COVSTU. \M> MMJIIIMi: CONDITIONS 



77 



of convection during the day and conditions of a tem- 

 perature inversion during the night. There is some 



phase shift in that the atmospheric conditions lag 

 aboul three to four hours behind the sun. The amount 

 of nocturnal cooling caused by infrared radiation of 

 the ground is very nearly independent of its constitu- 

 tion. It is, however, strongly reduced by the presence 

 of clouds which in turn radiate toward the ground, 

 canceling part of the cooling effect. High moisture 

 content in the lower atmosphere acts partly in the 

 same way and somewhat reduces the heat lost by the 

 ground. With a full overcast, nocturnal cooling is 

 negligible and normally no temperature inversion 

 will be formed. 



In temperate climates temperature inversions alone 

 can produce only weak ducts because the effect of 

 temperature upon the refractive index is relatively 

 small. In the fairly common case, however, where the 

 inversion is accompanied by sufficient moisture 

 gradient, a strong duct will result. This occurs when 

 the air is dry enough to allow evaporation into it 

 from the ground. In warmer climates where the transi- 

 tion between night and day is rapid, evaporation may 

 set in early in the morning before the nocturnal in- 

 version has been completely destroyed by the action 

 of the sun. A strong duct will then be formed for a 

 short period. 



Fog. Contrary to what might perhaps be expected, 

 the formation of fog results generally in a decrease of 

 refractive index. For instance, when fog forms by 

 nocturnal cooling of the ground, the total amount of 

 water in the air remains substantially unchanged, 

 although part of the water changes from the gaseous 

 to the liquid state. It is found that water suspended 

 in the air in the form of drops contributes less to the 

 refractive index than the equivalent amount of vapor. 

 The formation of fog, therefore, reduces the effective 

 contribution of the water vapor to the refractive index. 

 If there is a temperature inversion in the fog layer, 

 the vapor pressure required for saturation increases 

 with height, and a substandard M curve usually 

 results. 



With a substandard M curve the electromagnetic 

 field near the earth surface is diminished instead of 

 increased, a case opposite to that of superrefraction. 

 In practice this weakening of the field not uncom- 

 monly leads to a more or less complete radio blackout. 



Fog, however, does not always produce a sub- 

 standard M curve although that is usually the case. 

 In certain less frequent types of fog, the temperature 

 and saturation vapor pressure may be constant or 



increase with height through the fog layer. In this 

 event propagation will be standard, or ducts may 

 even form occasionally within the fog layer. 



9 - 4 COASTAL AND MARITIME CONDITIONS 



Advection is of prime importance near a coast 

 where the wind may blow the air from land to sea 

 or vice versa. The former case, which is the more 

 important in practice, will be considered. A tempera- 

 ture inversion is formed, if the air from above a 

 warmer land surface flows out over a cooler ocean 

 surface. Over the land the air will usually have 

 attained a state of convective equilibrium with 

 correspondingly slow variations of temperature and 

 humidity with height. When this air comes in contact 

 with the cold water surface a temperature inversion 

 is formed which increases gradually as the air 

 proceeds over the water. Thus the inversion is the 

 more pronounced, the greater the distance from the 

 shore. Eventually, however, at very large distances, 

 equilibrium between the air and the water surface 

 will again be reached. 



The temperature inversion formed during this 

 process would in itself give rise to only a compara- 

 tively weak duct. When, however, the air is dry, 

 evaporation from the sea surface takes place simul- 

 taneously with heat transfer, and a fairly strong 

 negative humidity gradient is established in the 

 lowest layers. This combination of temperature inver- 

 sion and moisture gradient is very favorable for the 

 formation of a pronounced duct off shore. 



340 350 



M 



Figure 2. Successive M curves resulting from modifica- 

 tion of warm dry air over cool moist surface. Zero time 

 corresponds to the coastline; 1/4 hr, 1/2 hr, etc. refer to 

 the time the air has been over water. 



The progressive formation of an advection duct, 

 created by the mechanism just outlined, is shown 



