40 INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 



in lining up the points parallel to the initial line, determine the 

 direction for the same minute. This method is recommended as a 

 time saver. 



122. Graphing board. — The wind-direction and velocity data are 

 represented graphically on a sheet of cross-section paper (Form 

 No. 1115-Aer.). A simple graphing board is furnished to all pilot- 

 balloon stations to aid in the preparation of these graphs. (See 

 fig. 12.) It consists of an ordinary clipboard, 14 inches square, on 

 one side of which a metal strip is mounted. The metal strip aids in 

 holding the form in place and acts as a guide for a celluloid triangle, 

 on which wind directions in degrees are graduated. An altitude scale, 

 indicating the assumed height of the balloon at the end of each 

 minute, is also graduated on the metal strip. 



123. Preparation of velocity-direction graph. — In preparing the 

 velocity-direction graph. Form No. 1115-Aer. is first placed on the 

 graphing board with the left-hand edge inserted beneath the metal 

 strip and the top inserted beneath the spring clip so that the left- 

 hand edge of the cross-section ruling is in line with and directly beneath 

 the right-hand edge of the metal strip. In order to graph the data 

 for levels above sea level, as is Weather Bureau practice, the form 

 must be adjusted downward until the zero or surface elevation on the 

 metal strip coincides with the altitude of the observation point above 

 sea level, as indicated on the altitude scale along the left-hand edge 

 of the form. The graphs are then constructed from this point as a 

 base. The first step in preparing the graphs is to plot pencil points 

 on the cross-section paper representing the direction (in degrees) and 

 velocity (in meters per second) of the surface wind and then the 

 same data for each successive minute. The pencil points are plotted 

 along the edge of the graduated triangle by placing the short leg of 

 the triangle flush against the metal altitude scale and then by referring 

 to the degree and velocity scales on the triangle to locate the points, 



124. When all points have been plotted, a smooth, curved line is 

 drawn in ink through the points. Decided irregularities and bumps 

 in either the velocity curve or the direction curve are to be checked 

 over by reference to the original data on Form No. lllOA-Aer. to see 

 that no errors have been made in computation. The velocity curve 

 is likely to be more irregular than the direction curve ; in fact, it may 

 be said that the actual direction curve will nearly always be smooth, 

 though irregularities, when at all present, are generally very prominent. 



125. Care should be exercised in locating the points for the direction 

 graph when changes in direction of 360° or more occur during the 

 observation in order to avoid breaking the graph at the right-hand 

 edge of the form. On some occasions it will, of course, be impossible 

 to avoid breaking the graph. In such cases the last point nearest the 

 edge of the form is to be plotted again at the proper location to the 

 left and the two points should then be joined by a horizontal dotted 

 line. The remaining points are then plotted with reference to the 

 same portion of the direction scale as that used in plotting the latter 

 of the above two points. 



126. In case the maximum altitude is over 7,000 meters above sea 

 level, that portion of the graph above 7,000 meters should be drawn 

 downward from the top of the form, and if the maximum altitude 

 is over 14,000 meters above sea level, that portion above 14,000 

 should be drawn upward from the bottom of the form. The down- 



