movement of air masses across 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 those 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 low 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 originate as cP air over Siberia, but are modified to mP masses as they move over the Pacific and increase their 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.

Fronts

The term “front” was suggested by the Bjerkneses because the collisions of two air masses reminded them of a battlefront during a military operation. Fronts develop when two air masses with different temperatures and, in most cases, different moisture contents come into contact with each other. The result depends on the relative temperature and moisture content of the two air masses and the relative movement of the two masses.

When a mass of cold air moving across Earth’s surface comes into contact with a warm air mass, the denser cold air mass may force its way under the lighter warm air mass. The boundary formed between these two air masses is a cold front. Cold fronts are generally accompanied by a decrease in atmospheric pressure and the development of large cumulonimbus clouds that bring rain and thunderstorms. Cold fronts are represented on weather maps as solid lines with solid triangles at regular distances along them. The direction in which the triangles point shows the direction in which the cold front is moving.

KEY TERMS

Anticyclonic— Referring to an area of high pressure around which winds blow in a clockwise direction in the northern hemisphere.

Continental— Referring to very large land masses.

Cyclonic— Referring to an area of low pressure around which winds blow in a counter-clockwise direction in the northern hemisphere.

Humidity— The amount of water vapor contained in the air.

Maritime— Referring to the oceans.

Topographical— Referring to the surface features of an area.

The opposite situation 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 comes into contact with 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 shown on weather maps as 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 complicated situation occurs when a cold front overtakes a slower-moving warm front to create 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 whereas the latter is a warm-type occluded front. 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 shown on a weather map with a solid line that contains, alternatively, both triangles and half circles on the same side of the line.

In some instances, a collision occurs in which neither mass is strong enough to displace the other and 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 generally accompanied by fair, clear weather, although some light precipitation may occur.