40 TECHNICAL SURVEY 
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 Weurve. 
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 
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. 
T GROUND T 
Ficure 1. Air temperature versus height for a tempera- 
ture inversion. 
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 
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 
about 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 scmewhat 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 temperatureinversions 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 
