KVDIO METEOROLOGY 



219 



The Bay ok Bengal 



Such reports as are available from this region 

 indicate that the seasonal trend is the same as in 

 the Arabian Sea, with normal conditions occurring 

 during the season of the southwest monsoon, while 

 superrefraction is found during the dry season. It 

 appears, however, that superrefraction is much less 

 pronounced than on the northwest side of the 

 peninsula. 



The Pacific Ocean 



This region appears to be the one where, up to 

 the present, least precise knowledge is available. 

 There seems, however, to be definite evidence for the 

 frequent occurrence of superrefraction at some loca- 

 tions; e.g., Guadalcanal, the east coast of Australia, 

 around New Guinea, and on Saipan. Along the 

 Pacific coast of the United States observations indi- 

 cate frequent occurrence of superrefraction, but no 

 statement as to its seasonal trend seems to be 

 available. The same holds good for the region near 

 Australia. 



In the tropics there is found a very strong and 

 persistent seasonal temperature inversion, the so- 

 called trade wind inversion. It has no doubt a very 

 profound influence on the operation of radar and 

 short-wave communication equipment in the Pacific 

 theater. 



Fluctuations in Signal Strength 

 with Time 



A number of different causes tend to produce 

 variations of signal strength with time. These are 

 discussed briefly in the following paragraphs. 



Target Modulation 



Very rapid fluctuations having periods of only a 

 small fraction of a second frequently are encountered 

 in radar observations, especially with centimeter 

 waves. These fluctuations arise as a consequence of 

 the internal motions of the target and are especially 

 noticeable for aircraft. Similar effects have been 

 observed with reflection of microwaves from wooded 

 hills, the fluctuations in signal probably being caused 

 by foliage moving in the wind. 



Effect of Waves on the Sea 



A similar phenomenon is observed when the trans- 

 mitter and receiver are so situated that reflection 



from a water surface contributes to the received 

 signal strength. Owing to irregularities of the water 

 surface and their rapid change with time, variations 

 in signal strength will appear. The fluctuations 

 arising in this way have a time scale of the order of 

 a second, in the case of a lightly ruffled sea (see 

 Figure 34). Evidently rays reflected from different 



Figure 34. Variation in signal strength with time in 

 radiation reflected from the sea (direct radiation cut off). 

 X = 9 cm. 



parts of the water surface interfere, and with the 

 changing form of the surface the interference pattern 

 at the place of the receiver changes accordingly. The 

 time scale of these changes must be connected with 

 the speed, wavelength, and amplitude of the waves, 

 but the exact relation is not known thus far. 



Tidal Effects 



The rise and fall of the tide produces a gradual 

 variation in signal strength by changing the inter- 

 ference between the direct and the reflected rays. 

 The path difference between these rays is 2hih,2/R, 

 where hi, hi are the heights of the transmitter and 

 receiver relative to the instantaneous water level and 

 R is the range. The corresponding difference in phase 

 between the two rays is equal to 



2h x hi _ 2w 

 R ' A 



4> , 



(26) 



measured in radians. The variation in the signal 

 strength depends upon the variation in </>. It is small 

 when the change in </> is small and increases to a 

 maximum for a change in <f> of x radians. It follows 

 from equation (26) that the tidal effect increases 

 with the variation in the water level of the tide and 

 with the heights hi and hi and decreases with the 

 range and the wavelength. 



Scintillations 



The really conspicuous fluctuations in propagation 

 conditions, however, are due to changing meteoro- 

 logical conditions. A characteristic type is an irregular 

 fluctuation in signal strength on a time scale of the 

 order of a minute and with an amplitude rarely 



