where m is the mass of liquid vapor per cubic meter, A is the wave length 

 in centimeters, and e, and e, are the real and imaginary parts of the di- 

 electric constant at the temperature in question for the wave length of 

 radiation. Humphreys [/,9] gives a value of 0.006 gm m 3 as the liquid 

 water content of fog. As the concentration of 0.0 (jm m 3 is rarely ex- 

 ceeded [5] this value is used in the computations below. Based on the 

 18°C. pure water complex dielectric constants of (63.6 ,/32.7) for A'-band 

 and (79.0 — ,/12.3) for the N-band, the maximum attenuation to be ex- 

 pected on a radar range of 5 miles is 0.55 decibels for the A'-band and 

 0.06 decibels for the S-band. Although there is a ten-fold difference be- 

 tween the two frequencies the values are insignificant when other factors 

 are considered. The attenuation due to water vapor (7.5 gm/m' A ) and 

 oxygen over the same range is approximately 0.10 decibels for both fre- 

 quencies \_20~\. Of interest here is the fact that water suspended in the air 

 in the form of drops contributes less to the refractive index than an equiva- 

 lent amount of vapor. The formation of fog, therefore, reduces the prefog 

 attenuation by water vapor; however, the importance of diminished 

 visibility far outweighs this slight advantage in fog over prefog conditions. 

 If there is a temperature inversion in the fog layer, the vapor pressure 

 required for saturation increases with height and substandard conditions 

 usually result. As mentioned above, this is a common occurrence on the 

 Grand Banks during southerly winds. In summary, it is concluded that 

 the attenuation of S- and X-band radar waves due to fog is not significant 

 compared to other factors. 



Icebergs Hidden by Weather 



Although fog attenuation is not significant, attenuation of radar waves 

 due to rain drops is of considerable importance for frequencies in the 

 A r -band and above. The back scattering from rain squalls, as shown in 

 Table I, is often sufficient to obliterate a small target. 



Table I 



This tabulation taken from curves presented by Goldstein [_21~\ is quite 

 revealing, and would make high frequency radars prohibitive in areas of 

 continuous heavy rain. Fortunately, the passage of frontal systems and 

 their associated rainstorms or sometimes cloudbursts is rather rapid com- 

 pared to other weather phenomena and if necessary a mariner may stop 

 his vessel until the masking effect has passed. On two occasions during 

 the held experiments growlers were masked by moderate to heavy rain 



82 



