Figure 6-3-11 is an example of a refractive index worksheet. Use the worksheet, in conjunc-tion with an upper-air sounding, to provide a quick reference to all of the data used in refractivity computations. Complete the work-sheet as follows:

PRESSURE COLUMN Enter surface, man-datory, and significant levels through 500-milli-bars.

HEIGHT Enter height, in feet, of all mandatory levels. Remember, heights in the radiosonde code are encoded in meters. To convert meters into feet, multiply the height in meters by 3.3 feet. Next, estimate the height values of all significant levels. This is done using the "feet per millibar" conversion factor: 45 feet = 1 millibar. For example, to find the height of the significant level at 565 millibars, find the difference, in millibars, between 565 and the close   mandatory level. The closest mandatory level to 565 millibars is the 500-millibar level. 565 500 = 65 millibars. Multiply the difference (65 millibars) by 45 feet (the conversion factor). This gives a conversion figure of 2,925 feet. Next, simply subtract the 2,925 feet from the height of the 500-millibar level. This gives you an approximate height of the 565-millibar level.

TEMPERATURE Enter the temperature in degrees Celsius.

DEW POINT Enter the dew point in degrees Celsius.

N-UNITS Compute N-units for each level, using the Refractive Index Nomogram or Skew T.

LAYER THICKNESS Enter, in feet, the difference between each height for which N was computed.

N-DIFFERENCE Enter the difference in N-units between each level. Label these values with a + or to indicate an increase or a decrease with height.

N-GRADIENT Enter the N-gradient per 1,000 feet by dividing the N-difference by the layer thickness for each layer. Round off to the nearest whole number. For example, if N decreases 42 N -units in a layer 560 feet, you must figure out the decrease per 1,000 feet. Using the ratio formula 42 is to 560 as X is to 1,000, we see that the gradient is 75 N-units per 1,000 feet.

CLASSIFICATION Enter the refractive classification for each level according to the N-gradient value.