30 INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 



up the value in the tables and record the distance on Form No. 11 lOA- 

 Aer. For such observations a hook or rack, which can be attached to 

 the theodohte, should be used to hold the clipboard, to which Forms 

 lllOA and 1115-Aer. are attached; also the slide rule or distance 

 tables, so that the observer's hands may be free to follow the balloon 

 when not engaged in recording data. 



89. Disappearance of balloon. — The cause of disappearance will be 

 recorded according to the following reasons. They stand in their 

 order of frequency and relative importance. 



1. Clouds: 



Against. 

 In base of. 

 Obscured by. 



2. Burst. 



3. Distance. 



4. Haze, smoke, fog, dust, etc. 



5. Sun. 



6. Obscured by — 



Tower. 



Chimney. 



Etc. 



7. Overhead: 



High elevation angle. 

 Rapid change of angles. 



8. Accident: 



Kicking of theodolite. 

 Allowed to pass off field. 

 Vibration of theodolite, etc. 



9. Abandoned. 



90. Disappearance of balloon due to clouds. — When the disappear- 

 ance is due to clouds it will be specifically stated whether against, 

 in, or beliind clouds. If this is not known, a statement to that effect 

 should be made. If the balloon is seen to enter the base of clouds, 

 particular attention will be given to the azimuth and elevation angles 

 of the balloon, and the fractional part of the minute of the balloon's 

 disappearance after the last minute reading. The product of this 

 fraction of a minute into the rate of ascent, when added to the alti- 

 tude of balloon for the last minute observed, will give the altitude of 

 the cloud base. The direction and velocity of clouds is then com- 

 puted in the same manner as for any other specific point of the pro- 

 jection. Care should be taken that the cloud direction as recorded 

 on the various forms agrees with the computed direction. If for 

 any reason it does not, a note to that effect should be made on the 

 forms. 



91. Wlien the cloud bases are dense and compact the balloons 

 disappear into them immediately upon reaching the cloud level, but 

 when there is no definite cloud base, as with fog-like stratus, the 

 moment of disappearance of the balloon is not the moment of reacliing 

 the cloud base. In fumisliing data for aviation purposes, the height 

 at wliich the balloon first enters cloud should be considered as the 

 true "ceihng" for the aviator. 



92. Disappearance of balloon due to distance, haze, etc. — Disappear- 

 ance due to distance will not occur during a short flight, except in 

 rare cases where there- is a very strong wind at the surface and aloft. 

 A distance of 10 kilometers is the minimum value for this entry. 



