METEOROLOGICAL ASPECTS OF PROPAGATION PROBLEMS 
By H. G. BOOKER 
Cornell University 
Introduction 
We shall be concerned with the branch of radio- 
meteorology that deals with refraction of radio waves 
in the atmosphere. Refraction phenomena depend 
mainly on the profiles of temperature and humidity 
near, or comparatively near, the earth’s surface. The 
first section is a description of the phenomenon of extra 
downward refraction, or swperrefraction, as experienced 
at radar stations. The second section explains in broad 
outline how superrefraction comes about, while the 
third section focuses attention on the meteorological 
phenomena mainly responsible for it. In the last section 
some meteorological investigations that have been made 
especially in connection with radio propagation are 
discussed. 
Description of Supperrefraction 
The expression superrefraction is used because, even 
under conditions of orthodox propagation, there is some 
downward bending of radio waves in the atmosphere 
due to the normal decrease of air density with height. 
Radio people have a simple way of allowing for this 
normal downward refraction, so that it is with de- 
partures from orthodox atmospheric refraction that we 
have to deal. The most striking phenomena are as- 
sociated with increases in downward refraction, and 
particularly with situations in which the downward 
refraction of radio rays exceeds the curvature of the 
earth, although reductions in downward refraction— 
subrefraction—also occur. 
The phenomenon of superrefraction, although known 
before 1940 [8, 9], was brought vividly to attention 
during World War II by the development of radar. 
Under orthodox propagation conditions the region 
within which a radar may detect aircraft, ships, coast 
lines, etc., does not normally extend quite as far as the 
geometrical horizon, and radar vision of targets below 
the geometrical horizon is usually impossible. When 
radars came into operation, at first on a wave length of 
13 m, the expectation that they would not see beyond 
the geometrical horizon was broadly fulfilled, although 
there was some evidence that echoes from objects on 
the ground were dependent upon atmospheric condi- 
tions. It was especially noticed that, during an intense 
anticyclone in January 1940 (with snow on the ground), 
echoes were obtained from tall towers some distance 
beyond the geometrical horizon, and at the same time 
the normal ability of the station to detect aircraft was 
somewhat modified. As time went on, radars were de- 
veloped on progressively shorter wave lengths—7 m, 
1.5m, 50 em, 10 em, 3 em—and it was found that the 
shorter the wave length in use the more frequently were 
conditions of unorthodox propagation encountered and 
the more intense was the degree of superrefraction ex- 
perienced. Thus, when 10-cm radars were installed 
along the south coast of England, it was found that 
they were often able to see the coast of France even 
where the width of the English Channel was too great 
for ordinary visual observation of the opposite shore. 
The phenomenon, when it occurred, was usually most 
pronounced in the evening, though it sometimes started 
in the afternoon and extended far into the night. After 
a while it became clear that these conditions of unusual 
refraction were associated with fine weather and that 
they were a good deal more pronounced in summer than 
in winter. In addition it was realized that radars looking 
over land instead of over sea also experienced unortho- 
dox propagation conditions in fine weather, but not 
usually in the afternoon. 
Understanding of superrefraction was greatly as- 
sisted by the progressive deployment of radars in the 
various theatres of war. When 7-m and 1.5-m radars 
were established on coastal sites in the Mediterranean, 
it soon became clear that superrefraction in this area 
was much more intense than on the same wave lengths 
around Britain. The entire south coast of Sicily could 
frequently be seen with 1.5-m radars on Malta, and 
from time to time echoes from Greece and Sardinia at 
ranges of the order of 400 miles were obtained. Ships 
were sometimes plotted to a range of 200 miles, the 
range of the geometrical horizon being less than 20 
miles. In contrast, ranges on aircraft were occasionally 
said to be subnormal, though these reports were con- 
fused by the possibility that the performance of the 
equipment may not have been up to standard. It soon 
became clear that unorthodox propagation was intense, 
frequent, and widespread over the Mediterranean in 
summer, though conditions largely returned to normal 
in winter. 
During the course of the war many other areas of 
the world were found to be subject to superrefraction. 
These included: 
1. Coast of French West Africa in the neighbourhood 
of the Canary and Cape Verde Islands. 
2. Gulf of Aden in summer. 
3. Persian Gulf in summer. 
4. Northern part of the Arabian Sea during the 
Indian hot season. 
5. West coast of India except during the southwest 
monsoon. 
6. Bay of Bengal. 
7. North coast of Australia except during the north- 
west monsoon. 
8. North west coast of Australia all the year round, 
but particularly in summer. 
9. West and south coasts of Australia and coast of 
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