INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 37 



result if the 10,000-meter distance scale is used for the early portion 

 of the observation and the 20,000-meter scale during the latter part. 

 In changing from one scale to the other it is necessary to repeat the 

 last two points on the first scale and plot them again as the first two 

 pomts on the second scale. When very hght, variable winds prevail 

 it sometimes becomes necessary, in order to avoid the crowding of 

 the points on the protractor, to multiply the horizontal distances by 

 some convenient factor such as 2, 5, or 10 before plotting. When 

 this is done the velocities as measured by the special velocity scale 

 must, of course, be divided by the same factor before recording them 

 on Form No. lllOA-Aer. 



114. The horizontal 'projection oj the balloon. — The immediate result 

 of the horizontal projection is to furnish a plan of the horizontal 

 movement of the balloon throughout the period during which it was 

 observed. On a very much smaller scale, each point represents the 

 actual horizontal position of the balloon at the time when the angles 

 were read on the theodolite. Since the balloon travels with the wind, 

 and since the horizontal projection is a scaled plan of the horizontal 

 movement, the direction and velocity of the wind movement may be 

 measured from the scaled plan. Special scales, graduated with 

 reference to the distance scales on the plotting board, are furnished 

 each station for use in measuring wind velocities. Wind directions 

 are taken directly from the protractor, as will be explained later. 



115. The horizontal projection will nearly always be a smooth, 

 even curve, though decided bends and sharp angles often exist. 

 The difference between an actual bend of the projection, even though 

 it be sharp, and a wavering of the projection due to poor data can 

 generally be detected. The projection should be examined care- 

 fully for errors in computation and angle readings before the wind 

 data is taken from it. Any points that look doubtful should be 

 checked. In this manner many errors may be corrected which 

 otherwise would not be detected. 



116. Velocity scales. — The scales used to determine the wind velocity 

 from the horizontal projection of the balloon on the plotting board, 

 are small cardboard scales, similar to those used to determine the 

 surface wind velocity from the triple-register sheets (Form No. 1017- 

 Metl.). They are graduated to indicate velocities in meters per 

 second when 2 minute intervals are measured, as will be explained in 

 the next paragraph. The graduations are at intervals of 0.6 cm. on 

 one edge of the scale and at intervals of 0.3 cm. on the other edge, 

 the former being used with the 10,000-meter distance scale and the 

 latter with the 20,000-meter distance scale. The division on the 

 velocity scales are subdivided in halves in order that the velocity 

 may be more readily read to meters and tenths. 



117. Obtaining wind data jrom horizontal projection. — The method 

 adopted by the Weather Bureau to obtain the wind data from the 

 horizontal projection for any point or minute is to take the mean 

 resultant direction and velocity over a 2-minute interval of time 

 extending from the beginning of the preceding minute to the end of 

 the following minute, thus placing the minute in question between 

 these limits. For instance, the direction and velocity for the fifth 

 minute is assumed to be the mean of the resultant from the fourth to 

 the sixth minute. 



