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 normal. What this means for communication purposes is that the range of frequencies on a given circuit is smaller than normal and that communications are possible only at lower working frequencies.

Weather

Wind, air temperature, and water content of the atmosphere can combine either to extend radio communications or to greatly attenuate wave propagation. making normal communications extremely difficult. Precipitation in the atmosphere has its greatest effect on the higher frequency ranges. Frequencies in the hf range and below show little effect from this condition.

RAIN.— Attenuation because of raindrops is greater than attenuation for any other form of precipitation. Raindrop attenuation may be caused either by absorption, where the raindrop acts as a poor dielectric, absorbs power from the radio wave and dissipates the power by heat loss; or by scattering (fig. 1-13). Raindrops cause greater attenuation by scattering than by absorption at frequencies above 100 megahertz. At frequencies above 6 gigahertz, attenuation by raindrop scatter is even greater.

Figure 1-13.—Rf energy losses from scattering.

'FOG.—Since fog remains suspended in the atmosphere, the attenuation is determined by the quantity of water per unit volume (density of the fog) and by the size of the droplets. Attenuation because of fog has little effect on frequencies lower than 2 gigahertz, but can cause serious attenuation by absorption at frequencies above 2 gigahertz.

SNOW.—Since snow has about 1/8 the density of rain, and because of the irregular shape of the snowflake, the scattering and absorption losses are difficult to compute, but will be less than those caused by raindrops.

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

When layers of warm air form above layers of cold air, the condition known as temperature inversion develops. This phenomenon causes ducts or channels to be formed, by sandwiching cool air either 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, 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. These long distances are possible because of the different densities and refractive qualities of warm and cool air. The sudden change in densities when a radio wave enters the warm air above the duct causes the wave to be refracted back toward earth. When the wave strikes the earth or a warm layer below the duct, it is again reflected or refracted upward and proceeds on through the duct with a multiple-hop type of action. An example of radio-wave propagation by ducting is shown in figure 1-14.

Figure 1-14.—Duct effect caused by temperature inversion.

TRANSMISSION LOSSES

All radio waves propagated over the ionosphere undergo energy losses before arriving at the receiving site. As we discussed earlier, absorption and lower 1-12