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Skew T, Log P Diagram There are two methods of determining refraction from the Skew T. One method uses the modified Skew T (DOD-WPC-9-16-2), which includes a built-in refractivity grid, while the other
Figure 6-3-2.-Refractive Index Nomogram. uses a standard Skew T, with a separate refrac-tivity overlay. The overlay and the over-print on the Modified Skew T are very similar. Computa-tions are done the same way on both. Both Skew T methods involve computations of the N -value for dry air (Nd) and moist air (Nw) The overall N -value is determined by adding Nd and Nw Nd is represented on the refractivity grid by black, nearly horizontal lines that slope down toward the right. These lines are labeled from 350 to 40. Nd is read directly from the point of intersection with the temperature curve. Nwis represented on the grid by black diagonal lines that slope steeply upwards toward the right. These lines are labeled across the bottom and right edge of the chart as follows: from 1 to 10, in increments of 1; from 10 to 40, in increments of 5; from 40 to 120, in increments of 10; and from 120 to 350, in increments of 20. Follow the example given in figure 6-3-3 to determine Nw for any given level; the example uses the 850-millibar level. The procedure is as follows: 1. Find the mixing ratio line that intersects the dewpoint. 2. From the temperature plot, follow the thin black line, sloping upward to the left to the point where it intersects the mixing ratio line determined in step 1. 3. Read Nw at the intersection of the thin black line and the mixing ratio line. 4. After determining Nd and Nw, add them together to obtain the value of N. Learning Objective: Identify the four types of refractive conditions, and the effects of each on radar. |
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