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Frontogenesis is the formation of a new front or the regeneration of an old one. Frontogenesis takes place only when two conditions are met. First, two air masses of different densities must

Table 4-2-1.—Numerical Characteristics of the Life Cycle of an Unstable Wave Cyclone

Figure 4-2-2.—Life cycle of a stable wave cyclone.

Figure 4-2-3.—Life cycle of an unstable frontal wave.

exist adjacent to one another; and second, a prevailing wind field must exist to bring them together.

There are three basic situations which are con-ducive to frontogenesis and satisfy the two basic requirements. They are as follows:

. The windflow is cross-isothermal and flow-ing from cold air to warmer air. The flow must be cross-isothermal, resulting in a concentration of isotherms (increased temperature gradient). The flow does not have to be perpendicular; however, the more perpendicular the cross-isothermal flow, the greater the intensity of frontogenesis.

. The winds of opposite air masses move toward the same point or line in that cross-isothermal flow. A classic example of this situa-tion is the polar front where cold polar air moves southward toward warmer temperatures and warm tropical air moves northward toward colder temperatures.

. The wind flow has formed a deformation field. A deformation field consists basically of an area of flat pressure between two opposing highs and two opposing lows (also called a COL or sad-dle).

It has two axes that have their origin at a neutral point in the COL (view A in fig. 4-2-4). The y axis, or axis of contraction, lies between the high and low that bring the air particles toward the neutral point. (Note the flow arrows in fig. 4-2-4.) The x axis lies between the high and low that take air particles away from the neutral point and is known as the axis of dilation. 

The distribution and concentration of isotherms T1 through T6 in this deformation field determine whether frontogenesis results. If the isotherms form a large angle with the axis of con-traction, frontogenesis results. If a small angle ex-ists, frontolysis (the dissipation of a front) results. It has been shown that in a perpendicular defor-mation field, isotherms must form an angle of 45° or less with the axis of dilation for frontogenesis to occur as shown in views A and B of the figure. In a deformation field not perpendicular, the critical angle changes correspondingly as il-lustrated in views A and B of figure 4-2-5. In most cases, frontogenesis occurs along the axis of dilation. AT ANY RATE, FRONTOGENESIS OCCURS WHERE THERE IS A CONCEN-TRATION OF ISOTHERMS WITH THE CIRCULATION TO SUSTAIN THAT CON-CENTRATION.

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