2-35
Figure 2-24.—Rf energy losses from scattering.
Fog
In the discussion of attenuation, fog may be considered as another form of rain. Since fog remains
suspended in the atmosphere, the attenuation is determined by the quantity of water per unit volume and
by the size of the droplets. Attenuation because of fog is of minor importance at frequencies lower than 2
gigahertz. However, fog can cause serious attenuation by absorption, at frequencies above 2 gigahertz.
Snow
The scattering effect because of snow is difficult to compute because of irregular sizes and shapes of
the flakes. While information on the attenuating effect of snow is limited, scientists assume that
attenuation from snow is less than from rain falling at an equal rate. This assumption is borne out by the
fact that the density of rain is eight times the density of snow. As a result, rain falling at 1 inch per hour
would have more water per cubic inch than snow falling at the same rate.
Hail
Attenuation by hail is determined by the size of the stones and their density. Attenuation of radio
waves by scattering because of hailstones is considerably less than by rain.
TEMPERATURE INVERSION
Under normal atmospheric conditions, the warmest air is found near the surface of the Earth. The air
gradually becomes cooler as altitude increases. At times, however, an unusual situation develops in which
layers of warm air are formed above layers of cool air. This condition is known as TEMPERATURE
INVERSION. These temperature inversions cause channels, or ducts, of cool air to be sandwiched
between the surface of the Earth and a layer of warm air, or between two layers of warm air.
If a transmitting antenna extends into such a duct of cool air, or if the radio wave enters the duct at a
very low angle of incidence, vhf and uhf transmissions may be propagated far beyond normal
line-of-sight distances. When ducts are present as a result of temperature inversions, good reception of
vhf and uhf television signals from a station located hundreds of miles away is not unusual. These long
