METEOROLOGICAL ASPECTS OF PROPAGATION PROBLEMS 
New South Wales in summer (but obliterated by pas- 
sage of a meridional front). 
10. Southeast coast of South Island, New Zealand, 
during a nor’wester (foehn wind from the Southern 
Alps). 
11. Around North Island, New Zealand, im summer. 
Inland it was found almost everywhere that condi- 
tions of unorthodox propagation, when they occurred, 
were largely associated with radiation nights. On such 
a night moderate superrefraction usually develops fairly 
quickly after sunset. If there is no tendency for forma- 
tion of fog, superrefraction usually continues all night, 
disappearing after sunrise. But if fog forms, superrefrac- 
tion may disappear during the night and may even be 
replaced by subrefraction before sunrise, after which 
conditions begin to return to normal. Fog, even over 
the sea, is often associated with unorthodox propaga- 
tion, but this may vary all the way from subrefraction 
to quite intense superrefraction. 
Most of the phenomena of unorthodox propagation 
outlined above were initially encountered in the course 
TEMPERATURE 
EXCESS 
HEIGHT 
VALUE FOR SURFACE 
VALUE FOR AIR MASS 
(a) POTENTIAL TEMPERATURE 
Fre. 1.—Modification of an air mass by surface conditions. 
of operational use of ordinary radar equipment. Some 
examples of the observational material from which it 
was necessary to work have been given in Weather [2]. 
The qualitative results obtained in this way, however, 
were followed by quantitative observations made by 
specially designed equipment using one-way propaga- 
tion mainly on wave lengths of 9 and 3 cm [10, 12, 
16, 17, 20]. 
It is clear therefore that the simple idea that radars 
(which operate on metre, decimetre, and centimetre 
wave lengths) cannot see targets beyond the geometrical 
horizon may, under suitable conditions of weather and 
climate, be violated on a stupendous scale. 
Explanation of Superrefraction 
From the outset there was, of course, no doubt that 
the phenomena described in the previous section were 
due to refraction or reflection in the troposphere. In 
the early days, some confusion was caused by repeated 
suggestions that reflection from the interfaces con- 
1291 
stituting ordinary meteorological fronts was the cause. 
In fact, however, reflections from ordinary frontal sur- 
faces, although sometimes observable at grazing inci- 
dence, are not the primary cause of superrefraction. 
But there is another way in which striking gradients of 
temperature and humidity can occur in the troposphere. 
Owing to advection, radiation, subsidence, or some com- 
bination of these phenomena, it can easily happen that 
the surface of the earth has a temperature substantially 
different from that representative of the superincum- 
bent air mass. An important gradient of temperature 
then occurs near the surface of the earth. In the same 
way, the humidity representative of the superincumbent 
air mass may differ appreciably from that existing close 
to the earth’s surface, and then there is an important 
gradient of humidity near the bottom of the air mass. 
Gradients of temperature and humidity that occur in 
this way are the main cause of unorthodox radio 
propagation. 
Two extremely important parameters im radio- 
meteorology are: (1) the temperature excess of the ap- 
HUMIDITY 
I DEFICIT | 
HEIGHT 
VALUE FOR AIR MASS 
VALUE FOR SURFACE 
| 
| 
I 
| 
| 
| 
| 
| 
| 
| 
| 
| 
| 
| 
I 
| 
I 
! 
t 
1 
(b) SPECIFIC HUMIDITY 
propriate air mass in relation to the earth’s surface, and 
(2) the humidity deficit of the appropriate air mass in 
relation to the earth’s surface. The first is the excess of 
the potential temperature of the air mass over the 
value corresponding to the earth’s surface, and the 
second is the deficit of the specific humidity of the air 
mass below that corresponding to the earth’s surface 
(see Fig. 1). Quite a good qualitative idea of the degree 
of superrefraction can often be derived purely from 
knowledge of the values of these two parameters, large 
positive values indicating marked superrefraction. For 
a more detailed understanding of superrefraction, how- 
ever, it is necessary to know not merely the temperature 
excess and humidity deficit but also the associated pro- 
files of potential temperature and specific humidity. 
From these it is possible, by means of a formula due to 
Englund, Crawford, and Mumford [8], to deduce the 
profile of the radio refractive index and hence the 
propagational properties of the atmosphere. 
The formula giving the radio refractive index n as a 
function of atmospheric pressure p in millibars, tem- 
