UPPER AIR CHARTS THE FORMATION OF NEW PRESSURE SYSTEMS

Computer drawn charts also provide the forecaster with another tool for forecasting the development of new pressure systems. These prognostic charts maybe used to directly prepare a forecast, or the forecaster may use a sequence of them to construct other charts. A very beneficial chart used in determining changes in surface pressure, frontogenesis, frontolysis, and development of new pressure systems is the advection chart. The normal methods of

Figure 3-1.-A well developed, comma-shaped cloud is the result of a moving vorticity center to the rear of the polar front. The comma cloud is composed of middle and high clouds over the lower-level cumulus and is preceded by a clear slot.

construction are time-consuming; however, by using computer charts, the chart may be more readily constructed.

The 700-hPa, 1000-hPa, and 500-hPa thickness charts should be used in construction of the advection chart. The 700-hPa contours approximate the mean wind vector between the 1000- and 500-hPa levels. On a 1000- to 500-hPa thickness chart, the contours depict thermal wind, which blows parallel to the thickness lines. Meteorologically speaking, we know that lines of greater thickness represent relatively warmer air than lines of less thickness. If the established advection pattern is replacing higher thickness values with lower thickness values, then it must be advecting cooler air (convergence and divergence not considered). The opposite of this is also true. The changing of thickness values can be determined by the mean wind vector within the layer of air. The 700-hPa contours will be used as the mean wind vectors.

The advection chart should be constructed in the following manner:

1. Place the thickness chart over the 700-mb chart and line it up properly.

2. Remember that the 700-hPa contours represent the mean wind vector. Place a red dot indicating warm air at all intersections where the mean wind vector is blowing from higher to lower thickness values.

3. Use the same procedure to place a blue dot at all intersections where the mean wind vector is blowing from lower to higher thickness values.

Now, to use this advection chart, it should be compared against the chart from the preceding 12 hours. From comparison of the red and blue dots, you can determine if there has been an increase or decrease in the amount of warm or cold air advection in a particular area, as well as any change in the intensity of advection. Then, the advection type and amount, as well as change, can be applied to determine the possibility of new pressure system development.

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