TROPOSPHERIC PROPAGATION AND RADIO METEOROLOGY 159 
conditions preventing mixing of the lowest layers by 
frictional turbulence and maintaining the thermal 
stratification caused by radiative cooling or local 
breezes; clear skies producing nocturnal cooling 
over land. 
The conditions in a barometric Low, on the other 
hand, generally favor standard propagation. A lifting 
of the air, the opposite of subsidence, usually occurs 
in such regions and is accompanied by strong winds. 
The combined effect is to destroy any local thermal 
stratification and to create a deep layer of frictional 
turbulence. The air is therefore well mixed, and 
nonstandard vertical temperature and moisture 
gradients are wiped out in the early stages of their 
creation. Moreover, the sky is usually overcast in a 
low-pressure area and nocturnal cooling, therefore, 
is negligible. 
To summarize, high-pressure regions, clear skies, 
and calm air are conducive to duct formation, while 
low-pressure areas, cloudy skies, and winds favor 
standard refraction. 
Fronts in the atmosphere are possible sources of 
refractive effects. A front is a surface of discontinuity 
which separates two air masses of different tempera- 
tures. The surface slants at an angle of 1° to 2° with 
the horizontal, with the colder air forming a wedge 
under the warmer air. Fronts are a common occur- 
rence in the atmosphere, and it might be thought 
that they should have a considerable influence on 
wave propagation. This is, however, not borne out 
by English radar experience, which shows very little 
superrefraction connected with fronts. The explana- 
tion is probably that fronts are invariably accom- 
panied by low-pressure areas, and turbulence along 
a front is usually so strong that the transition from 
the cold air to the overlying warm air takes place 
continuously over a vertical distance of about a 
kilometer. Propagation conditions might, however, 
be somewhat different with fronts in sub-tropical 
climates, although our knowledge is still inadequate 
on this point. In one-way transmission frontal effects 
have been studied to a limited extent (pages 160- 
164). 
Seasonal and Global Aspects 
of Superrefraction 
Although the general picture is still incomplete, 
enough is now known about the geographical and 
seasonal aspects of superrefraction to warrant a 
general summary. 
ATLANTIC Coast OF THE UNITED STATES 
Along the northern part of this coast superrefrac- 
tion is common in summer, while in the Florida 
Tegion the seasonal trend is the reverse, with a 
maximum 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 pro- 
nounced maximum in the summer months. Conditions 
in the Irish Sea, the Channel, and East Anglia have 
been studied by observing the appearance or non- 
appearance of fixed echoes (see Figure 33). Additional 
2 212 ie O 6 1212 ie Oo 6 12 
TIME GMT 
SEPTEMBER 
OCTOBER 
Fiaur# 33. Diurnal frequency of long-range fixed echoes 
at North Foreland, Kent. Wavelength 10 em. 
data based on one-way communication confirmed the 
radar investigations. 
MEDITERRANEAN REGION 
The campaign in this region provided good oppor- 
tunities for the study of local propagation conditions. 
The seasonal variation is very marked, with super- 
refraction 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 beyond the normal radio 
range. 
Superrefraction in the central Mediterranean area, 
is caused by flow of warm, dry air from the south 
(sirocco) which moves across the ocean and thus 
provides an excellent opportunity for the formation 
of ducts. In the winter time, however, the climate in 
the central Mediterranean is more or less a reflection 
of Atlantic conditions and hence is not favorable for 
duct formation. 
Tue ARABIAN SEA 
Observations covering a considerable 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 that blows from early June to mid-September 
and covers the whole Arabian Sea with moist equa- 
torial air up to considerable heights. Where this 
meteorological situation is fully developed, no occur- 
rence of superrefraction is to be expected. In accord- 
ance with this expectation the stations along the 
west side of the Deccan all report normal conditions 
during the wet season (middle of June to middle of 
September). During the dry season, on the other 
hand, conditions are very different. Superrefraction 
then is the rule rather than the exception, and on 
