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Streamline-Isotach Analysis

This method consists of two sets of lines: streamlines, representing the wind direction, and isotachs (labeled in knots), representing the wind speed. The two sets of lines give a continuous representation of the wind field, from which wind direction and wind speed can be determined at any point on the chart. Familiarity with circulation patterns is a necessity when using this method of analysis.

ASYMPTOTES.— These are streamlines in the wind field away from which neighboring streamlines diverge (positive asymptotes) or toward which they converge (negative asymptotes). We are primarily interested in the negative asymptotes lying between converging currents since, in the lower levels, they are most frequently associated with bad weather. Significant negative asymptotes are frequently found in the convergent flow around well-developed cyclonic indrafts and in the broad zone of convergence between the trade wind currents of the two hemispheres (ITCZ of the doldrums). Asymptotes may or may not represent lines of true horizontal mass divergence and convergence. Typical examples of asymptotes

Figure 9-2-6.—Streamline asymptotes of difluence

(divergence) left; of confluence (convergence) right. 

are shown in figure 9-2-6. Difluent asymptotes are drawn in blue and confluent asymptotes in red. They begin and end 1 inch before and after the first and last converging or diverging streamline. See figure 9-2-7.

WAVES.— These are perturbations in the streamlines similar to the wavelike arrangements of troughs and ridges in isobaric patterns. Waves that do not extend across the entire width of the current in which they are embedded are called damped waves. In this case, the streamlines on one or both sides of the current have smaller amplitude than those in which the wave is more pronounced. Figure 9-2-8 illustrates a damped wave in the streamlines.

SINGULAR POINTS.— These are points into which more than one streamline can be drawn or about which streamlines form a closed curve. The


Figure 9-2-8.—A damped wave in the streamlines.


Figure 9-2-9.—Cusps.

wind speed is zero at singular points, and the

Cusps.— Cusps represent an intermediate pattern in the transition between a wave and a vortex. They are relatively unimportant in synoptic wind analysis since they are short-lived, and there is normally insufficient data to determine their presence. Figure 9-2-9 illustrates two variations of this class of singular points.


Figure 9-2-7.—Coloring asymptotes.


Figure 9-2-10.—Vortices in the streamlines (Northern Hemisphere).

Vortices.— These are centers, outdrafts, and indrafts of anticyclonic or cyclonic circulation. Figure 9-2-10 shows six of the most commonly seen vortices. Anticyclonic outdrafts and cyclonic indrafts are frequently found at low levels in the atmosphere. Data is usually too sparse at the upper levels to determine outdraft or indraft characteristics of vortices. Therefore, many upper level vortices are drawn as pure cyclones or anti-cyclones, for lack of more detailed information. Satellite pictures reveal even the weaker, smaller-scale vortices, but conventional data is required to determine the level of most intense circulation.

Neutral Points.— These are the points at which two asymptotes, one of directional confluence (convergence) and one of directional difluence (divergence), come together. They are similar to cols of isobaric analysis in that they represent a saddle between two areas of anticyclonic flow and two areas of cyclonic flow. Neutral points in the streamlines are shown in figure 9-2-11.

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