BACKGROUND 135 



sea breeze a duct may be formed over the water due to subsidence. The 

 land breeze is accompanied by subsiding air over the land with resultant 

 duct formation. 



The formation of fog results in a decrease of superrefractive or ducting 

 possibilities. When a fog forms by nocturnal radiation, the water content 

 of the air remains practically the same; however, part of the water 

 changes from the gaseous to the liquid phase thus reducing the vapor 

 pressure. The resulting humidity lapse rate tends to counteract the 

 temperature inversion and cause above standard refraction. However, 

 the temperature inversion may be strong enough to keep the layer stand- 

 ard or superrefractive. This process may also occur with advection fogs. 



The nighttime temperature profile is a result of the interaction between 

 nocturnal radiation, turbulence, and heat conduction. The associated 

 refractive index profiles are such that a radar duct begins to form about 

 the time of sunset, developing quickly during the early evening, more 

 slowly after midnight, and dissipating rapidly after sunrise. This is 

 mainly an inland effect resulting from large diurnal temperature varia- 

 tions observed in the interiors of large continents. However, a shallow 

 body of water may have an appreciable diurnal temperature variation, as 

 compared to the open ocean so that superrefraction may occur over such 

 a location from time to time. 



It is generally recognized that radiosonde observations (RAOB's) do 

 not have a sufficiently high degree of accuracy to be completely acceptable 

 for use in observing changes in the degree of stratification of the very 

 lowest layers of the atmosphere; however, until more accurate methods 

 such as meteorological towers and refractometer measurements are more 

 commonly used, the RAOB will continue to be used as a basis for fore- 

 casting the occurrence of superrefractive conditions. 



4.4.4. Background 



The property of the atmosphere basic to radio ray tracing is the radio 

 refractive index of the atmosphere, n, which for VHF-UHF frequencies 

 at standard conditions near the surface, is a number of the order of 1.0003. 

 Although the refractive index is used in ray tracing theory, it is more 

 convenient when evaluating refraction effects from common meteoro- 

 logical observations to use the refractivity A'', which, for the frequency 

 range to 30,000 Mc/s, is given by (1.20). 



When evaluating the meteorological conditions that give rise to refrac- 

 tive phenomena, it is frequently instructive to examine separately the 

 behavior of the dry component, D, and wet component, W, of N . 



