GENERAL METEOROLOGY AND FORECASTING 43 
gation does not seem to be very pronounced. This is 
probably due to the fact that in a front the transition 
between warm and cold air is comparatively gradual 
extending over a height of perhaps 1 km. In the Eng- 
lish propagation experiments some effects of fronts 
have indicated slightly substandard conditions with 
warm fronts and slightly superstandard conditions 
with cold fronts. Often, however, the effect of fronts 
upon radio propagation is negligible. This, of course, 
refers only to the frontal region itself and not to the 
change in air mass and attendant propagation con- 
ditions connected with the passage of a front. 
WORLD SURVEY 
It clearly appears from the preceding sections 
that climate has a fundamental influence on the 
nature of propagation conditions. A systematic 
attack on the problem of the occurrence of ducts over 
the ocean has been made in England on a world-wide 
scale.2°° Monthly maps based on estimates. drawn 
from general low-level weather data, giving regions 
of the most frequent occurrence of superrefraction 
and substandard refraction, were issued. However, 
these need much further checking by actual observa- 
tions. The propagation features of some important 
parts of the world where some knowledge has been 
accumulated is outlined briefly below. 
Atlantic Coast of the United States. Along the north- 
ern part of this coast superrefraction is commonin 
summer, while in the Florida region the seasonal 
trend is reversed, a maximum occurring in the winter 
season. 
Western Europe. On the eastern side of the Atlantic, 
around the British Isles and in the North Sea, there 
is a pronounced maximum in the summer months. 
Conditions in the Irish Sea, the Channel, and East 
Anglia have been studied by observing the appear- 
ance or nonappearance of fixed echoes. Additional 
data based on one-way communication confirmed the 
radar investigations. 
Mediterranean Region. The campaign in this region 
provided good opportunities for the study of local 
propagation conditions. The seasonal variation 1s 
very marked, with superrefraction more or less the 
rule in summer, while conditions are approximately 
standard in the winter. An illuminating example is 
provided by observations from Malta, where the 
island of Pantelleria was visible 90 per cent of the 
time during the summer months, although it lies be- 
yond the normal radar range. 
Superrefraction in the Central Mediterranean area 
is caused by the flow of warm, dry air from the south 
(sirocco) which moves across the ocean, thus pro- 
viding an excellent opportunity for the formation of 
ducts. In the winter, however, the climate in the 
Central Mediterranean is more or less a reflection of 
Atlantic conditions and hence is not favorable for 
duct formation. 
The Arabian Sea. Observations covering a con- 
siderable period are available from stations in India, 
the inlet to the Persian Gulf, and the Gulf of Aden. 
The dominating meteorological factor in this region 
is the southwest monsoon which blows from early 
June to mid-September and covers the whole Arabian 
Sea with moist equatorial air up to considerable 
heights. Where this meteorological situation is fully 
developed, no occurrence of superrefraction is to be 
expected. In accordance with this expectation, all the 
stations along the west side of the Deccan report 
normal conditions during the southwest monsoon 
season. During the dry season, on the other hand, 
conditions are very different. Superrefraction then is 
the rule rather than the exception, and on some oc- 
casions very long ranges, up to 1,500 miles (Oman, 
Somaliland), have been observed with fixed echoes 
on 200-me radar, based near Bombay. 
When the southwest monsoon sets in early in 
June, superrefraction disappears on the Indian side 
of the Arabian Sea. However, along the western 
coasts conditions favoring superrefraction may still 
linger. This has been reported from the Gulf of 
Aden and the Strait of Hormuz, both of which lie 
on the outskirts of the main region dominated by 
the monsoon. The Strait of Hormuz is particularly 
interesting as the monsoon there has to contest 
against the “‘shamal’’ from the north. The Strait 
itself falls at the boundary between the two wind 
systems, forming a front, with the dry and warm 
shamal on top, and the colder, humid monsoon 
underneath. As a consequence, conditions are favor- 
able for the formation of an extensive radio duct, 
which is of great importance for radar operation in 
the Strait. 
The Bay of Bengal. Such reports as are available 
from this region indicate that the seasonal trend is 
the same as in the Arabian Sea, with normal condi- 
tions occurring during the season of the southwest 
monsoon, while superrefraction is found during the 
dry season. It appears, however, that superrefraction 
is much less pronounced than on the northwest side 
of the peninsula. 
The Pacific Ocean. This region appears to be the 
one where, up to the present, least precise knowledge 
is available. There seems, however, to be definite 
evidence for the frequent occurrence of superrefrac- 
tion at some locations, e.g., Guadalcanal, the east 
coast of Australia, around New Guinea, and on 
Saipan. Along the Pacific coast of the United States, 
observations indicate frequent occurrence of super- 
refraction, but no statement as to its seasonal trend 
seems to be available. The same holds good for the 
region near Australia. 
In the tropics there is a very strong and persistent 
seasonal temperature inversion, the so-called trade 
wind inversion. It has no doubt a very profound 
influence on the operation of radar and short wave 
communication equipment in the Pacific theater. 
