236 REFLECTION AND TRANSMISSION OF RADIO WAVES 



usually give rise to elevated ducts, since inversion levels commonly occur 

 at 5000 to 10,000 ft. In some cases subsidence inversions descend low 

 enough to form a strong surface duct of the type shown in Fig. 4-50d. 



Inversions can also be produced by cooling of the ground at night through 

 the process of radiation. In the absence of wind, the radiation inversion 

 grows upward as the night progresses, forming a surface duct of the type 

 shown in Fig. 4-50c. 



Strong surface ducts are formed when warm air from a large land mass 

 moves out over water. The air in contact with the water is cooled and 

 moistened. This cooling and pickup of moisture works its way upward with 

 time by eddy diffusion. As a result, during the formative process an inclined 

 duct usually results, which can extend 200 miles or more out to sea. Duct 

 heights can extend up to 1000 feet or so, and hence can influence airborne 

 radar operation. 



Weak surface ducts are formed over the open oceans in the trade-wind 

 regions. Here the air is colder than the water, so that an increase of 

 temperature with height is accompanied by a decrease of vapor pressure 

 with height. Their effects on the refractivity thus oppose, as can be seen 

 from Equation 4-87, but the influence of the moisture predominates. These 

 ducts are very persistent, lasting almost all year round, incident to the 

 persistence of the trade winds. The duct height is about 50-75 ft, so that 

 they are not very important for airborne radar, except possibly in unusual 

 situations. 



An adverse effect on airborne radar can occur when an elevated layer lies 

 below the radar and the target. Then, in addition to a direct ray, a ray 

 refracted by the layer can be received. At certain ranges, well within the 

 horizon, the two rays can interfere destructively, resulting in a decrease 

 in field. This is referred to as a radio hole. Radio holes have been observed 

 in which the field strength falls by as much as 15 db over a one-way path, 

 which would mean a 30-db drop for a radar path. Radio holes extend in 

 range for 20 to 50 miles, and so can seriously decrease the range of an 

 airborne radar. 



Radio holes have been shown''^ to be caused by only small departures of 

 the M curve from a straight line. A layer in which the slope changes by 

 as little as 10 per cent of the slope in adjoining regions can produce a radio 

 hole. It has been estimated that layers of this kind are present at altitudes 

 between about 5000 and 10,000 ft between 50 and 95 per cent of the time. 

 Thus this phenomenon can have a profound effect on airborne radar. 



Many of the effects of the varying refractivity of the atmosphere can be 

 deduced, and to a certain extent predicted, from climatological considera- 

 tions. However, most of the propagation measurements which have been 



'^''Investigation of Air-to-Air and Air-to-Ground Experimental Data, Final Report Part III, 

 Contract AF33(038)-U)91, School of Electrical Engineering, Cornell University, 10 Dec. 1951. 



