20 INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 



the station. A plan of these points will be constructed to some con- 

 venient scale and mailed to the central office for file along with a brief 

 description of the arrangement of equipment, obstructions to view, 

 etc. At two-theodolite stations an additional report will show the length, 

 bearing, and arrangement of base lines, and the exact altitude of all 

 secondary stations. A copy of the above reports should be retained 

 for the station record. 



Note that the bearing of orientation points and base lines should 

 always be measured in degrees from the north toward the right. In 

 orienting the theodolite for an observation, however, the azimuth 

 angle must read zero, or 360.0°, when the telescope points toward the 

 south. Consequently the theodolite setting on all orientation points 

 will be 180°, plus or minus the bearing of these points from north. 

 A list and diagramatic plan of the theodolite setting on all orientation 

 points should be posted in the balloon inflation shelter and a copy 

 kept in the station file for reference. 



III. PILOT BALLOONS 



47. Size, color, efc, oj balloons. — The balloons generally used in 

 pilot-balloon work are 6 inches in diameter when uninflated and 

 weigh approximately 30 grams. They are of five different shades — 

 uncolored, or pure gum, yellow, orange, red, and black. They are 

 manufactured in such a manner that they are wdthout seams and are 

 nearly spherical in shape when inflated. An extension of the longer 

 axis, about 2 inches in length, forms the neck or appendix tlu'ough 

 which inflation is accomplished. \\'lien these balloons are inflated to 

 ascend at the rate of 180 meters per minute, they will ascend at a 

 fairly constant rate up to 15 or 16 kilometers. For higher ascensional 

 rates larger balloons are used, weighing approximately 100 grams each. 



48. Selection of color of balloon for observation. — The proper color 

 of balloon to use for a particular observation is determined by the 

 appearance of the sky. ^Mien the sky is free from clouds, a pure gum 

 balloon gives the most satisfactory results. This is true, even when 

 haze, smoke, or dust is present in the lower levels, due to the fact that 

 this type of balloon gives the maximum reflection of sunlight, which 

 is the most important factor in observing balloons to great distances. 

 \yhen the sky is partly or completely overcast with thin cirrus clouds, 

 giving a wliite or gray background, yellow or orange balloons are best 

 suited for the condition. In this connection it is desired to point out 

 that there seems to be a tendency on the part of some observers to 

 use the yellow or orange balloons when there are no clouds but w^hen 

 haze, dust, or smoke are present. As previously stated, it is believed 

 that the pure gum balloons will, on the average, give better results 

 in such cases. The red balloons are best adapted for all-round use. 

 However, they are most easily seen against a background of white or 

 gray clouds. The black balloons should be used when the sky is 

 completely overcast wdth clouds or when the portion of the sky 

 serving as a background is overcast. They are also found to be 

 especially satisfactory for observations made in the enTly morning, 

 just before sunrise. In general, the uncolored, yellow, and orange 

 balloons are to be used upon comparatively clear days when there is 

 assurance that the sun will shine on the balloon throughout the ascen- 

 sion, and the red and black balloons are to be used on days when 

 clouds form the background for the observation. 



