218 



TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 



continuously over a vertical distance of about a 

 kilometer. Propagation conditions might, however, 

 be somewhat different with fronts in sub-tropical 

 climates, although our knowledge is still inadequate 

 on this point. In one-way transmission frontal effects 

 have been studied to a limited extent (see Section 

 17.3.9). 



Seasonal and Global Aspects 

 of Superrefraction 



Although the general picture is still incomplete, 

 enough is now known about the geographical and 

 seasonal aspects of superrefraction to warrant a 

 general summary. 



Atlantic Coast of the United States 



Along the northern part of this coast superrefrac- 

 tion is common in summer, while in the Florida 

 region the seasonal trend is the reverse, with a 

 maximum in the winter season. 



Western Europe 



On the eastern side of the Atlantic, around the 

 British Isles and in the North Sea, there is a pro- 

 nounced maximum in the summer months. Conditions 

 in the Irish Sea, the Channel, and East Anglia have 

 been studied by observing the appearance or non- 

 appearance of fixed echoes (see Figure 33) . Additional 



Figure 33. Diurnal frequency of long-range fixed echoes 

 at North Foreland, Kent. Wavelength 10 cm. 



data based on one-way communication confirmed the 

 radar investigations. 



Mediterranean Region 



The campaign in this region provided good oppor- 

 tunities for the study of local propagation conditions. 

 The seasonal variation is very marked, with super- 

 refraction more or less the rule in summer, while 



conditions are approximately standard in the winter. 

 An illuminating example is provided by observations 

 from Malta, where the island of Pantelleria was 

 visible 90 per cent of the time during the summer 

 months, although it lies beyond the normal radio 

 range. 



Superrefraction in the central Mediterranean area 

 is caused by flow of warm, dry air from the south 

 (sirocco) which moves across the ocean and thus 

 provides an excellent opportunity for the formation 

 of ducts. In the winter time, however, the climate in 

 the central Mediterranean is more or less a reflection 

 of Atlantic conditions and hence is not favorable for 

 duct formation. 



The Arabian Sea 



Observations covering a considerable period are 

 available from stations in India, the inlet to the 

 Persian Gulf, and the Gulf of Aden. The dominating 

 meteorological factor in this region is the southwest 

 monsoon that blows from early June to mid-September 

 and covers the whole Arabian Sea with moist equa- 

 torial air up to considerable heights. Where this 

 meteorological situation is fully developed, no occur- 

 rence of superrefraction is to be expected. In accord- 

 ance with this expectation the stations along the 

 west side of the Deccan all report normal conditions 

 during the wet season (middle of June to middle of 

 September). During the dry season, on the other 

 hand, conditions are very different. Superrefraction 

 then is the rule rather than the exception, and on 

 some occasions very long ranges, up to 1,500 miles 

 (Oman, Somaliland), have been observed on 200-mc 

 radar on fixed echoes. 



When the southwest monsoon sets in early in 

 June, superrefraction disappears on the Indian side 

 of the Arabian Sea. However, along the western 

 coasts conditions favoring superrefraction may still 

 linger. This has been reported from the Gulf of Aden 

 and the Strait of Hormuz, both of which lie on the 

 outskirts of the main region dominated by the 

 monsoon. The Strait of Hormuz is particularly inter- 

 esting as the monsoon there has to contest against 

 the shamal from the north. The Strait itself falls at 

 the boundary between the two wind systems, forming 

 a front, with the dry and warm shamal on top, and 

 the colder, humid monsoon underneath. As a conse- 

 quence, conditions are favorable for the formation of 

 an extensive radio duct, which is of great importance 

 for radar operation in the Strait. 



