FRONTAL ELEMENTS 

From our previous discussion and definitions of fronts, it was implied that a certain geometrical and meteorological consistency must exist between fronts at adjoining levels. It can also be inferred that the data at no one particular level is sufficient to locate a front with certainty in every case. We must consider the horizontal and vertical distribu-tion of three weather elements (temperature, wind, and pressure) in a frontal zone.

Temperature

Typical fronts always consist of warm air above cold air. A radiosonde observation taken through a frontal surface often indicates a relatively narrow layer where the normal decrease of temperature with height is reversed. This temperature inversion is called a within the inversion layer and the thickness of the layer can be used as a rough indication of the intensity of a front. Strong fronts tend to have a distinct inversion; moderate fronts have isother-mal frontal zones; and weak fronts have a decrease in temperature through the frontal zone. Frontal zones are often difficult to locate on a sounding because air masses become modified after leaving their source region and because of turbulent mixing and falling precipitation through the frontal zone. Normally, however, some indica-tion does exist. The degree to which a frontal zone appears pronounced is proportional to the temperature difference between two air masses. The primary indication of a frontal zone on a Skew T diagram is a decrease in the lapse rate somewhere in the sounding below 400 mb. The decrease in lapse rate maybe a slightly less steep lapse rate for a stratum in a weak frontal zone to a very sharp inversion in strong fronts. In addition to a decrease in the lapse rate, there is usually an increase in moisture (a concurrent dew-point inversion) at the frontal zone. This is especially true when the front is strong and abundant cloudiness and precipitation accompany it. View A of figure 4-2-8 shows the height of the

Figure 4-2-7.Chart showing world air masses, fronts, and centers of major pressure systems in July.


Figure 4-2-8.Inversions.

inversion in two different parts of a frontal zone, and view B of figure 4-2-8 shows a strong frontal inversion with a consequent dew-point inversion. A cold front generally shows a stronger inver-sion than a warm front, and the inversion appears at successively higher levels as the front moves past a station. The reverse is true of warm fronts. Occluded fronts generally show a double inver-sion. However, as the occlusion process continues, mixing of the air masses takes place, and the inversions are wiped out or fuse into one inver-sion. It is very important in raob analysis not to con-fuse the subsidence inversion of polar and arctic air masses with frontal inversions. Extremely cold continental arctic air, for instance, has a strong inversion that extends to the 700-mb level. Sometimes it is difficult to find an inversion on a particular sounding, though it is known that a front intersects the column of air over a given station. This may be because of adiabatic warm-ing of the descending cold air just under the fron-tal surface or excessive local vertical mixing in the vicinity of the frontal zone. Under conditions of subsidence of the cold air beneath the frontal sur-face, the subsidence inversion within the cold air may be more marked than the frontal zone itself. Sometimes fronts on a raob sounding, which might show a strong inversion, often are accom-panied by little weather activity. This is because of subsidence in the warm air, which strengthens the inversion. The weather activity at a front in-creases only when there is a net upward vertical motion of the warm air mass.

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