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Air Masses and Fronts CHAPTER 4 AIR MASSES AND FRONTS Temperature, in the form of heating and cooling, contrasts and produces a homogeneous mass of air. The plays a key roll in our atmosphere’s circulation. energy supplied to Earth’s surface from the Sun is Heating and cooling is also the key in the formation of distributed to the air mass by convection, radiation, and various air masses. These air masses, because of conduction. temperature contrast, ultimately result in the formation Another condition necessary for air mass formation of frontal systems. The air masses and frontal systems, is equilibrium between ground and air. This is however, could not move significantly without the established by a combination of the following interplay of low-pressure systems (cyclones). processes: (1) turbulent-convective transport of heat Some regions of Earth have weak pressure upward into the higher levels of the air; (2) cooling of gradients at times that allow for little air movement. air by radiation loss of heat; and (3) transport of heat by Therefore, the air lying over these regions eventually evaporation and condensation processes. takes on the certain characteristics of temperature and The fastest and most effective process involved in moisture normal to that region. Ultimately, air masses establishing equilibrium is the turbulent-convective with these specific characteristics (warm, cold, moist, transport of heat upwards. The slowest and least or dry) develop. Because of the existence of cyclones effective process is radiation. and other factors aloft, these air masses are eventually subject to some movement that forces them together. During radiation and turbulent-convective When these air masses are forced together, fronts processes, evaporation and condensation contribute in develop between them. The fronts are then brought conserving the heat of the overlying air. This occurs together by the cyclones and airflow aloft. This because the water vapor in the air allows radiation only produces the classic complex frontal systems often seen through transparent bands during radiational cooling on surface weather maps. and allows for the release of the latent heat of condensation during the turbulent-convective processes. Therefore, the tropical latitudes, because of AIR MASSES a higher moisture content in the air, rapidly form air LEARNING OBJECTIVE: Determine the masses primarily through the upward transport of heat conditions necessary for the formation of air by the turbulent-convective process. The dryer polar masses and identify air mass source regions. regions slowly form air masses primarily because of the An air mass is a body of air extending over a large loss of heat through radiation. Since underlying area (usually 1,000 miles or more across). It is surfaces are not uniform in thermal properties during generally an area of high pressure that stagnates for the year and the distribution of land and water is several days where surface terrain varies little. During unequal, specific or special summer and/or winter air this time, the air mass takes on characteristics of the masses may be formed. The rate of air mass formation underlying surface. Properties of temperature, moisture varies more with the intensity of insolation. (humidity), and lapse rate remain fairly homogeneous EFFECTS OF CIRCULATION ON ALL AIR throughout the air mass. Horizontal changes of these MASS FORMATION properties are usually very gradual. There are three types of circulation over Earth. CONDITIONS NECESSARY FOR AIR MASS However, not all of these are favorable for air mass FORMATION development. They are as follows: Two primary factors are necessary to produce an air 1. The anticyclonic systems. Anticyclonic mass. First, a surface whose properties, essentially systems have stagnant or slow-moving air, which temperature and moisture, are relatively uniform (it allows time for air to adjust its heat and moisture may be water, land, or a snow-covered area). Second, a content to that of the underlying surface. These large divergent flow that tends to destroy temperature 4-1 anticyclones have a divergent airflow that spreads the Newfoundland. These two areas act as source regions properties horizontally over a large area; turbulence and for maritime polar air. convection distribute these properties vertically. Subsidence (downward motion), another property of AIR MASS SOURCE REGIONS anticyclones, is favorable for lateral mixing, which results in horizontal or layer homogeneity. The ideal condition for the production of an air mass is the stagnation of air over a uniform surface Warm highs, such as the Bermuda and Pacific (water, land, or ice cap) of uniform temperature and highs, extend to great heights because of a lesser humidity. The length of time an air mass stagnates over density gradient aloft and thereby produce an air mass its source region depends upon the surrounding of relatively great vertical extent. Cold highs, such as pressures. From the surface up through the upper levels, the Siberian high, are of moderate or shallow vertical such air acquires definite properties and characteristics. extent and produce air masses of moderate or shallow The resulting air mass becomes virtually homogeneous height. throughout, and its properties become uniform at each 2. Cyclonic systems. Cyclonic systems are not level. In the middle latitudes, the land and sea areas conducive to air mass formation because they are with the associated steep latitudinal temperature characterized by greater wind speeds than anticyclonic gradient are generally not homogeneous enough for systems. These wind speeds prevent cyclonic systems source regions. These areas act as transitional zones for from stabilizing. An exception is the stationary heat air masses after they have left their source regions. low. The source regions for the world’s air masses are 3. Belts of convergence. Belts of convergence are shown in figure 4-1. Note the uniformity of the normally not conducive to air mass formation since underlying surfaces; also note the relatively uniform they have essentially the same properties as cyclonic climatic conditions in the various source regions, such systems. However, there are two areas of convergence as the southern North Atlantic and Pacific Oceans for where air masses do form. These are the areas over the maritime tropical air and the deep interiors of North north Pacific, between Siberia and North America, and America and Asia for continental polar air. the Atlantic, off the coast of Labrador and A ARCTIC AIR MA WINTER MASSES SIBERIAN 60 CP CP CP SOURCE SOURCE SOURCE MP MP SOURCE SOURCE 40 CT SOURCE NUMBER MT MT CT 20 MT MT SOURCE SOURCE SOURCE CT SOURCE MT 0 EQUATORIAL AIR CT 20 MT CT SOURCE MT MT SOURCE SOURCE MT SOURCE 40 MP MP SOURCE MP SOURCE SOURCE 100 120 140 160 180 160 140 120 100 80 60 40 20 0 20 40 60 80 100 AG5f0401 Figure 4-1.—Air mass source regions. 4-2 Characteristics of Air Masses significant land areas lying in the tropical regions; generally these tropical regions are located between The characteristics of an air mass are acquired in latitudes 25°N and 25°S. The large land areas located in the source region, which is the surface area over which these latitudes are usually desert regions (such as the the air mass originates. The ideal source region has a Sahara or Kalahari Deserts of Africa, the Arabian uniform surface (all land or all water), a uniform Desert, and the interior of Australia). The air over these temperature, and is an area in which air stagnates to land areas is hot and dry. form high-pressure systems. The properties MARITIME TROPICAL (mT) AIR.—The (temperature and moisture content) an air mass maritime tropical source regions are the large zones of acquires in its source region are dependent upon a open tropical sea along the belt of the subtropical number of factors—the time of year (winter or anticyclones. High-pressure cells stagnate in these summer), the nature of the underlying surface (whether areas most of the year. The air is warm because of the land, water, or ice covered), and the length of time it low latitude and can hold considerable moisture. remains over its source region. EQUATORIAL (E) AIR.—The equatorial source ARCTIC (A) AIR.—There is a permanent region is the area from about latitudes 10°N to 10°S. It high-pressure area in the vicinity of the North Pole. In is essentially an oceanic belt that is extremely warm this region, a gentle flow of air over the polar ice fields and that has a high moisture content. Convergence of allows an arctic air mass to form. This air mass is the trade winds from both hemispheres and the intense characteristically dry aloft and very cold and stable in insolation over this region causes lifting of the unstable, the lower altitudes. moist air to high levels. The weather associated with ANTARCTIC (A) AIR.—Antarctica is a great these conditions is characterized by thunderstorms source region for intensely cold air masses that have throughout the year. continental characteristics. Before the antarctic air SUPERIOR (S) AIR.—Superior air is a high-level reaches other land areas, it becomes modified and is air mass found over the south central United States. properly called maritime polar. The temperatures are This air mass occasionally reaches the surface; because colder than in the arctic regions. Results of Operation of subsidence effects, it is the warmest air mass on Deepfreeze have revealed the coldest surface record in the North American continent in both seasons. temperatures in the world to be in the Antarctic. CONTINENTAL POLAR (cP) AIR.—The Southern Hemisphere Air Masses continental polar source regions consist of all land areas dominated by the Canadian and Siberian Air masses encountered in the Southern high-pressure cells. In the winter, these regions are Hemisphere differ little from their counterparts in the covered by snow and ice.
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