After N -units are determined, the refractive conditions can be determined. The four basic classifications of refraction are Normal, Sub-refractive, Super-refractive, and Trapping. These classifications are based on refractive conditions in the atmosphere.

Figure 6-3-3.-The Skew T refractive overlay and over-print.

For effective evaluation of refractive condi-tions, changes in N over known vertical distances are required. This is the reason for computing N-units at different levels in the atmosphere. Determining change in N with height is known as computing the N-gradient.

Under NORMAL-REFRACTION conditions, N decreases with height at a rate between 0 and 24 N -units per 1,000 feet. The normal N-gradient overland shows a decrease of 12 N -units per 1,000 feet; while over water, N decreases at a rate of 18 N -units per 1,000 feet. When there is no change in N -values (0 N -units per 1,000 feet), there is no refraction; the radar waves travel in a straight line. As the gradient decreases toward 24 N -units per 1,000 feet, the radar waves bend increasingly downward, toward Earths surface. Figure 6-3-4 illustrates the effect normal-refraction conditions have on radar.