Chapter 3 
METEOROLOGICAL MEASUREMENTS 
INTRODUCTION 
Aes DIRECT MEASUREMENT of the refractive index 
of air is carried out in the laboratory under 
closely controlled conditions. The variations of the 
refractive index in the atmosphere which are of 
paramount importance for propagation problems are 
determined indirectly by measurements of the tem- 
perature and humidity. From the values of these 
latter the refractive index is computed by equation 
(9) of Chapter 1. There has been no reason, so far, 
to doubt the reliability of this procedure, and specu- 
lative assumptions of the failure of this relation 
which have been brought forward at times during 
the war have not been accepted. 
This chapter describes measuring equipment that 
was especially developed during 1943 to 1945 to 
study refractive index variations. Following this 
description is a collection of actual M curves which 
have been measured in different parts of the world. 
TEMPERATURE AND HUMIDITY 
ELEMENTS 
The value of the refractive index n, or of M as 
defined by equation (4),Chapter 2, is sensitive to 
relatively small changes in temperature and especially 
in humidity. Both accuracy and speed in determina- 
tion of M are required. Speed is especially necessary 
because a considerable number of points generally 
are needed to determine the shape of an M curve. 
Electrical methods have been used almost exclusively 
for these measurements, though an ordinary psychro- 
meter will do in the absence of more specialized 
equipment. 
There is no particular difficulty in measuring the 
temperature with suitable accuracy, such as +0.2 C. 
The electric resistance element used in the Bureau 
of Standards radiosonde is well suited to the purpose 
and is commercially available. More recently ther- 
mistors have been used. At stationary installations 
in England ordinary nickel or platinum resistance 
thermometers have been installed, primarily for 
recording purposes. 
Humidity may be measured either directly, or 
indirectly by measuring the wet bulb temperature. 
Hair hygrometers are unsuitable because of their 
large time lag. For the direct measurement of 
humidity electrolytic resistance elements, such as 
are standard in the U.S. Weather Bureau radiosonde, 
are used. The active agent in this type of element is 
an aqueous solution of lithium chloride which is 
18 
deposited as a film on a small cylinder. The resist- 
ance of the solution is highly sensitive to changes in 
relative humidity of the surrounding air. In England 
a variant of this principle has been employed where 
the lithium chloride solution is absorbed in a cotton 
cloth. 
In the indirect method of measuring humidity a 
thermistor of cylindrical form is surrounded by a 
moist wick which, with proper aeration, indicates 
the. wet bulb temperature. To insure insulation the 
element is covered with several coats of insulating 
lacquer before the wick is attached. 
The main problem in all these devices is that of 
time lag. When mobile carriers such as captive 
balloons, kites, airplanes, or ships are employed, it 
is in general necessary to obtain an individual reading 
within less than a minute, and the response of the 
measuring elements to the temperature and humidity 
of the ambient medium must be reasonably close 
within the time available. 
The time lag constant is the time required to 
attain the fraction 1 — (1/e) = 0.63 of the total 
change, if the temperature (or humidity) is changed 
suddenly. For the temperature elements the time 
lag constant is several seconds in an air stream with 
a velocity of 2 to 5 m per sec. The lag depends 
somewhat on the position of the element relative to 
the air stream and is a maximum when the element 
is perpendicular to the stream. The lag constant of 
the same element, used as wet bulb indicator with 
wick applied, is only slightly larger than that of the 
dry element. The lag constant of the Bureau of 
Standards humidity element has been measured in 
several laboratories, and there seems to be some 
controversy as to its exact value, the results varying 
from a few seconds to about 45 sec,??8 the latter in 
an air stream of 2 to 5m per sec.?38 
THE WIRED SONDE 
Temperature and humidity elements of the type 
described are combined in a lightweight assembly 
which can be moved rapidly through the lower 
atmosphere. Such equipment, when first built in 
‘England, used dry and wet thermoriles,??” and 
soon thereafter the same method was adopted by 
the State College of Washington, ??°'23?)234 and, with 
slight modification, by the Navy Radio and Sound 
Laboratory [NRSL] at San Diego. 35 238 This design 
uses a combination of a resistance temperature 
element and an electrolytic humidity element. The 
instrument developed by the Radiation Laboratory 
