98 



METEOROLOGICAL EQUIPMENT FOR SHORT WAVE 



rather slow-flying plane is essential, in order to keep 

 the dynamic temperature correction small and also in 

 order to insure a not too excessive rate of evaporation. 

 Thermocouples, thermopiles, and temperature-sen- 

 sitive resistors frequently are used as temperature 

 responsive elements in place of actual wet and dry 

 bulh thermometers. They are incorporated in spe- 

 cially designed electrical bridge circuits in which the 

 temperatures are read on either indicating or record- 

 ing meters. 



^■^ ' Temperature and Humidity 



Resistance Elements 



Temperature 



Temperature-sensitive resistors arc satisfactory both 

 with regard to accuracy and the absence of lag. The 

 British have used platinum resistance thermometers 

 very successfully in stationary installations. In the 

 United States electrolytic or ceramic resistance ele- 

 ments are commonly used. The latter can be made to 

 change their resistance several fold over a relatively 

 narrow temperature interval. Their accuracy is tliei'e- 

 fore limited, not so much liy the accuracy of the cur- 

 rent measurement as by their intrinsic stability after 

 calibration, proper radiation shielding, etc. 



The electrolytic element developed for the Bureau 

 of Standards radiosonde^'- has a time-lag constant 

 (time required to attain the fraction (1 — e~M = 0.(33 

 of the total change) of 8 sec at an airspeed of 3 m per 

 second, of 14 sec at an airspeed of 1 m per second, and 

 of 40 sec in still air. 



Eecently the ceramic Sanborn element" has come 

 into use; it has about the same lag characteristics as 

 the electrolytic element but is practically free from 

 aging. The following time-lag constants have been 

 measured :* 8 sec at an airspeed of 3 m per second and 

 12 sec at an airspeed of 1 m per second. Another 

 source reports 20 sec at an airspeed of 5 m per second 

 (this value seems too large in comparison with the 

 others) and 42 sec in still air. (See footnote h, pg. 97.) 



Moisture 



The Bureau of Standards resistance element as well 

 as the Gregory humidiometer (a British development) 

 uses a dilute solution of lithium chloride. 



In the Bureau of Standards element the lithium 

 chloride film is deposited on the surface of a thin cyl- 

 inder on which there is a bifilar winding of two thin 



"Manufactured by Paul H. Sanborn, 2602 Riverview Drive, 

 Parkersburg, W. Va. 



wires. The stability and aging characteristics of this 

 element are described in the literature. ^'^ An average 

 actual accuracy of 5 per cent relative humidity is 

 claimed for the ordinary radiosonde when used under 

 routine conditions. Higher accuracy (1 per cent EH) 

 is claimed, at least at temperatures above freezing, 

 when used with captive balloon equipment,"^" partially 

 Ijecause the current is frequently reversed to cut down 

 polarization effects and partially because the calibra- 

 tion can be more closely watched. Tests^ show that at 

 an airspeed of 2.5 m per second the time lag constant 

 is 3 sec at 24 C and 11 sec at C. 



The Gregory humidiometer''''^ uses a lithium chlo- 

 ride solution soaked in a clean cotton cloth. The re- 

 sistance of the clement changes from over 100,000 

 ohms at 30 per cent RH to as little as 50 ohms at 100 

 per cent EH. It undergoes pronounced aging during 

 the first several days and then remains sensibly con- 

 stant for a number of weeks. The instrument is in an 

 experimental stage and is at present being tried out at 

 the Eye towers in Sussex (see Section T.1.6, below). 



"•'■* Circuit Design for Resistor Elements 



Thermocouples or thermopiles are commonly used 

 in a conventional bridge circuit. In connection with 

 tlie electrolytic and ceramic type of resistance ele- 

 ments, circuits have recently been developed that in- 

 clude certain features novel in the technique of atmos- 

 pheric measurements. 



In the equipment developed liy Washington State 

 College"" the standard radiosonde temperature ele- 

 ment was originally used, but in a more recent type 

 they have combined the Sanborn temperature element 

 and the radiosonde electrolytic humidity element. The 

 electric equipment (Figure 1) consists of a dry cell 

 with potentiometer supplying about % volt, two 

 double-pivot microammeters, one in series with each 

 of the elements, and a 6-volt d-c motor. The relay re- 

 verses the current through the elements at a rate of 

 50 cycles (100 reversals) per minute while maintain- 

 ing constant polarity at the meters. The current is 

 smoothed by large condensers in parallel with the 

 meters. The commutation eliminates polarization of 

 the electrolytic elements and greatly increases their 

 accuracy and useful life. The commutation period is 

 so selected that it is long enough to prevent inductive 

 and capacitative interaction between the two circuits 



''Information supplied to the U. S. Propagation Mission to 

 England. 



''Instruments made by Negretti and Zamba, Ltd., London. 



