14 A WORLD ATLAS OF ATMOSPHERIC RADIO REFRACTIVITY 



(2) Hawaiian Islands all months except 

 May; 



(3) South Africa all months except Novem- 

 ber; 



(4) Southeast coast of South America in No- 

 vember and February ; 



(5) Southern California in November ; 



(6) North Indian Ocean in May ; 



(7) Isthmus of Panama in November. 



5.3. Discussion of Gradient Map Contours 

 (Superrefraction and Ducting) 



Superrefractive and ducting gradients in 

 ground-based layers are most often associated 

 with temperature inversions (temperature in- 

 creasing with height within the layer) , not only 

 because a positive temperature height-gradient 

 causes a negative A r -gradient, but also because 

 the low eddy diffusion qualities associated with 

 a temperature inversion often lead to a steep 

 negative gradient of humidity through the in- 

 version. However, previous investigations 

 [Bean, 1959] have shown that there are at least 

 two other typical situations encountered in the 

 formation of strong ground-based gradients: 

 The first of these is the arctic situation, where, 

 with surface temperatures below about — 20°C, 

 a strong temperature inversion (typical of con- 

 tinental arctic air masses) produces a superre- 

 fractive or ducting layer, while the vapor pres- 

 sure may actually increase with height. More 

 often, in this case, the wet term is negligible 

 throughout the layer. The second case is typical 

 of very humid tropical areas when the surface 

 temperature is 30°C or greater. In these loca- 

 tions (which are usually coastal) a common 

 occurrence is a slight decrease of temperature 

 with height, accompanied by a very strong 

 lapse of absolute humidity. Such profiles may 

 show only a slight decrease of relative humidity 

 with height, but, because the saturation vapor 

 pressure is nearly an exponential function of 

 temperature, the resulting vapor pressure gra- 

 dient may be very large, thus causing a steep 

 iV-gradient. 



Figures C-41, C-45, C-49, and C-53 show that 

 persistent ducting (D) or superrefractive (SR) 

 initial gradients can be found more than 25 

 percent of the time for at least two seasons in 

 seven general areas of the world : 



(1) Dakar -Fort Lamy transitional strip in 

 Africa (D : all seasons) , 



(2) northern Arabian Sea including coastal 

 areas of the Gulf of Aden and the Persian Gulf 

 (D: all seasons), 



(3) India, Bay of Bengal, southeast Asia, 

 Indonesia, and north tip of Australia (SR: all 

 seasons), 



(4) southwest coast of North America, in- 

 cluding portions of the North Pacific (SR : Feb- 



ruary, May, November) , 



(5) Gulf of Mexico and Caribbean region 

 (SR: May, August, November), 



(6) northwest coast of Africa and western 

 Mediterranean (SR: May, August), 



(7) Antarctica (D: May, August). 



Area (1 ) : Tropical west coast locations in the 

 vicinity of 15°-22°N or S are affected annually 

 by three or four latitudinal weather zones [Tre- 

 wartha, 1961]. In winter the Dakar-Fort Lamy 

 region is under the influence of dry anticyclonic 

 Saharan air, but even at the time of low sun, the 

 prevailing surface air movement is onshore 

 from the southwest. The vertical depth of this 

 maritime current is more shallow than in sum- 

 mer, but during the early morning hours, the 

 surface relative humidity is 80 to 90 percent 

 compared with 40 to 60 percent in the dry sub- 

 siding air above. Even with radiational cooling, 

 the night temperatures throughout the marine 

 layer (from 50 to 600 m thick) still remain over 

 20°C. This combination of temperature and hu- 

 midity creates trapping conditions for frequen- 

 cies below 300 Mc/s about 30 percent of the 

 time in February (fig. C-31). 



The weather zones advance rapidly north- 

 ward [Thompson, 1965], so that by July the 

 Dakar-Fort Lamy strip is in the wet tropical 

 regime associated with the fluctuating, unstable 

 Intertropical Convergence Zone (ITC). The 

 marine current of the westerlies becomes much 

 deeper, but the ducting layers are shallower 

 and can exist only intermittently between the 

 turbulent, showery periods common to the re- 

 gion. Figures C-22, C-24, C-26, and C-28 indi- 

 cate that more than 30 percent of the ducting 

 layers are over 100 m thick for all seasons ex- 

 cept summer. 



Area (2) : The coastal areas of Arabia ex- 

 perience high surface humidities all year from 

 monsoonal and sea-breeze effects, but during 

 May and August these values are reinforced by 

 temperatures above 25 °C in a marine layer 

 which may extend up to a height of 800 to 900 

 m before it meets the overrunning dry north- 

 easterlies [Tunnell, 1964]. The percentage oc- 

 currence of ducts at Bahrain seems much high- 

 er than at Aden because all observations at 

 Bahrain were taken at 0300 LST (when the 

 surface humidity is at its maximum of 75 to 90 

 percent) , whereas the Aden observations, taken 

 twice a day, include as many observations at 

 1500 LST (when the relative humidity value is 

 much less) as at 0300 LST. For instance, 50 of 

 the 66 ducts recorded in August at Aden were 

 from early morning observations. However, the 

 fact that ducting gradients at Bahrain trap fre- 

 quencies below 300 Mc/s over 75 percent of the 

 time as compared to 5 percent at Aden (fig. 

 C-37) is due to another factor: the thickness of 

 the moist marine layer, when ducting is present 



