WORLD MAPS OF EXTREME iV-GRADIENTS 15 



at Bahrain, is greater than 300 m over half the 

 time. 



Area (3) : A moist surface layer is also found 

 in the monsoonal areas. Its temperature is 25 

 to 30°C and, during occurrences of ducting, the 

 surface relative humidity ranges from 85 to 100 

 percent, but the trapping incidence is much less 

 than in either area (1) or (2). The surface 

 layer is shallower and its gradient is less intense 

 because the humidity decrease between it and 

 the air mass directly above it is only 10 to 20 

 percent. Because brief periods of stable weather 

 occur even between surges of the summer mon- 

 soon, the ducting incidence remains over 10 per- 

 cent for all of area (3) . 



Area (4-) : Along the western coast of North 

 America, from Southern California to Central 

 Mexico, the most important month for unusual 

 radio propagation due to surface conditions is 

 February, when frequencies below 300 Mc/s are 

 trapped 10 percent of the time. During the pe- 

 riod studied, 30 percent of the superrefractive 

 gradients were at least 300 m thick in all 4 

 months. Closer examination of the ducting 

 structure in Mazatlan reveals that if near-sur- 

 face layers (bases of 100-300 m) were included, 

 the percentage of occurrence would be increased 

 by 20 to 40 percent for all months except Au- 

 gust. During February, May, and November 

 the surface temperature of 20 to 30 °C remains 

 nearly constant through the ducting layer, but 

 the relative humidity decreases from a surface 

 value of 70 to 80 percent to values ranging from 

 20 to 40 percent. The dry air in the upper layer 

 results from subsidence in the eastern margin 

 of the Pacific high pressure cell, which shifts 

 northwestward in August, thus decreasing the 

 ducting incidence in Mazatlan but increasing it 

 in lower California and the Hawaiian Islands 

 (figs. C-21, C-23, C-25, C-27). 



Area (5) : The center of most intense ducting 

 in the Caribbean Sea and Gulf of Mexico changes 

 with the seasons (figs. C-41 through C-56). The 

 smallest percentage of superrefractive ground- 

 based gradients is found in February, with the 

 stronger gradients concentrated near the east 

 coast of Central America. By May the super- 

 refractive area has shifted eastward into the 

 Caribbean and northward into Florida. In Au- 

 gust it includes parts of the eastern U.S. but is 

 still most intense in the Swan Island area, and 

 in November the area encompasses all of the 

 Caribbean. The ground-based superrefractive 

 layers are thicker than 100 m approximately 

 70 percent of the year, but the ducting layers are 

 never intense enough to exceed the 1-percent 

 trapping level for 300 Mc/s. 



Area (6) : The cause of superref raction in the 

 western Mediterranean and northwest Africa 

 is very similar to that in area (4) . During the 

 summer season, subsidence along the eastern 



edge of the Atlantic high-pressure cell superim- 

 poses a dry layer over the marine surface layer ; 

 during the winter season, the major subsidence 

 area shifts southward, the temperatures 

 throughout the surface layer are 5 to 10 °C low- 

 er, and the percentage of superrefraction and 

 trapping incidence decreases. 



Area (7) : During the long Antarctic night, in- 

 tense radiation from the snow-covered ground 

 keeps the surface temperature much lower than 

 that in the air several hundred meters above. 

 This temperature inversion of 10 to 25°C is the 

 cause of all the ducting gradients in May and 

 August, which trap frequencies <1000 Mc/s at 

 least 40 percent of the time (see appendix E). 



5.4. Discussion of Cumulative Distributions 

 of Ground-Based Gradients 



Data from 22 representative stations were 

 selected as a sample of the kinds of ground- 

 based gradient distributions from the surface 

 to 100 m which occur in various climates and 

 locations throughout the world. 



Interesting similarities which exist among 

 the gradient distributions imply that the re- 

 f ractivity climate of any station may be related 

 more to the season or month of the year than to 

 any particular latitudinal location. For in- 

 stance, consider the interesting relationships 

 between Bangui (a tropical station), Bordeaux 

 (temperate), and Amundsen-Scott (arctic). 

 The gradient distribution at Amundsen-Scott 

 in February resembles that of Bordeaux in 

 February, but its August distribution slope re- 

 sembles that of Bangui in May. However, Ban- 

 gui's distribution slope and range in August 

 resembles Bordeaux in May. Amundsen-Scott 

 forms another interesting climatic triad with 

 Saigon and Long Beach. In August the distri- 

 bution slope and range of Amundsen-Scott is 

 very similar to that of Saigon (a tropical sta- 

 tion), but the negative gradient intensity is 

 about 100 iV-units greater at all percentage lev- 

 els. However, Saigon in May, before the mon- 

 soon, resembles Long Beach in February. 



It was expected that a pronounced diurnal 

 effect would exist in the gradient structure near 

 the surface, so two stations, Aden, Arabia, and 

 Nicosia, Cyprus, where data were available at 

 two thermally opposite times of day — 2 and 3 

 a.m. and 2 and 3 p.m. (0000 and 1200 GMT 6 ) — 

 were studied. Figures C-57 and C-71 in appen- 

 dix C show the diurnal differences in the cumu- 

 lative distribution of initial gradients from 

 to 100 m for these two stations for the 4 months 

 studied. 



Superrefractive conditions normally accom- 

 pany nocturnal inversions. At Nicosia this 



6 GMT (Greenwich Mean Time) is the same as UT (Universal 

 Time). 



