TC 9-64 _________________________________________________________________________
Figure 2-24. RF Energy Losses from Scattering
2-127. Fog. In the discussion of attenuation, fog may be considered as
another form of rain. Because 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.
2-128. 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.
2-129. 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.
2-130. 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.
2-131. 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 distances are possible because of the
different densities and refractive qualities of warm and cool air. The sudden