Expected Average Propagation Conditions 



At this time it cannot be said with certitude to what extent the con- 

 ditions on the Grand Banks may reduce radar ranges; however, there 

 is little doubt that the subnormal propagation conditions are the rule 

 rather than the exception. It is possible to estimate the magnitude of 

 range reduction which might be expected due to the atmospheric condi- 

 tions prevalent on the Grand Banks and contiguous areas of the North 

 Atlantic Ocean. A qualitative consideration of the processes involved 

 indicates that the average temperature and humidity distribution from 

 the surface to 150 feet at the Tail of the Grand Banks during spring is 

 characterized by increase in temperature with height and near constant 

 relative humidity approaching 100 percent. An examination of radiosonde 

 observations from coastal stations and the computed average index of 

 refraction conditions presented in the U.S. Climatic Atlas of the Oceans 

 \_16~\ indicate that the expected average condition would be one of iso- 

 refractive index from the surface to a few hundred feet. Ray diagrams for 

 this condition and the condition of standard propagation (index of re- 

 fraction gradient of — 1.19X10 -8 units per foot) have been constructed 

 and are shown in figure 37. The top diagram represents the internationally 

 accepted standard conditions and the bottom diagram shows a comparison 

 between the standard conditions and the average subnormal conditions 

 defined here for the Grand Banks. The minima for both *S- and A r -band 

 for the standard conditions have been superimposed on those for the 

 "average" conditions. This comparison indicates that the magnitude of 

 decreased detection in the Grand Banks region is insignificant at short 

 ranges and that there is no preference to radar frequency for short range 

 detection during subnormal propagation conditions. The temperature 

 and humidity conditions upon which the lower ray diagram was con- 

 structed are considered to be conservative generalizations of subnormal 

 conditions; and it should be remembered that at times a rapid increase 

 in temperature in the first 100 feet accompanied by a constant high rela- 

 tive humidity or humidity increasing with height might lead to slightly 

 reduced ranges on small targets and relatively greatly reduced ranges on 

 large targets. 



Fog Attenuation 



The prevalence of fog in the potential ice areas has led to a theoretical 

 investigation of the attenuation to be expected due to fog. Observations 

 indicate that fair weather clouds and fog are composed of water droplets 

 whose diameters do not exceed 0.02 centimeters. For this size droplets 

 the attenuation formula becomes independent of the drop size distribu- 

 tion and takes on the remarkably simple form [5]: 



24. 55m e; 

 80 



