1-17
REFRACTION is the bending of electromagnetic waves caused by a change in the density of the
medium through which the waves are passing. A visible example of electromagnetic refraction is the
apparent displacement of underwater objects caused by the bending of light as it passes from the
atmosphere into the water. An INDEX OF REFRACTION has been established which indicates the
degree of refraction, or bending, caused by different substances. Because the density of the atmosphere
changes with altitude, the index of refraction changes gradually with height.
The temperature and moisture content of the atmosphere normally decrease uniformly with an
increase in altitude. However, under certain conditions the temperature may first increase with height and
then begin to decrease. Such a situation is called a temperature inversion. An even more important
deviation from normal may exist over the ocean. Since the atmosphere close to the surface over large
bodies of water may contain more than a normal amount of moisture, the moisture content may decrease
more rapidly at heights just above the sea. This effect is referred to as MOISTURE LAPSE.
Either temperature inversion or moisture lapse, alone or in combination, can cause a large change in
the refraction index of the lowest few-hundred feet of the atmosphere. The result is a greater bending of
the radar waves passing through the abnormal condition. The increased bending in such a situation is
referred to as DUCTING and may greatly affect radar performance. The radar horizon may be extended
or reduced, depending on the direction the radar waves are bent. The effect of ducting on radar waves is
illustrated in figure 1-15.
Figure 1-15.—Ducting effect on the radar wave.
Another effect of the atmosphere on radar performance is caused by particles suspended in the air.
Water droplets and dust particles diffuse radar energy through absorption, reflection, and scattering so
less energy strikes the target. Consequently, the return echo is smaller. The overall effect is a reduction in
usable range that varies widely with weather conditions. The higher the frequency of a radar system, the
more it is affected by weather conditions such as rain or clouds. In some parts of the world, dust
suspended in the air can greatly decrease the normal range of high-frequency radar.
Q13. What term is used to describe the ability of a radar system to distinguish between targets that are
close together?
Q14. The degree of bearing resolution for a given radar system depends on what two factors?
Q15. What happens to the speed of electromagnetic energy traveling through air as the altitude
increases?
