RELATION OF FRONTS TO AIR MASSES 

At this point you should have figured out that without air masses there would be no fronts. The centers of action are responsible for bringing the air masses together and forming frontal zones.

The primary frontal zones of the Northern Hemisphere are the arctic frontal zone and the polar frontal zone. The most important frontal zone affecting the United States is the polar front. The polar front is the region of transition between the cold polar air and warm tropical air. During the winter months (in the Northern Hemisphere), the polar front pushes farther southward, because of the greater density of the polar air, than during the summer months. During the summer months (in the Northern Hemisphere), the polar front seldom moves farther south than the central United States.

On a surface map a front is indicated by a line separating two air masses; this is only a picture of the surface conditions. These air masses and fronts extend vertically. (See fig. 4-2-1.)

A cold air mass, being heavier, acts like a wedge and tends to underrun a warm air mass. Thus, the cold air is below and the warm air is above the surface of discontinuity. This wedge of cold air produces a slope of the frontal surface. This slope is usually be-tween 1 to 50 (1 mile vertical for 50 miles horizontal) for a cold front and 1 to 300 (1 mile vertical for 300 miles horizontal) for a warm front. For example, 100 miles from the place where the frontal surface meets the ground, the frontal surface might be some-where between 2,000 feet and 10,000 feet above Earths surface, depending on the slope. The slope of a front is of considerable impor-tance in visualizing and understanding the weather along the front.

There is a systemic relationship between cyclones and fronts, in that the cyclones are usually associated with waves along fronts primarily cold fronts. Cyclones come into being or intensify because pressure falls more rapidly at one point than it does in the surrounding area. Cyclogenesis can occur anywhere, but in middle and high latitudes, it is most likely to occur on

Figure 4-2-1.Vertical view of a frontal system (clouds not shown).

a frontal trough. When a cyclone (or simply low) develops on a front, the cyclogenesis begins at the surface and develops gradually upward as the cyclone deepens. The reverse also occurs; closed circulations aloft sometime work their way downward until they appear on the surface chart. These cyclones rarely contain fronts and are quasi-stationary or drift slowly westward and/or equatorward.

Every front, however, is associated with a cyclone. Fronts move with the counterclockwise flow associated with Northern Hemisphere cyclones and clockwise with the flow of Southern Hemisphere cyclones. The middle latitudes are regions where cold and warm air masses con-tinually interact with each other. This interaction coincides with the location of the polar front. When the polar front moves southward, it is usually associated with the development and movement of cyclones and with outbreaks of cold polar air. The cyclonic circulation associated with the polar front tends to bring polar air southward and warm moist tropical air northward. During the winter months, the warm airflow usually occurs over water and the cold air moves southward over continental areas. In summer the situation is reversed. Large cyclones that form on the polar front are usually followed by smaller cyclones and are referred to as families. These smaller cyclones tend to carry the front farther southward. In an ideal situation these cyclones come in succession, causing the front (in the Northern Hemisphere) to lie in a southwest to northeast direction.

Every moving cyclone usually has two signifi-cant lines of convergence distinguished by ther-mal properties. The discontinuity line on the forward side of the cyclone where warm air replaces cold air is the warm front; the discon-tinuity line in the rear portion of the cyclone where cold air displaces warm air is the cold front. The polar front is subject to cyclonic develop-ment along it. When wind, temperature, pressure, and upper level influences are right, waves form along the polar front. Wave cyclones normally progress along the polar front with an eastward component at an average rate of 25 to 30 knots, although 50 knots is not impossible, especially in the case of stable waves. These waves may ultimately develop into full-blown low-pressure systems with gale force winds. The development of a significant cyclone along the polar front depends on whether the initial wave is stable or unstable. Wave formation is more likely to occur on slowly moving or stationary fronts like the polar front than on rapidly moving fronts. Certain areas are preferred localities for wave cyclogenesis. The Rockies, the Ozarks, and the Appalachians are examples in North America.

A stable wave is one that neither develops nor occludes, but appears to remain in about the same state. Stable waves usually have small amplitude, weak low centers, and a fairly regular rate and direction of movement. The development of a stable wave is shown in views A, B, and C of figure 4-2-2. Stable waves do not go into a growth and occlusion stage.

The unstable wave is by far the more common wave that is experienced with development along the polar front. The amplitude of this wave increases with time until the occlusion process occurs. The formation of a deep cyclone and an occluded front breaks up the polar front. When the occlusion process is complete, the polar front is reestablished. This process is shown in figure 4-2-3. Views A through G of figure 4-2-3, refer-red to in the next three paragraphs, show the life cycle of the unstable wave.

In its initial stage of development, the polar front separates the polar easterlies from the mid-latitude westerlies (view A); the small disturbance caused by the steady state of the wind is often not obvious on the weather map. Uneven local heating, irregular terrain, or wind shear between the opposing air currents may start a wavelike per-turbation on the front (view B); if this tendency persists and the wave increases in amplitude, a counterclockwise (cyclonic) circulation is set up. One section of the front begins to move as a warm front while the adjacent sections begin to move as a cold front (view C). This deformation is called a frontal wave.

The pressure at the peak of the frontal wave falls, and a low-pressure center is formed. The cyclonic circulation becomes stronger; the wind components are now strong enough to move the fronts; the westerlies turn to southwest winds and push the eastern part of the front northward as a warm front; and the easterlies on the western side turn to northerly winds and push the western part southward as a cold front. The cold front is moving faster than the warm front (view D). When the cold front overtakes the warm front and closes the warm sector, an occlusion is formed (view E). This is the time of maximum intensity of the wave cyclone.

As the occlusion continues to extend outward, the cyclonic circulation diminishes in intensity (the low-pressure area weakens), and the frontal move-ment slows down (view F). Sometimes a new frontal wave may begin to form on the westward trailing portion of the cold front. In the final stage, the two fronts become a single stationary front again. The low center with its remnant of the occlusion has disappeared (view G).

Table 4-2-1 shows the numerical average life cycle of a typical unstable wave cyclone from initial development to cyclolysis. It is only intended to be used as a guide in areas where reports are sparse.

Learning Objective: Describe the condi-tions necessary for frontogenesis and frontolysis, and identify the world fronto-genetical zones.

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