The isotherms of a constant-pressure level located near the middle of an isobaric layer are assumed to be representatiave of the mean iso-therms (thickness lines) of the layer. For example, lets use the isobaric layer between 1000 mb and 500 mb. The constant-pressure level closest to the middle of the 1000-500-mb layer is 700 mb. The 700-mb isotherms are assumed to be representa-tive of the mean temperature and thickness of the 1000-500-mb layer. Put another way, the 700-mb isotherm pattern is similar to the 1000500-mb thickness pattern. The isotherm-thickness relation-ship applies to other layers. For example, the 850-mb isotherms are representative of the mean isotherms of the 1000-700-mb layer.

ISOTACH ANALYSIS 

An isotach analysis provides a visual repre-sentation of the wind field at a given level. Isotachs are lines connecting points of equal wind speeds. They are drawn at 20-knot intervals. They form elongated ellipses that localize areas of highest wind speeds (fig. 8-1-10). These areas are known as centers of wind speed maxima in the overall isotach pattern. These centers move from west to east at speeds less than that of the winds themselves but greater than that of the waves in the long wave pattern. Their speed approximates that of the short waves. They move around the long wave troughs and ridges as these features move eastward. Occa-sionally, centers appear nearly stationary in the base of a trough or crest of a ridge, but they have

Figure 8-1-10.Common contour and isotach patterns. (A) Speed maximum at long-wave trough line; (B) Speed maximum at ridge line.

never been known to retrogress upstream. Isotach maxima on any constant-pressure chart represent the intersection of that level with a jet stream (jet).

Whatever level you analyze, begin the analysis in an area of dense reports. Sketch in the 60-knot isotach. Isotachs are drawn as a series of dashes. Begin at a point representing 60 knots and follow it downwind, interpolating between reported wind speeds. The isotachs will parallel, more or less, the height con-tours. However, at low speeds, isotachs cross contours at large angles, and where isoheights converge (upstream from speed maxima), isotachs tend to cross from high to low heights.

The reverse takes place downstream, where isoheights diverge. Sketch in the remaining isotachs at 20-knot intervals and identify the centers of speed maxima. The centers are labeled with a "J" followed by the estimated maximum speed; e.g., J120. Speed minima (centers of minimum wind speeds) are labeled with an "S".

Computing wind speeds in sparse-data areas is accomplished using gradient or geostrophic wind scales. For reasons mentioned previously we use the geostrophic wind scale, and only in the case of cyclonically curved isoheights would you need to correct geostrophic wind to gradient wind.

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