RADIO REFRACTIVE INDEX PROFILES 133 



the physical structure of the lowest layers of the atmosphere. This proc- 

 ess results in air with different refractive index characteristics being 

 brought into the area. The most common and important case of advec- 

 tion is that of dry air above a warm land surface flowing out over a cold 

 sea. This type of advection frequently appears in the English Channel 

 during the summer when the weather has been fine for several days. 



Advective duct formation depends on two quantities: (a) the excess of 

 the unmodified air temperature above that of the water surface, and (b) 

 the humidity deficit (the difference between the water vapor pressures of 

 the modified and unmodified air). If these quantities are large, especially 

 the humidity deficit, an intensive duct may form. 



Important advective processes can also occur over land, but the condi- 

 tions required for duct formation occur less frequently. However, a duct 

 may be formed when dry, warm air flows over cold, wet ground with 

 resultant temperature and humidity structure previously discussed. 



Radiation: Differences in daytime and nighttime radiation are the 

 causes of diurnal variation in refractive conditions. A subref ractive layer 

 may be present during the day, especially at the time of maximum surface 

 heating. Clear skies and light surface winds at night result in consider- 

 able cooling of the earth, thus causing the formation of temperature in- 

 versions. The surface heat loss produced by nocturnal radiation is a 

 prime factor in the formation of temperature inversions. Atmospheric 

 stratification formed by such a combination of meteorological parameters 

 may jjroduce a trapping layer. A temperature inversion is seldom strong 

 enough to produce a duct in the middle and low latitudes, but it is of 

 major importance in the formation of ducts in the northern latitudes. 

 Low stratus clouds or extreme amounts of moisture (as in the troj^ics) 

 tend to prevent loss by radiation which lessens the possibility of duct 

 formation. 



If the ground temperature in a nocturnal inversion falls below the 

 dewpoint temperature, the water vapor in the lowest layers of the air 

 condenses and the heat of condensation is released directly to the air. 

 Under conditions of radiative fog formation, the humidity lapse tends to 

 counteract any temperature inversion present and may cause substandard 

 refraction if the humidity "inversion" is sufficiently strong. However, 

 the temperature inversion may be strong enough to keep the layer 

 standard or superrefractive. 



Subsidence: Subsidence is the slow settling of air from a high-pressure 

 system. The air is heated by adiabatic compression as it descends and 

 spreads out in a layer well above the earth's surface. This process pro- 

 duces stable layers and inversions of temperature with an accompanying 

 decrease in relative humidity. Since the air has come from a high level 

 in the atmosphere, it is dry and may overlay a cooler, moist air mass. 



