INSTRUCTIONS FOR MAKING PILOT BALLON 

 OBSERVATIONS 



INTRODUCTION 



As the original edition (1928) of this pubhcation has become ex- 

 hausted and numerous amendments thereto are necessary, it is 

 deemed appropriate to pubhsh a revised edition at this time. 



Observations with pilot balloons are made for the purpose of 

 determining wind direction and velocity at various altitudes; also the 

 height, direction, and velocity of clouds whenever the balloons enter 

 their bases. The apparatus and methods used are described in detail 

 in the following sections. Briefly, the observations are made by 

 following with a theodolite the flight of small rubber balloons filled 

 with hydrogen or helium gas. The angles of azimuth and elevation 

 are observed and recorded, and these data, together with the balloon's 

 altitude at successive intervals of time, make possible the construction 

 of a horizontal projection of the path followed and the determination 

 from this projection of wind direction and velocity at various levels. 



Pilot-balloon stations are of two kinds, viz, "single-theodolite" and 

 "double- theodolite." Similarly, methods of observation are known 

 as "single-theodolite" and " double- theodohte" methods. 



By the first method the theodolite is set over a fixed point and 

 oriented with zero azimuth on the south point. The ballon is assumed 

 to rise at a constant rate of speed. Computation involving the ascen- 

 sional rate, elevation angle, and azimuth angle determines the position 

 of the balloon. By the double-theodolite method, two theodolites 

 are homogously oriented (that is, with zero azimuth in the same 

 direction), one at either end of a definite base line. Computation 

 involving the length and bearing of the base fine and the observed 

 azimuth and elevation angles determines the horizontal position of the 

 balloon and its height above the surface. 



Since the two methods have so much in common, and since the 

 majority of the stations now in operation are, and probably most of 

 those of the future will be, of the single-theodolite type, these instruc- 

 tions will treat chiefly of the single-theodolite method. However, 

 departures from this method applicable to the double-theodolite 

 method will be fully considered herein. 



In order to facilitate the work, most of the pilot-balloon stations of 

 the Weather Bureau are equipped with portable telephone^. ? ,(See 

 frontispiece.) By their use the wind data may be obtained %iiile the >, 

 balloon is being followed. :" 



^Ilibr.aryj S 

 58125 



