126 



METEOROLOGY— FORECASTING 



the wavelength alwve which trapping will not occur. 

 A rough generalization can be made by stating that, 

 when the wavelength of the radiation is around 3 cm, 

 duct widths of 20 ft or more will be sufficient to cause 

 simple surface trapping, and, when the wavelength is 

 around 10 cm, duct widths of at least 10 or .50 ft will 

 be sufficient, providt-d in each case that the transmitter 

 is located witliin the duc't. Thus, for a particuhir radai- 

 frcquencv and location, an estimation of duct width 

 becomes a critical factor in anticipating whether an 

 appreciable amount of trapping will occur and con- 

 sequently whether radar ranges will be appreciably 

 larger than under stanihird conditions. 



Table 2. Values of the maximum wavelength (in cm) 

 for which waves can be trapped in a surface duct for 

 given values of M deficit (AM) and duct width id). 



d(h) 



AM (A/ units) 

 10 15 



20 



Dependence on Extraneous Factors. Meteorological 

 conditions other than ducts often have important 

 effects on propagation. Surface fog frequently causes 

 sid)standard conditions. Rain and clouds may attenu- 

 ate the propagated energy and effectively decrease 

 the range. Heavy rain and sometimes cumulo nimbus 

 clouds cause radar echoes. 



Opek.vtional Use and Limitations 



The restrictions surrounding the possible utilization 

 of this prediction technique have been covered earlier 

 in this section, where it is indicated in what respects 

 applicability is limited. Subject to these limitations, 

 the method can be employed to advantage. Short-term 

 predictions (a day or so ahead) of duct formation and 

 radar range can aid in estimating the coverage of a 

 particular radar set. Information of this nature should 

 lead to the more efficient use of radar facilities. 



^■^•* Computed Climatological Information 

 on Surface Ducts 



PuKPOSE OF This Information 



Inasmuch as the present report is designed to aid 

 radar and meteorological officers in the Pacific theater 



of war, it has been thought expedient to include 

 climatological information on that area which gives 

 an iiulication of the occurrence of surface ducts. For 

 this purpose, computations have been carried out, 

 using the methodology presented in Section 8.3.3, to 

 yield the following: 



1. Estimation of the per cent of time surface ducts 

 of certain widths are likely to occur at various times 

 of the year and at various places in the western Pacific 

 theater. 



3. E.stimation of the variation in duct width with 

 the time of year, geographical location, etc. 



3. Estimation, from (1) and (3) above, of the 

 amount of trapping to be expected for specified radio 

 frequencies and specified elevations of sites. 



Peoioxs Chosen for Study 



The area chosen for investigation was that bounded 

 approximately by 10° and 50° N latitude and by 120° 

 and l."iO° E longitude. These regions include the Japa- 

 nese islands, the coasts of Korea, Manchuria, and 

 China, northern Philippines, the Marianas, the 

 Bonins, and the Eyukyu Islands. The computations 

 were carried out for representatively selected 5x5 degree 

 sectors or "squares." The results for several regions 

 in this area are summarized and are given below. 



Souu('f:s of Ci.imatoeogical Data 



The charts and atlases used in compiling the data 

 can be found in references 11 to 15. 



■MicTiioii OF Computation 



The procedures described in Section 8.3.3 were ap- 

 plied as follows : 



1. A determination of the monthly mean tempera- 

 ture excess was made for each 5x5 degree square for 

 each month (1) by taking the difference between the 

 mean temperature of the air over the sea and the mean 

 sea surface temperature, and (2) by taking the differ- 

 ence between the mean temperature of the air at a land 

 station (when the given square was near a coast) and 

 I be mean sea surface temperature. 



2. A determination of the monthly mean M deficit"^ 

 was made for each square for each month using the 



''These calculations do not represent e.xactly the correct 

 mean value of the M deficit, since M is not a linear function 

 of the teniijerature and humidity, and so the value of M com- 

 puted from mean temperature and himiidity data is not quite 

 the same as the mean of all .1/'s com])uted from individual 

 tcmiieratures and humidities. A cursory evaluation of this 

 error has indicated that the values computed are if anything 

 conservative, i.e., that the actual mean M deficits are prob- 

 ably larger than those computed. 



