762 



AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



clouds (owing to the lesser equivalent water density of the latter hydro- 

 meteors). Values of attenuation have been calculated, but experimentally 

 determined values have been difficult to collect and correlate either with 

 each other or with the theoretical values. Fig. 14-14 gives some typical 



30 40 50 60 70 

 RAINFALL RATE (R) in mm/hr 



100 



Fig. 14-14 Theoretical Curves of Two-Way Attenuation 7 Versus Rainfall Rate 

 (R). Summarization of Data Derived by Ryde and Ryde "Attenuation of Centi- 

 meter and Millimeter Waves by Rain, Hail, Fog, and Clouds," GEC Report No. 



8670 (1954). 



theoretical values for two-way attenuation 7 and illustrates the interrela- 

 tion of attenuation and rainfall rate. It is implied in these curves that a 

 given theoretical drop size distribution is typical for weather targets. 



The nature of meteorological scatterers has just been discussed in a 

 general way. It is also important in the design of weather radar to have 

 some knowledge of the nature of weather disturbances and how they might 

 affect the utilization of the radar. Most of this discussion will be devoted to 

 the thunderstorm problem for two reasons. It creates one of the most severe 

 turbulence problems for the pilot, and it is quite commonplace. Tornadoes 

 and hurricanes are obviously more dangerous, but not nearly so often 

 encountered. 



Thunderstorms and heavy showers normally stand out well, even among 

 large stable rainfall areas because of the high back-scattering of heavy 



