16 A WORLD ATLAS OF ATMOSPHERIC RADIO REFRACTIVITY 



proves to be the case for all seasons, with a very 

 definite maximum in August, when anticyclonic 

 upper air circulation intensifies the humidity 

 decrease aloft and radiational cooling lowers the 

 surface temperature 15 °C below that found 

 during the day. 



Aden, a coastal station with less change in its 

 diurnal temperature cycle than Nicosia (an in- 

 terior valley station on a fairly large island), 

 exhibits superrefraction day and night for all 



seasons. During August and November initial 

 gradients have a wide range of values, with the 

 largest variation occurring at 0000 GMT, but 

 in February and May the nocturnal stability 

 apparently is seldom destroyed by convective 

 mixing, and the 1200 GMT (1500 LST) initial 

 refractivity gradient may exceed the 0000 GMT 

 (0300 LST) gradient. However, the early 

 morning inversion is usually more significant 

 from a radio-meteorological viewpoint because 



i 



-10 



-50 -60 -70 



N-GRADIENT 



-100 



-110 



-120 



FIGURE 5. Five-year mean vertical refractive gradient profile: Aden. 



the refractivity gradient is much more intense 

 from 250 m to 750 m, thus affecting more radio 

 frequencies. This can be noted in figure 5, rep- 

 resenting a 5-year mean of the vertical gradient 

 observed from to 4.5 km during May. 



Figure 6 presents the same data for Nicosia 

 during a 5-year August period. Because scale 

 heights are also a measure of stability and strat- 

 ification, figures 5 and 6 not only give the differ- 

 ences in the mean total iV-gradient values in the 

 lower atmosphere, but also the wet (H w ) and 

 dry {H\) scale heights. Five-year mean values 

 of N s at Aden in May were found to be 382 



(surface wet term of 123) at 0000 GMT and 

 374 (surface wet term of 119) at 1200 GMT. 

 The corresponding values at Nicosia in August 

 were 341 (wet term of 83) and 310 (wet term 

 of 62). 



The percentage of occurrence of subrefraction 

 (iV/km>0) is larger at night (0000 GMT) in 

 February and November at both locations. 

 However, this diurnal trend is much more pro- 

 nounced at Nicosia, particularly in November 

 when the percentage of night subrefraction is 

 over 30 percent larger than the daytime per- 

 centage of occurrence. 



