Chapter 9 

 GENERAL METEOROLOGY AND FORECASTING 



INTRODUCTION 



I 



refractive index as 



N chapter 5, equation (9) was given for the 



V .n 



(ft - 1 ) • 10 6 = y ( P ~ e + ~^l ■ 



On adding to this the term (/i/a)10 6 the "modified 

 refractive index M" of equation (4), Chapter 6 is 

 obtained, namely 



M = ( ft - 1 + - ) • 10 6 



a, 



(2) 



When the temperature increases with height, other 

 things being constant, n — 1 decreases with height 

 and when this decrease is strong enough 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 7 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. 



In connection with the transmission experiment 

 across Massachusetts Bay, which was described in 

 Chapter 8, 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 244 and in this country. 

 The most extensive American texts on the subject 

 have been issued by Headquarters, Weather Divi- 

 sion, AAF, 245 and by the Columbia University Wave 

 Propagation Group. 157 The latter report, Tropos- 

 pheric Propagation and Radiometeorology is published 

 in Volume 2 of the Summary Technical Report of the 

 Committee on Propagation. 



92 ATMOSPHERIC STRATIFICATION 



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 



75 



