158 TECHNICAL SURVEY 
formation of fog results, in general, in a decrease of 
refractive index. When fog forms, e.g., by nocturnal 
cooling of the ground, the total amount of water in 
the air remains substantially unchanged, but part 
of the water changes from the gaseous to the liquid 
state. The contribution of a given quantity of water 
to the refractive index is found to be far less when 
the water is contained in liquid drops than when it 
exists in the form of vapor. The formation of fog, 
therefore, results in a reduction of the amount of 
water vapor contributing to the value of M. If there 
is a temperature inversion in the fog layer, the 
saturation vapor pressure increases with height, and 
a substandard M curve frequently results (see Figure 
20, curve Ib). This occurs with radiative fog (caused 
by nocturnal cooling of the ground) and also with 
advective fog (caused by the advection of warmer 
air over a cooler surface). Advective fog is very 
common in the Aleutian Islands and off Newfound- 
land. 
If fog causes a substandard M curve, it is to be 
inferred that the rays will be bent upward, instead 
of downward as with superrefraction, and lead to a 
weakening of the field in the lowest layers, even to 
the point of producing a complete fade-out of radio 
reception. Appreciable reduction of radar ranges and 
interruption of microwave transmission have 
frequently been observed in such cases. 
Fog, however, does not always produce a sub- 
standard M curve, though this is the most common 
case. In certain other less frequent types of fog, the 
temperature (and thereby the vapor pressure) may 
be constant or increase with height through the fog 
layer. In this event near-standard propagation will 
prevail, or a duct may develop when the temperature 
inversion is strong enough. An example is steam fog, 
formed when cold air passes over a warm sea (see 
also pages 160-164). 
Subsidence— Dynamic Effects 
The temperature inversions discussed so far owe 
their existence to the modification of air by contact 
with the ground, but subsidence inversions are 
produced by a mechanism of an entirely different 
nature. By subsidence is meant the sinking of air, - 
that is, a vertical displacement, which must of course 
be accompanied by a lateral spreading (divergence) 
in the lower part of the subsiding column of air; 
otherwise there would be an accumulation of air in 
the lower levels. The thermodynamic analysis of this 
complex process shows that if the effect of subsidence 
is strong enough a temperature inversion will be 
created. Since this process does not require the 
presence of a ground surface, it may occur, and in 
fact often does occur, aloft in the atmosphere. The 
effects of subsidence frequently are the most pro- 
nounced at an elevation of the order of a kilometer 
or more. 
As a general rule, subsidence occurs in regions of 
high barometric pressure. In fact, subsidence always 
does occur in such regions, but it may not always be 
intense enough to give rise to a strong temperature 
inversion. The flow of air in a barometric High is 
shown in Figure 32 as it appears on a weather map 
ILLUSTRATING SUBSIDENCE (SINKING) IN HIGH PRESSURE AREA 
IGH PRES-) 
SURE AREA 
THE AR_AS IT SINKS GETS WARMER—MORE SO IN THE HIGHER LEV 
ELS-AND A TEMPERATURE INVERSION IS CREATED 
A aa 
ei, LA \ De vertical 
EE INVERSION REGION Te SECTION 
ee 
UNAFFECTED AIR S~ LSS 
SD 
SURFACE 
FiGureE 32. Characteristics of subsidence. 
in horizontal projection, and also in a vertical cross 
section. 
With subsidence, the air as a rule is very dry, and 
there is nothing in the process which can change the 
moisture content or produce moisture gradients. If, 
however, the dry air finds itself over a surface capable 
of evaporation, such as the sea surface, a steep 
moisture gradient may be established and a duct 
will be created. It is thus seen that subsidence in 
itself does not produce a duct, except in extreme 
cases, but it can act as an auxiliary factor and greatly 
enhance the formation of a duct whenever other 
conditions are favorable. Thus, forecasts of super- 
refraction based on a purely advective mechanism, 
or purely on radiative cooling or evaporation, may 
have to be modified in the presence of subsidence; 
an otherwise very weak duct may be converted into 
a strong duct by the effect of subsidence upon the 
lower strata. 
Strong subsidence effects are of frequent occur- 
rence on the southern California coast where they 
may continue with little change for days at a time. 
At times the duct is elevated, giving an elevated 
S-shaped M curve like IIa in Figure 20. Again the 
duct may extend practically from the ground up 
with M curves similar to curves II or IIIb in Figure 
20. The elevation of the top of the duct may vary 
from 300 to 5,000 ft, and the thickness may lie 
between a few feet and 1,000 ft. Coverage diagrams 
and the corresponding M curves for several typical 
situations are illustrated in Figure 24. 
For a number of reasons the meteorological condi- 
tions in a barometric High are favorable for the 
formation of ducts. Among the favorable factors 
are: subsidence, creating very dry air into which 
evaporation from the surface can take place; again 
subsidence, creating temperature inversions; calm 
