Chapter 7 

 METEOROLOGICAL MEASUREMENTS 



7.1 



INTRODUCTION 



The 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 5. 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 



72 TEMPERATURE AND HUMIDITY 

 ELEMENTS 



The value of the refractive index n, or of M as 

 defined by equation (4), Chapter 6, 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 

 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, 228 the latter in 

 an air stream of 2 to 5 m per sec. 238 



50 



