\>()KI,I) SI H\ IA 



79 



generally favor standard propagation. Winds are 

 usually strong or at least moderate resulting in a 

 well-mixed layer of frictional turbulence. Local 

 thermal stratifications are destroyed, and abnormal 

 moisture gradients will not develop because of the 

 intense turbulent mixing. The sky is frequently 

 overcast in the low pressure area and nocturnal 

 cooling therefore is often negligible. 



On the other hand, meteorological conditions in 

 a high pressure area are frequently favorable for 

 the formation of ducts. The sky is commonly clear, 

 thus giving rise to pronounced nocturnal cooling of 

 the ground and to the attendant formation of a 

 temperature inversion in the lowest layers. This, 

 again, often gives rise, by evaporation, to steep 

 moisture gradients within the inversion layer result- 

 ing in the formation of ducts in the manner already 

 described. Winds in high pressure areas are often 

 slight, or a calm prevails, resulting in a formation 

 of local thermal stratifications and of land and sea 

 breezes. 



One of the prime phenomena conducive to non- 

 standard propagation conditions in a barometric 

 high is subsidence, already described. Subsidence is 

 closely connected to high pressure areas on the 

 weather map and is always found in such areas, but 

 it is not always intense enough to produce an 

 inversion. The typical pattern of air flow in a baro- 



ILLUSTRATING SUBSIDENCE (SINKING) IN HIGH PRESSURE AREA 



AS IT APPEARS 

 ON THE WEATH- 

 ER MA" 



THE AIR AS IT SINKS GETS WARMER -MORE SO IN THE HIGHER LEV- 

 ELS-AND A TEMPERATURE INVERSION IS CREATED 



/ 



\ 



INVERSION REGION 



VERTICAL 

 CROSS 

 SECTION 



UNAFFECTED AIR 



Figure 3. Schematic diagram illustrating subsidence in 

 a region of high barometric pressure. 



metric high is shown in Figure 3 in both horizontal 

 projection and vertical cross section. 



The air in the lower parts of a region of subsidence 

 is very dry because it has descended from a high 

 level in the atmosphere where the temperature is low 

 and hence the saturation vapor pressure is small. 



If such air is located over a surface capable of evap- 

 oration such as the ocean, a steep moisture gradient 

 may be established at some level above the ground. 

 This is the most common mechanism for the forma- 

 tion of elevated ducts. Quite often subsidence com- 

 bines with some or the other effects mentioned 

 earlier enhancing their tendency toward the forma- 

 tion of the duct. The elevated ducts found in the San 

 Diego region are perhaps the most outstanding exam- 

 ple of this type of dynamically induced stratification. 

 The effect of fronts in the atmosphere upon propa- 

 gation does not seem to be very pronounced. This is 

 probably due to the fact that in a front the transition 

 between warm and cold air is comparatively gradual 

 extending over a height of perhaps 1 km. In the Eng- 

 lish propagation experiments some effects of fronts 

 have indicated slightly substandard conditions with 

 warm fronts and slightly superstandard conditions 

 with cold fronts. Often, however, the effect of fronts 

 upon radio propagation is negligible. This, of course, 

 refers only to the frontal region itself and not to the 

 change in air mass and attendant propagation con- 

 ditions connected with the passage of a front. 



96 WORLD SURVEY 



It clearly appears from the preceding sections 

 that climate has a fundamental influence on the 

 nature of propagation conditions. A systematic 

 attack on the problem of the occurrence of ducts over 

 the ocean has been made in England on a world-wide 

 scale. 250 Monthly maps based on estimates drawn 

 from general low-level weather data, giving regions 

 of the most frequent occurrence of superrefraction 

 and substandard refraction, were issued. However, 

 these need much further checking by actual observa- 

 tions. The propagation features of some important 

 parts of the world where some knowledge has been 

 accumulated is outlined briefly below. 



Atlantic Coast of the United States. Along the north- 

 ern part of this coast superrefraction is common in 

 summer, while in the Florida region the seasonal 

 trend is reversed, a maximum occurring 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 pronounced maximum in the summer months. 

 Conditions in the Irish Sea, the Channel, and East 

 Anglia have been studied by observing the appear- 

 ance or nonappearance of fixed echoes. Additional 



