Vertical structure of secondary circulations (pressure centers)

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VERTICAL STRUCTURE OF SECONDARY CIRCULATIONS (PRESSURE CENTERS)

To better understand the nature of the pressure centers of the secondary circulation, it is necessary to consider them from a three-dimensional standpoint. With the aid of surface and upper air charts (covered in Units 7 and 8), you will be able to see the three dimensions of these pressure systems as well as the circulation patterns of the secondary circulation as estab-lished at higher levels in the troposphere and lower stratosphere.

In Unit 2, the study of gas laws showed that volume is directly proportional to temperature. Stated another way, we might say that the thickness of a layer between two isobaric surfaces is directly proportional to the mean virtual temperature of the layer. Because the atmosphere is always moist to some degree, virtual temperature is used. Mean virtual temperature is defined as the average temperature at which dry air would have the same pressure and density as moist air. Thus, lines representing thickness are also isotherms of mean virtual temperature. The higher the mean virtual temperature, the thicker the layer, or vice versa. The thickness between layers is expressed in geopotential meters. The shift in location, as well as the change of shape and intensity upward of atmospheric pressure systems, is dependent on the temperature distribution.

An example of the effects of virtual temperature can be demonstrated by two columns of air placed side by side—one cold and one warm. The constant pressure surfaces in the cold column are closer together than those in the warm column of air. Figure 3-2-4 shows an increase in thickness between two given pressure surfaces for an increase in mean virtual temperature. Note the increase in the distance between the constant pressure surfaces; P, P1, etc., from column A to column B.

The thickness between two pressure surfaces can be derived by using the hypsometric equation.

Figure 3-2-4.-—Thickness of two strata as a function of mean virtual temperature.

Thickness may also be determined from tables, graphs, etc.

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