76 



GENERAL METEOROLOGY AND FORECASTING 



heat from the ground by infrared radiation. The 

 cooling thus effected is communicated to the lower 

 strata of the atmosphere by means of turbulence. 

 Nocturnal cooling occurs to an appreciable degree 

 only if the sky is clear. Any layer of clouds will exert 

 a "blanketing" effect which reduces the cooling of 

 the ground to a small fraction of that for clear nights. 



Subsidence is a meteorological term for the slow 

 vertical sinking of air over a very large area. It is 

 usually found in regions where barometric highs are 

 located. By a dynamic process, too complicated to be 

 described here, subsidence often produces a temper- 

 ature inversion, the air in a subsiding stratum being, 

 as a rule, very dry. Subsidence is usually strongest in 

 a layer somewhat elevated from the ground, and 

 when the dry subsiding mass overlies a moist stratum 

 near the ground, a sharp moisture gradient is created 

 which is favorable for the formation of the duct. The 

 elevated ducts at San Diego are of this type. 



Convection occurs whenever the vertical tempera- 

 ture gradient exceeds in absolute value the critical 

 gradient of about — 1 C per 100 m. It is usually the 

 result of the heating of the ground by the sun's rays, 

 and over land on a hot summer day it may extend to 

 great heights in the atmosphere. Since convection 

 mixes the air thoroughly, it establishes small and 

 constant moisture gradients throughout the lower 

 atmosphere, resulting in a very nearly linear Mcurve. 

 Consequently standard conditions of propagation 

 prevail on summer days over land from late morning 

 until late afternoon, this being the time when con- 

 vection is most likely to be present. Often this applies 

 also to summer days with a light overcast. 



Frictional turbulence occurs normally in the lowest 

 1,000 m of the atmosphere even when convection is 

 absent. It is caused by the wind, requires at least 

 light winds, and is fully developed with moderate or 

 strong winds over land. Since turbulence is caused by 

 the roughness of the ground it is less well developed 

 over the sea surface. It can safely be assumed that 

 over land with moderate or strong winds standard 

 propagation conditions prevail because of the reg- 

 ularizing action of turbulence. 



Temperature inversions occur when the temperature 

 of the sea or land surface is appreciably lower than 

 that of the air. The temperature transition from the 

 ground to the free air takes the form shown in Figure 

 1. The heat and moisture transfer caused by turbu- 

 lence in a temperature inversion is less simple than 

 that in a frictional layer. The turbulent processes in 

 inversion regions are highly complex and, as yet, are 



T GROUND 1 



Figure 1. Air temperature versus height for a tempera- 

 ture inversion. 



not very well explored. It is known, however, that 

 the intensity of the vertical transfer of heat and 

 moisture is much less than the rate of transfer with 

 frictional turbulence and decreases with the vertical 

 increase of temperature. In a steep inversion the rate 

 of transfer may be many times less than in a fric- 

 tional layer. This tends to produce a vertical stabiliz- 

 ation of the air layers in the inversion region. As soon, 

 therefore, as a temperature inversion has begun to 

 form, the rapid mixing in the lowest layers, usually 

 effected by frictional turbulence, stops and is re- 

 placed by a much more gradual diffusion. 



Assuming that the rate of diffusion has become so 

 slow that the transfer of moisture over a height of a 

 few hundred feet takes many hours or, perhaps, a 

 day or two, when the air in the inversion is dry to 

 begin with and flows over the sea or moist land there 

 will be established, in such an air mass, a steep mois- 

 ture lapse, since the water vapor that has been taken 

 up by the air near the ground will only gradually 

 diffuse into the dry air aloft. Conditions are then 

 favorable for the formation of an evaporation duct, 

 in addition to whatever tendency toward duct forma- 

 tion may be caused by the temperature inversion 

 itself. 



9.3 



CONDITIONS OVER LAND 



Because of the considerable variation of the ground 

 temperature by cooling at night and heating during 

 the day, there is to be found over land an alternation 



