METEOROLOGY — FORECASTING 245 
application of the method can therefore be of direot 
assistance in anticipating radar performance for short 
periods in advance or for regions where detailed mete- 
orological observations may be limited. Enough par- 
ticulars, together with charts and nomograms, are 
given to enable one with meteorological training to 
apply these prediction techniques and thus facilitate 
daily or hourly adjustments to make optimum use of 
radar equipment. 
Computed Climatological Information on Surface 
Ducts. To obtain a broad picture of the variation in 
radio and radar ranges likely to be encountered in the 
western Pacific region, average duct widths (height 
from the base to the top of the duct) have been com- 
puted. These computations are based on the relation- 
ships between meteorological elements and radar per- 
formance mentioned above and utilize climatological 
data consisting of monthly averages of air tempera- 
ture, humidity and sea temperature, and monthly 
frequencies of winds with specified direction and 
speed. 
The area covered: includes the Japanese islands, the 
coasts of Korea, Manchuria, and China, the northern 
Philippines, the Marianas, the Bonins, and the Ryukyu 
Islands—approximately 10° to 50°N latitude and 120° 
to 150° E longitude. The computations indicate the 
percentage of time surface ducts of various widths may 
be expected at different times of the year and at dif- 
ferent locations within the region. This information is 
summarized in tabular form. The results are not in- 
tended to represent an accurately detailed picture but 
do give a sufficiently close approximation of average 
conditions influencing certain aspects of radio and 
radar performance so that they may be used as a 
guide in long-term operational planning. 
Radio-Meteorology 
Evipencrs or NonsTaNDARD PRoPaGATION 
Since the start of the war, cases of very long radio 
ranges and radar coverages, together with extreme 
variations of these quantities, have become well known 
to personnel working at microwave frequencies. Such 
phenomena, when due to influences acting on the 
propagated electromagnetic waves and not to freak 
behavior in set performance, have been classed under 
the term nonstandard propagation. It has been found 
that nonstandard propagation (such as is illustrated 
by the behavior of microwaves when they are con- 
strained to follow a path of such curvature that the 
Tays remain close to the surface of the earth and hence 
reach otherwise inaccessible targets—a phenomenon 
frequently referred to as trapping) is directly associ- 
ated with certain conditions that occur in the lower 
levels of the atmosphere (normally below 5,000 ft) 
which have been given the name of ducts. Detailed 
analyses of the structure of ducts have been presented 
adequately in the previously mentioned reports. Hence 
the paragraphs immediately following give only a brief 
and somewhat simplified description of the meteor- 
ological elements associated with ducts. 
METEOROLOGICAL ConpiTIONS AssocIATED 
witH Ducts 
Remarks on Pressure, Temperature, and Humidity. 
The meteorological situations in which trapping of 
microwaves occurs involve certain types of stratifica- 
tion in the lower levels of the atmosphere. The amount 
of stratification is dependent on the vertical distribu- 
tions of pressure, temperature, and humidity. 
Although the atmospheric pressure at any particular 
elevation and, to a lesser extent, the rate at which 
pressure decreases with altitude may vary from one 
time to another, these variations are relatively unim- 
portant as far as their direct influence on propagation 
is concerned and so-may be neglected in practical 
considerations. 
On the other hand, temperature and its change with 
altitude do have an immediate bearing on duct forma- 
tion. Under more or less average conditions through- 
out the troposphere, the temperature decreases with 
increasing altitude and the term “temperature lapse 
rate” is defined as the rate of decrease of temperature 
with height (and consequently is usually expressed in 
degrees Fahrenheit per 1,000 ft or degrees centigrade 
per kilometer). For reference purposes a “standard” 
lapse rate has been taken as 3.47 EF ner 1.000 ft. 
(Further details of the National Advisory Committee 
on Aeronautics standard atmosphere are given in 
the Appendix on page 258.) Under certain conditions 
the temperature throughout a layer of the atmosphere 
may increase with height, in which case a temperature 
inversion (Figure 10) is said to exist. 
ty) 
ELEVATED TEMPERATURE 
INVERSION 
ELEVATION ————> 
GROUND 
TEMPERATURE 
INVERSION 
TEMPERATURE —————-> 
Ficure 10. Vertical variation of temperature showing 
@ ground inversion EF and an elevated inversion BC. 
The slopes of the portions FG, AB, and CD denote 
standard conditions, a decrease of temperature with 
elevation. 
