42 TECHNICAL SURVEY 
an important part of the radiation trajectory of 
coastal radars. These breezes develop only under 
fairly calm conditions; they are wiped out by a 
moderate or strong wind. 
The advective ducts of the types described here 
are by their very nature of only limited horizontal 
extent. The horizontal variation of refractive index 
presents a problem that till now has not been sys- 
tematically studied from either the experimental or 
the theoretical angle. 
A particular type of duct has been discovered in 
purely maritime air, that is, air which has had an 
extremely long sea trajectory and thus should have 
reached an approximately steady state of diffusion 
relative to the underlying sea surface. The Antigua 
experiments described in the preceding chapter reveal 
the existence of a type of low duet which seems to be 
characteristic of maritime air. It appears probable 
that similar ducts are permanent in the oceanic 
regions of many parts of the earth. The relative 
humidity of the air at Antigua was found to be 60 
to 80 per cent, indicating that a continuous upward 
diffusion of moisture must take placé, since the air 
immediately adjacent to the water surface is always 
practically saturated. On the other hand, there is 
little difference between the air and sea tempera- 
tures in this case, the ocean being about 25 C while 
the air temperature varies between 23 and 26 C. 
The ducts are therefore caused solely by the varia- 
tion of water vapor in the lowest layers and are 
much lower than the advective ducts described 
before, their height rarely exceeding 40 ft. Typical 
M curves have been shown in Chapter 3, and, for 
the particular effects caused by the low height of 
these ducts, we refer to the discussion of the experi- 
‘mental results. 
The diurnal change of ocean temperature is 
insignificant, except in extremely shallow water, and 
therefore, at some distance from the coast, propaga- 
tion conditions do not show any appreciable diurnal 
variation. 
DYNAMIC EFFECTS 
The physical processes in the lower strata of the 
atmosphere which determine the formation of ducts 
are to a considerable extent controlled by the large- 
scale dynamics of the atmosphere. It is therefore 
often possible to make at least a qualitative forecast 
of propagation conditions on the basis of a knowledge 
of the synoptic weather situation. An example in 
point is the diurnal variation over land in clear 
weather from standard conditions during the day 
to duct conditions in the latter part of the night 
and the early morning hours. 
Conditions in a barometric low pressure area 
generally favor standard propagation. Winds are 
usually strong or at least moderate resulting in a 
well-mixed layer of frictional turbulence. Local 
thermal stratifications are destroyed, and abnormal 
moisture gradients will not develop because of the 
intense turbulent mixing. The sky is frequently 
overcast in the low pressure area and nocturnal 
cooling therefore is often negligible. 
On the other hand, meteorological conditions in 
a high pressure area are frequently favorable for 
the formation of ducts. The sky is commonly clear, 
thus giving rise to pronounced nocturnal cooling of 
the ground and to the attendant formation of a 
temperature inversion in the lowest layers. This, 
again, often gives rise, by evaporation, to steep 
moisture gradients within the inversion layer result- 
ing in the formation of ducts in the manner already 
described. Winds in high pressure areas are often 
slight, or a calm prevails, resulting in a formation 
of local thermal stratifications and of land and sea 
breezes. 
One of the prime phenomena conducive to non- 
standard propagation conditions in‘a barometric 
high is subsidence, already described. Subsidence is 
closely connected to high pressure areas on the 
weather map and is always found in such areas, but 
it is not always intense enough to produce an 
inversion. The typical pattern of air flow in a baro- 
ILLUSTRATING SUBSIDENCE (SINKING) IN HIGH PRESSURE AREA 
IGH PRES~ 
SURE AREA 
AS IT APPEARS 
ON THE WEATH- 
ER MAP 
THE AIR AS IT SINKS GET: Ri N 
ELS-AND A TEMPERATURE INVERSION IS CREATED 
-_— f v v Y \ Sa VERTICAL 
Sea Re ~~ EctiON 
se INVERSION REGION | cel 
(A ey 
UNAFFECTED AIR 
Ficure 3. Schematic diagram illustrating subsidence in 
a region of high barometric pressure. 
metric high is shown in Figure 3 in both horizontal 
projection and vertical cross section. 
The air in the lower parts of a region of subsidence 
is very dry because it has descended from a high 
level in the atmosphere where the temperature is low 
and hence the saturation vapor pressure is small. 
If such air is located over a surface capable of evap- 
oration such as the ocean, a steep moisture gradient 
may be established at some level above the ground. 
This is the most common mechanism for the forma- 
tion of elevated ducts. Quite often subsidence com- 
bines with some or the other effects mentioned 
earlier enhancing their tendency toward the forma- 
tion of the duct. The elevated ducts found: in the San 
Diego region are perhaps the most outstanding exam- 
ple of this type of dynamically induced stratification. 
The effect of fronts in the atmosphere upon propa- ~ 
