As a radar pulse travels through the atmosphere, various physical actions cause the energy of the pulse to decrease. In this section, we will describe these physical actions and their effect on radar systems. REFRACTION A common misconception about a radar beam is that it travels in a straight line, much like that of a laser beam. In reality, the beam (electromagnetic wave) is actually  bent  due  to  differences  in  atmospheric density. These density differences, both vertical and horizontal,   affect   the   speed   and   direction   of electromagnetic waves. In some regions, a wave may speed up, while in other regions it 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. This bending is known as refraction.  Refraction  in  the  atmosphere  is  ultimately caused by variations in temperature, moisture, and pressure, with changes in moisture having the greatest impact. Refractive Index and Refractivity In free space, an electromagnetic wave will travel in a straight line because the velocity of the wave is the same everywhere. The ratio of the distance a wave would travel in free space to the distance it actually travels  in  the  earth’s  atmosphere  is  called  the  refractive index. The refractive index is symbolized by "n" and a typical value at the earths surface would be 1.000300. Thus,  "n"  would  gradually  decrease  to  1.000000  as you move upward toward the theoretical interface between the atmosphere and free space. For example, in the time it takes for electromagnetic energy to travel a distance of one wavelength in air at 1000 hPa, 15°C temperature, and 40 percent relative humidity, it could have traveled 1.0003 wavelengths in free space, which makes 1.0003 the refractive index. The normal value of n for the atmosphere near the earths surface varies between 1.000250 and 1.000400. Since the refractive index produces a somewhat unwieldy number, we use a scaled refractive index called  refractivity.  Refractivity  is  symbolized  by  "N" and is a function of pressure, temperature, and vapor pressure  (moisture).  A  result  is  that  atmospheric refractivity  near  the  earth’s  surface  normally  varies between 250 and 400 N units (the smaller the N-value, the faster the propagation; the larger the N-value, the slower the propagation). Refractivity values become smaller  with  decreasing  pressure  and  decreasing moisture, but larger with decreasing temperature. All of these variables usually decrease with increasing altitude. However, the increase in N due to decreasing temperature is not sufficient to offset the decrease in N due to a decrease in moisture and pressure. As a result, refractivity   values   will   normally   decrease   with increasing  height. NOTE:  It is sometimes advantageous to compute refractivity in terms of waves traveling in a straight line. This may be approximated by replacing the actual earth’s radius (curved earth) with one approximately four-thirds  as  great  ("flat  earth").  The  refractivity using  this  orientation  is  called  modified  refractivity and is expressed in M units. Several  software  programs  such  as  GFMPL automatically  compute  N-units  and  M-units  from radiosonde data. N-units can also be computed from a special Skew-T, Log P diagram with a refractivity overprint  (DOD-WPC  19-16-2).  A  refractivity nomogram, such as the one in Appendix II, can also be used. Refractive  Conditions Under normal atmospheric conditions, when there is a gradual decrease of pressure, temperature, and humidity with height, a radar beam’s curvature is slightly less than the earth’s curvature. This causes it to gradually climb higher with distance and is called standard or normal refraction (fig. 2-15, view A). When there is an unusual or other-than-normal vertical distributions   of   moisture   and/or   temperature, nonstandard refraction or anomalous propagation (AP) takes place. This causes exaggerated bending of the beam either up or down. There are three categories of   anomalous   propagation:   subrefraction, superrefraction,  and  ducting. SUBREFRACTION.—-Occasionally, motions in the   atmosphere   produce   a   situation   where   the temperature  and  humidity  distributions  create  an increasing value of N with height. This occurs when density contrast in the atmosphere is weak, such as when   water   vapor   content   increases   and/or temperature decreases rapidly with height. The beam bends   less   than   normal   and   climbs   excessively skyward. This phenomenon is known as subrefraction. Subrefraction causes the radar to overshoot targets that are   normally   detected   (fig.   2-15,   view   B). Subrefractive  conditions  are  generally  rare,  and usually occur in desert regions and on the lee sides of mountain  ranges. 2-12