216 



TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 



pendent of the physical constitution of the ground 

 but is dependent upon its temperature and increases 

 very rapidly with a rise in ground temperature. 



The atmosphere has a "blanketing" effect upon 

 the infrared radiation emitted by the ground. The 

 atmosphere itself absorbs and emits infrared radia- 

 tion, and the cooling of the ground may be greatly 

 reduced by the action of the atmosphere. The 

 blanketing effect is least with a clear sky and dry, 

 cool air; it is somewhat stronger when, with a clear 

 sky, the atmosphere is very warm and humid, as in 

 the tropics. A cloud will produce a distinct blanket- 

 ing effect, and with a complete overcast of low cloud 

 the blanketing is so pronounced that the nocturnal 

 cooling of the ground is reduced to only a small 

 fraction of its value with clear skies. 



The loss of heat from the ground is distributed by 

 turbulence over the lowest layers of the atmosphere, 

 thus giving rise to a temperature inversion. Inver- 

 sions of this type are strongest in temperate and 

 cold climates with a clear sky and cold, dry air 

 overhead; they are less pronounced in the tropics 

 with humid air and a clear sky and are practically 

 absent with an overcast sky. A meteorologist, after 

 some experience, can estimate the magnitude of an 

 inversion to be expected with given local weather 

 conditions. 



Temperature inversions, by themselves, can at 

 best produce only weak ducts, but strong ducts may 

 result when the inversion is accompanied by a suffi- 

 cient moisture lapse. This requires that the air be 

 dry enough to allow evaporation into it from the 

 ground. In warmer climates where the transition 

 between night and day is rapid, evaporation may 

 set in in the early hours of the morning before the 

 nocturnal inversion has been completely destroyed 

 by the action of the sun. A strong duct will then be ■ 

 formed for a short period. This condition seems to 

 be frequent during certain seasons in Florida. 



It is obvious that the shape of the M curve, when 

 it deviates from the normal, may undergo rapid 

 variations with the period of a day. One example 

 has just been quoted; another is illustrated by the 

 advective ducts over the North Sea produced by the 

 mechanism described in Section 17.3.3. These ducts 

 usually form in the hours before midnight and last 

 until the early hours of the morning. 



Fog 



Contrary to what might perhaps be expected, the 

 formation of fog results, in general, in a decrease of 



refractive index. When fog forms, e.g., by nocturnal 

 cooling of the ground, the total amount of water in 

 the air remains substantially unchanged, but part 

 of the water changes from the gaseous to the liquid 

 state. The contribution of a given quantity of water 

 to the refractive index is found to be far less when 

 the water is contained in liquid drops than when it 

 exists in the form of vapor. The formation of fog, 

 therefore, results in a reduction of the amount of 

 water vapor contributing to the value of M . If there 

 is a temperature inversion in the fog layer, the 

 saturation vapor pressure increases with height, and 

 a substandard M curve frequently results (see Figure 

 20, curve lb). This occurs with radiative fog (caused 

 by nocturnal cooling of the ground) and also with 

 advective fog (caused by the advection of warmer 

 air over a cooler surface). Advective fog is very 

 common in the Aleutian Islands and off Newfound- 

 land. 



If fog causes a substandard M curve, it is to be 

 inferred that the rays will be bent upward, instead 

 of downward as with superrefraction, and lead to a 

 weakening of the field in the lowest layers, even to 

 the point of producing a complete fade-out of radio 

 reception. Appreciable reduction of radar ranges and 

 interruption of microwave transmission have 

 frequently been observed in such cases. 



Fog, however, does not always produce a sub- 

 standard M curve, though this is the most common 

 case. In certain other less frequent types of fog, the 

 temperature (and thereby the vapor pressure) may 

 be constant or increase with height through the fog 

 layer. In this event near-standard propagation will 

 prevail, or a duct may develop when the temperature 

 inversion is strong enough. An example is steam fog, 

 formed when cold air passes over a warm sea (see 

 also Section 17.3.9). 



17 37 Subsidence — Dynamic Effects 



The temperature inversions discussed so far owe 

 their existence to the modification of air by contact 

 with the ground, but subsidence inversions are 

 produced by a mechanism of an entirely different 

 nature. By subsidence is meant the sinking of air, 

 that is, a vertical displacement, which must of course 

 be accompanied by a lateral spreading (divergence) 

 in the lower part of the subsiding column of air; 

 otherwise there would be an accumulation of air in 

 the lower levels. The thermodynamic analysis of this 

 complex process shows that if the effect of subsidence 



