Chapter 5 
GENERAL METEOROLOGY AND FORECASTING 
INTRODUCTION 
I“ CHAPTER 1, equation (9) was given for the 
refractive index as 
Pe, age 7 8006) 
ries T 
On adding to this the term (h/a)10® the ‘‘modified 
refractive index M”’’ of equation (4), Chapter 2 is 
obtained, namely 
*) 10°. 
a 
(1) 
M=(n-1+ (2) 
When the temperature increases with height, other 
things being constant, n — 1 decreases with height 
and when this decrease is strong encugh it will 
outweigh the increase of M caused by the term h/a. 
Similarly, a decrease of moisture with height will 
produce a decrease of n — 1 which, if strong enough, 
will again produce a negative slope of the M curve. 
In Chapter 3 we have dealt with these changes 
purely from the observational viewpoint. Now the 
origin of these variations owing to the physics and 
dynamics of the lower atmosphere will be considered. 
A knowledge of general meteorological conditions 
may enable a trained weather forecaster to predict, 
from weather maps and other pertinent data relat- 
ing to the structure of the lower atmosphere, the 
presence of ducts and other meteorological factors 
affecting transmission. 
The first attempts at radio forecasting were made 
as early as 1943 by the British Meteorological Office 
in conjunction with the services operating the radar 
sets along the North Sea and Channel Coast. While 
the correlation between forecasts and observed results 
was imperfect, results were promising enough to 
encourage further studies. Since then, the forecasting 
technique in the British home waters has been 
developed to a considerable degree of effectiveness. 
Studies regarding the relationship between the 
dynamics of the lower atmosphere and radio wave 
propagation have been initiated by the interested 
Services in various parts of the British Empire, 
particularly in Australia where a number of interest- 
ing correlations have been discovered. In the United 
States the problem was first systematically attacked 
by the propagation group of the Massachusetts 
Institute of Technology Radiation Laboratory 
([MIT-RL], and at about the same time by the 
Army Air Forces Tactical School in Florida. The 
latter established a training course for radio meteoro- 
logical forecasters, a number of whom participated 
in offensive operations in the Pacific at Leyte and 
later. 
39 
In connection with the transmission experiment 
across Massachusetts Bay, which was described in 
Chapter 4, a forecasting unit was established coopera- 
tively by MIT-RL, the AAF, and the U.S. Weather 
Bureau at Boston. Regular forecasts were made and 
checked by both meteorological and radio observa- 
tions. The pertinent information required for radio 
meteorological forecasting was assembled by a 
number of agencies in England”“4 and in this country. 
The most extensive American texts on the subject 
have been issued by Headquarters, Weather Divi- 
sion, AAF, 24 ‘and by the Columbia University Wave 
Propagation Group.!? The latter report, Tropos- 
pheric Propagation and Radiometeorology is published 
in Volume 2 . 
ATMOSPHERIC SFRATIFICATION 
From the meteorological viewpoint it is convenient 
to distinguish three factors which tend to affect the 
temperature and moisture distribution in the lower 
part of the atmosphere. These factors are known to 
meteorologists as (1) advection, (2) nocturnal cooling 
(over land) and (3) subsidence. 
Advection is a term that designates the horizontal 
displacement of an air mass of specific properties 
over an underlying surface which tends to modify 
the structure of the mass. Thus one speaks of the 
advection of dry polar air over a warm water surface. 
Advection is not the modification of air mass proper- 
ties but merely a preliminary to such modification. 
Advection changes the physical characteristics of 
the lower strata of the atmosphere through transfer 
of heat or moisture between the air and the under- 
lying ground or sea surface. The operating factor in 
this exchange is turbulence, and a brief review of its 
effects in the atmosphere will be given. 
Nocturnal cooling over land is caused by a loss of 
heat from the ground by infrared radiation. The 
cooling thus effected is communicated to the lower 
strata of the atmosphere by means of turbulence. 
Nocturnal cooling occurs to an appreciable degree 
only if the sky is clear. Any layer of clouds will exert 
a “blanketing”’ effect which reduces the cooling of 
the ground to a small fraction of that for clear nights. : 
Subsidence is a meteorological term for the slow 
vertical sinking of air over a very large area. It is 
usually found in regions where barometric highs are 
located. By a dynamic process, too complicated to be 
described here, subsidence often produces a temper- 
ature inversion, the air in a subsiding stratum being, 
as a rule, very dry. Subsidence is usually strongest in 
