METEOROLOGY — FORECASTING 249 
1. The amount by which J/ decreases through the 
M inversion. 
2. The duct width, for the wider the duet the 
more energy will be trapped. 
3. The elevation of the transmitter with respect to 
the duct, the trapping being most complete when the 
transmitter is at the base of the M inversion. 
4, The angle at which the rays are propagated from 
the transmitter; the smaller the angle made with the 
top of the duet, the greater the range. 
5. The frequency of the propagated waves; in 
general, the higher the frequency, the greater the 
extent of trapping. 
Specific Relationships Between 
Meteorological Elements and 
Radar Performance 
ResearncH ON Forrcastine or Rapio 
AND Rapar RANGES 
Army Air Force Board Project. Realizing the im- 
portant and direct effects of temperature and humidity 
distributions on microwave propagation, several proj- 
ects have been undertaken in the attempt to develop 
a systematic method of forecasting the meteorological 
conditions leading to nonstandard propagation. Of 
these methods, one which has met with a considerable 
degree of success is described below. The methodology, 
developed by the Army Air Force Board working in 
conjunction with the Radiation Laboratory at MIT,? 
is designed to predict the formation of surface ducts 
over water. Its fundamental concepts are quite similar 
to those used in other methods of radio and radar 
forecasting.*° 
General Procedure. In essence the method consists 
of an analysis of the modification that air undergoes 
in the lower 1,000 ft as it moves from a large land mass 
out over the ocean. The study was carried out in the 
vicinity of Cape Cod, but indications are that the 
numerical factors entering into the procedure are 
much more generally applicable. In the modification 
of the air moving over the sea, the following assump- 
tions are made. 
1. The air initially (before moving off the land 
mass) is well mixed, i.e., it exhibits conditions close 
to neutral equilibrium (see Figure 11). 
2. The stability conditions of the air as it moves out 
over water are determined by its initial temperature 
(over land) relative to that of the sea surface. 
3. The modified air at the sea surface acquires the 
same temperature as the sea. 
4. In the modified air at the sea surface the mois- 
ture content becomes that corresponding to satura- 
tion at the sea surface temperature, except for a cor- 
rection owing to the salinity of the sea. 
5. The resulting M curve is determined by the 
quantities : 
a. Temperature excess.* 
b. M deficit.® 
e. Wind speed and direction. 
d. Distance of over-sea travel (in some cases). 
Thus the method attempts to relate duct formation 
to a limited number of easily determined meteoro- 
logical factors. It involves a simplified consideration 
of the upward diffusion of heat and moisture. It turns 
out, however, that the simplified assumptions yield 
results which in practical application are of sufficient 
accuracy to be of definite use in forecasting the ex- 
istence of nonstandard conditions. It should also be 
mentioned that, although the method is designed 
primarily for situations in which air over land moves 
out over the sea, it can also be satisfactorily applied 
to situations in which the air has a purely over-sea 
trajectory. 
The particular steps to be taken in carrying out the 
procedure follow. 
Metuop or DetermMiIninc Duct WiptH 
Observation of Initial Conditions. The necessary 
meteorological measurements to be taken should be 
as representative as possible, i.e., uninfluenced by 
purely local effects. Measurements are: 
1. Surface air temperature (of the unmodified air 
over land, in the case of air moving off a land mass) ; 
2. Surface air humidity (also of the unmodified 
air which can be expressed in terms of relative humid- 
ity, specific humidity, dew point, wet bulb tempera- 
ture, or vapor pressure) ; 
3. Sea surface temperature; 
4, Wind speed and direction (preferably at 1,000 ft- 
elevation) ; and sometimes 
5. Distance from land (of primary importance only 
in the case of stability conditions, when the air is 
warmer than the sea surface). ; 
All these data may, of course, be profitably supple- 
mented by aerological soundings, weather maps, and 
any other pertinent information available. 
Modification of Air by Sea Surface. As a qualitative 
description of the modification that the air undergoes 
in moving over water, three cases may be distinguished : 
1. Neutral equilibrium (resulting when the initial 
surface air temperature is the same as the sea temper- 
ature). The temperature structure of the air remains 
unchanged ; however, since the air is usually not com- 
pletely saturated, moisture is supplied to the lower 
layers by evaporation from the sea surface, in this way 
causing a greater decrease of humidity with height, 
which tends to establish an M distribution such that 
the modified refractive index is either constant or 
decreasing with height. In the case in which the air 
is initially completely saturated no modification takes 
place. 
®These terms are defined on page 250. 
