2 INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 

 I. STATIONS AND OBSERVATION POINTS 



1. Selection of observation point. — The selection of an observation 

 point for single-theodolite work will be determined by the following: 



(a) Geographical location. 



(6) Low horizon. 



(c) Angular altitude of obstructions. 



(d) Convenience to office. 



In addition to the above, double-theodolite work will also depend 

 upon — 



(e) Base hne, length, and bearing. 



(/) Unobstructed view along base line. 



(g) Common level of primary and secondary points. 



2. The shape of the earth's surface and the obstructions on it 

 influence the surface winds and the winds aloft in the lower levels. 

 When the surface is abnormal or the obstructions are pronounced, 

 the local influences will be met with through 1,000 or 1,500 meters 

 in elevation. Therefore, if these modifying influences are so marked 

 as to cause divergence from the normal wind conditions, the data 

 obtained will be abnormal and local rather than normal and of the 

 general wind circulation near the surface. And, since we are more 

 interested in the conditions of the general circulation than of the 

 local influences, a choice of station must be made which will give as 

 nearly as possible wind data little affected by local influences. 



3. An ideal observation point would be in the open, level country 

 or on the crest of a shght rise. The ground should be firm and the 

 position well removed from buildings and tall trees that might inter- 

 fere with the Hne of sight upon the balloon. The maximum angular 

 altitude of obstructions such as buildings and trees should never 

 exceed 6° above the sensible horizon. Smokestacks and chimneys 

 in close proximity to the station give much annoyance and should 

 be avoided, if possible, since even slight amounts of smoke therefrom 

 are sufficient to obscure the balloons. 



The observation point should be as convenient to the administra- 

 tion oflice as satisfactory location will permit. In single- theodolite 

 work this may well be on the roof of the office building or one near 

 by. In double- theodolite work either primary or secondary point, 

 and sometimes both, will necessarily be a little distance from the 

 administration office. 



4. The observation point for single-theodolite work may be any 

 convenient point from which, as far as possible, an unobstructed view 

 may be obtained. Since most of the administration offices are located 

 in or near cities or large towns, satisfactory ground conditions with 

 convenient location will seldom be found. The next in order will be 

 a position on a flat-roofed, well-exposed building. In the selection 

 of such a site due consideration must be given to superstructures 

 such as towers, penthouses, cupolas, etc. Where it becomes imprac- 

 ticable to observe from the top of the superstructure itself, a position 

 to one side of the structure, and sometimes one also on the opposite 

 side, will answer. Many instances will arise in which the range of 

 vision is affected only by the central structure. When this is not too 

 high, the difficulty may be overcome by erecting upon the main roof 

 a platform from which to observe, but, in general, two points of obser- 

 vation, one on either side, are preferred to one on top, inasmuch as 



