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A derivative of the thermal wind equation and advection arrows is the thermal wind rule. It simply states that in the Northern Hemisphere if the wind backs with height, cold advection is indicated; and if the wind veers with height, warm air advection is indicated. Figure 8-1-14 illustrates the thermal wind rule as it applies in the Northern Hemisphere.

Time Differential Analysis

Time differential charts show the amount and direction of change of a meteorological quantity over a given period of time. They are used to track upper-height centers. They are usually drawn at 24-hour intervals, vice every 12 hours, to minimize diurnal effects.

Construction is as follows: Take two charts, 24 hours apart and of the same level, and lay the most recent over the other. Over these lay a clean acetate or blank chart. Using a light table, algebraically subtract the contour values of the most recent chart from those of the earlier chart. This is done where the contours of the two charts intersect. Lines are then constructed connecting like height differences. Falling height lines are drawn in red, rising heights in blue, and lines of no height change are drawn in purple. These lines are labeled in decimeters. The centers of rising

Figure 8-1-13.—Explanation of advection arrow construction.

Figure 8-1-14.—The thermal wind rule (Northern Hemisphere).

and falling heights are then transposed onto a blank chart (if acetate was used). These centers are then tracked in the same manner as are the pressure centers on constant-pressure charts. See figure 8-1-15 for an illustration of time differential construction.

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