18 A WORLD ATLAS OF ATMOSPHERIC RADIO REFRACTIVITY 



time would be distributed throughout the day as 

 one traversed the globe. For instance, the fol- 

 lowing stations (in tropical areas where the 

 occurrence of either subrefractive or superre- 

 fractive layers is especially dependent upon the 

 time of day) were used in this report : 



Station 



Local time GMT 



0300 



0000 



2000 



0000 



*Aden, Federation of South 



Arabia 

 * Curacao, Netherlands 



Antilles 



Fort Lamy, Republic of Chad 0100 0000 



*Hilo, Hawaii 1400 0000 



Lae, Territory of New Guinea 1000 0000 



Majuro Island, Marshall 1200 0000 



Islands 

 Singapore 0700 0000 



Those stations marked with an asterisk also 

 take observations at 1200 GMT. However, when 

 evaluating the apparently low level of duct oc- 

 currence at some locations (e.g., Majuro) and 

 high occurrence at others (e.g., Fort Lamy), 

 and when checking the subref raction occurrence 

 at warm, dry continental locations, such as Ni- 

 amey (where no midday observation is taken), 

 the local time of the radiosonde ascent should be 

 considered. 



In addition to the spatial and temporal limita- 

 tions imposed by the use of available radiosonde 

 data, there are instrument recording limitations 

 (see sec. 2) which must be considered when 

 evaluating A 7 -gradients. Although the alternat- 

 ing sequence system of observing the humidity 

 and temperature can put a lower limit on the 

 thickness and thus mask the true gradient of 

 atmospheric layers which can be detected by 

 radiosonde, the response of the radiosonde tem- 

 perature and humidity elements is a more se- 



rious problem in the measurement of the in- 

 tensity and number of superref ractive gradients 

 at or near the surface. For example, in typical 

 ducting situations during May in a tropical 

 (Saigon) and in a temperate climate (Bor- 

 deaux), correction of both humidity and tem- 

 perature sensor time lags as suggested by Bean 

 and Dutton [1961] would intensify gradients of 

 —293 N/km (Saigon) and —212 N/km (Bor- 

 deaux) to —377 A7km and —362 N/km, re- 

 spectively. This type of correction also would 

 have increased the percentage occurrence of 

 superrefractive and ducting gradients in the 

 majority of cases. Such extensive recalcula- 

 tions were not possible in this study, but the 

 possibility that more intensive gradients may 

 occur in larger percentages at some locations 

 (particularly in temperate, humid climates) 

 should be kept in mind when applying values 

 obtained from any of the figures in appendix C. 

 Another limitation which applies primarily to 

 the detection of subrefractive layers (figs. C-l 

 through C-12) in hot, dry regions is the high 

 electrical resistance of the lithium chloride hu- 

 midity element at very low humidities which 

 causes open-circuit signals ("motorboating"). 

 At stations such as Aoulef, Algeria, where the 

 daytime surface temperature often exceeds 

 30°C, the relative humidity may be below the 

 motorboating boundary at all levels from sur- 

 face upward, and all relative humidity values 

 (except the surface) are estimated, usually in 

 values which are equal to, or less than, the sur- 

 face value. However, it is quite probable in 

 these highly convective conditions that the abso- 

 lute humidity remains constant with height, 

 instead of rapidly decreasing (as the estimated 

 relative humidity values would indicate) . If it 

 did remain constant, fairly persistent subre- 

 fractive gradients would be found in such areas 

 during the hours of most intense solar heating. 



