INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 25 



windy weather it would probably be possible to have only one window 

 or door open, but even this would provide for some ventilation. 



A sign, somewhat as follows, "Danger— No Smoking," should be 

 posted in a conspicuous place where the pilot balloons are inflated. 

 If practicable, both the inflation balance and the hydrogen cylinder 

 should be grounded to carry off any accumulation of static electricity 

 and thereby eliminate as far as possible the occurrence of static sparks. 

 The need for the above precautions is, of course, greatly reduced if 

 helium gas is used instead of hydrogen. 



IV. THE OBSERVATION 



69. Time of observation. — Pilot-balloon observations are usually 

 made four times daily at 5:00 and 11:00 a. m. and p. m., seventy-fifth 

 meridian time. A leeway of 1 hour either way from these scheduled 

 times is permitted, however, for the purpose of adjusting the obser- 

 vation times as closely as possible to airway flight schedules and also 

 for the purpose of obtaining the maximum number of observations, 

 even when adverse weather conditions prevail or are expected to pre- 

 vail at the scheduled starting times. Furthermore, if adverse weather 

 conditions prevent an observation being made during the scheduled 

 2-hour periods (4:00 to 6:00 and 10:00 to 12:00 a. m. and p. m., E. 

 S. T.), a delayed observation is to be made at the first opportunity 

 thereafter, but not later than 9:00 a. m. or p. m., E. S. T., for the 

 scheduled 5:00 o'clock observations and not later than 3:00 a. m. or 

 p. m. for the special 11:00 o'clock observations. A delayed observa- 

 tion will not be made, however, unless an altitude of at least 2,000 

 feet above the surface can be attained, as previously determined by 

 ceiling balloon or ceiling light. 



70. Steps in observation. — A pilot-balloon observation may be 

 divided into the following parts: Collection of data, computation and 

 plotting, and reduction and tabulation. 



The first part, collection of data, is the making of the observation 

 itself, and involves the taking and recording the balloon data, mete- 

 orological data, and observed readings of azimuth and elevation angles. 



Computation and plotting, the second part of the observation, 

 involves the work necessary to prepare the observed data for obtaining 

 the wind direction and velocity; that is, computing the horizontal dis- 

 tance of the balloon and plotting these distances on the plotting board. 



The third part of the observation consists of determining the wind 

 direction and velocity from the plotted data and of recording wind 

 data on the various forms, including the preparation of Form No. 

 1115-Aer. 



71. Recording meteorological data. — After the setting up of the theod- 

 olite is completed, the observer will record the current meteorological 

 data comprising clouds, the velocity of the wind, the current and 

 wet-bulb temperatures, the barometric pressure, and the relative 

 humidity. The surface wind direction will be determined from the 

 azimuth of the balloon a few seconds after it has been released and 

 should be recorded both in degrees and to the nearest 16 points of the 

 compass. Where a regular meteorological observation has been taken 

 within 15 minutes of the actual starting time of the balloon ascension 

 (time of balloon release) that meteorological observation may be used 

 instead of taking another. But in the event that a period of more 



