When a radar is radiating, electromagnetic energy in the form of radio waves is transmitted. As the waves move through the atmosphere, they often encounter layers or regions having different densities. Density differences, both vertical and horizontal, affect wave speed and direction. In some regions, the waves may speed up, while in other regions they may slow down. When one portion of a wave is slowed and another portion is not, the wave bends in the direction of the slower portion of the wave. These variations in wave velocity bend the radio waves either toward or away from Earths surface. This bending process is called REFRACTION.

Refraction is controlled by temperature, moisture, and pressure; it is measured in "N -units."

TEMPERATURE An increase in tempera-ture causes a decrease in N -units. The actual ratio of change is very near one-to-one. A 1C increase in temperature produces a one-unit decrease in N, and a 1C decrease produces a one-unit increase in N. Latitudinally, N -units are higher at the poles and lower at the equator.

MOISTURE Changes in water vapor con-tent produce the greatest changes in refraction. The greater the moisture content in a given layer of the atmosphere, the greater the vapor pressure. Vapor pressure produces a directly proportional change in N. The ratio is 1 to 5; a 1-millibar increase in vapor pressure causes a 5-unit increase in N.

PRESSURE Although pressure is one of the properties that control refraction, its effects are minimal. Pressure variations alone provide no significant change in refractive conditions.

N-UNITS N-units are computed for various levels in the atmosphere. Like temperature, pressure, and moisture, N -units normally decrease with height. N -units are used to determine the speed of electromagnetic waves. The smaller the N -value, the faster the waves; the larger the N -value, the slower the waves.

N-values are determined for various levels within the atmosphere by using pressure, temperature, and dewpoint values and either a refractive index nomogram or a Skew T, Log P Diagram.

Refractive Index Nomogram The Refractive Index Nomogram is a reasonably accurate method of determining N-values. Directions are included on the nomogram. See figure 6-3-2.