132 TECHNICAL SURVEY 
HEIGHT IN FEET 
—= uP 
TEMPERATURE 
Ficure 9. Formation of temperature inversion over land 
due to nocturnal cooling. 
tion in the lowest layers is such as to reinforce or at 
least not to counteract the effect of the temperature 
distribution, that is, when the moisture decreases 
not too slowly with height. Nocturnal cooling is 
greatest with clear skies and is quite small under 
an overcast. Hence guided propagation over land. 
occurs at night almost exclusively with clear skies. 
This type of temperature inversion is strictly confined 
to land areas. It does not occur over the ocean because 
the sea temperature does not show appreciable daily 
variations. Temperature inversions caused by noc- 
turnal cooling are most pronounced over dry land 
(desert) but will occur almost anywhere over land 
with a clear sky and.a not too humid atmosphere. 
SUBSIDENCE 
Another weather phenomenon favorable to trap- 
ping is subsidence. By subsidence is meant the slow 
downward motion, combined with horizontal spread- 
ing, of air above the lowest layers of the atmosphere. 
This process, which most frequently occurs in the 
area of barometric high, will produce temperature 
inversions; the subsiding air moreover becomes rel- 
atively much drier than the unaffected air below. In 
general the subsidence inversion is quite high (e.g., 
above 4,000 to 5,000 ft). In the light of present 
knowledge it appears that high subsidence inversions 
do not generally affect guided propagation when the 
sets are situated at low altitudes. It appears, how- 
ever, that such subsidence inversions might materi- 
ally affect communications or airborne radar search 
aloft. Lower subsidence inversions (1,000 to 2,500 
ft) along the southwestern coast of the United States 
are known to produce stable duct layers affecting 
radar coverage at low angles. 
TURBULENCE OF THE AIR © 
This has a distinct normalizing effect in that it 
tends to smooth out the temperature and moisture 
variations which are conducive to guided propaga- 
tion. Moderate to strong winds produce a turbulent 
layer extending normally to a height of about 4,000 
ft. The air is well mixed within this layer, and 
consequently the standard type of refraction prevails. 
Regions of a barometric low are characterized by 
strong to moderate winds and pronounced turbu- 
132 
lence in the lower layers. In addition low pressure 
areas usually have overcast skies. Hence a barometric 
low will as a rule lead to propagation of the standard 
type. 
FREQUENCY OF OcCURRENCE 
It is extremely difficult to estimate in general 
terms the frequency of occurrence of guided propa- 
gation, since statistical data are almost nonexistent 
at present except for very limited regions in Europe 
such as the North Sea. In the central Mediterranean 
during the summer months of 1943, ducts have been 
observed on 9 days out of 10. Frequent trapping has 
also been observed in some parts of the Pacific. At 
other times and places guided propagation might be 
an unusual occurrence, especially if the barometric 
pressure is generally low and the winds strong. It 
seems advisable to consult a weather officer with 
regard to any given locality. 
MEASUREMENTS 
In order to determine weather’s influence upon 
radar in a quantitative way, the variation of refrac- 
tive index with height must be determined. This 
requires accurate knowledge of the temperature and 
moisture distribution in the lowest few hundred or 
thousand feet of the atmosphere. The ordinary 
radiosonde is not well adapted to measurements of 
this type because the measured points on an ascent 
are usually spaced several hundred feet apart. Among 
the methods which have been developed for this 
purpose during the past two years, the one most 
generally adopted uses a captive balloon (or kite) 
which carries aloft electrical temperature and moist- 
ure—measuring elements. These are connected to a 
meter on the ground by means of thin wires attached 
to the cable holding the balloon. This device permits 
measurements at intervals as closely spaced as 
desired. A psychrometer held out of the window of a 
slowly flying plane has been used with good success 
in the absence of more elaborate equipment. 
CLOUD ECHOES IN RADAR 
Cloud echoes (more precisely, precipitation echoes) 
are observed frequently on radar scopes. At times 
they have caused confusion by blotting out other 
targets. Their similarity, upon certain occasions, to 
actual targets have caused some difficulty in the 
interpretation. of the signals. 
These echoes are caused by a reflection of the radar 
pulse from the raindrops in the clouds (or in rain 
storms). The amount of reflection increases very 
rapidly with frequency. Cloud echoes are qnite 
exceptional below about 1,000 mc. In microwave 
radar they first appeared as a nuisance, but more 
recently they have been put to practical use. In 
tropical climates they are very helpful for aerial 
navigation. 
