38 INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 



118. Measuring wind velocity. — In using the wind velocity scales, 

 particular attention must be given that the wind scale used is that 

 corresponding to the scale upon which the plot was made. If the 

 projection is constructed upon the scale of 1 cm. = 200 m., then the 

 wind scale of the same base should be used. In applying the wind 

 scale always apply it to alternating points, or connect the points on 

 either side of the one being measured, from to 2, from 1 to 3, and 

 from 2 to 4, etc. The zero of the wind scale will be placed on the ear- 

 lier of the three points under consideration, and the graduated edge 

 will then be adjusted until it coincides with the last of the three points. 

 Note that the point being measured is the point between. At the 

 point of coincidence between the set scale and the last of the group, 

 the velocity for the intermediate point will be read off to tenths of 

 meters per second. The velocity of the last point may be determined 

 either by placing an auxiliary point in a position beyond the last point, 

 which will approximately satisfy the character of the last portion of 

 the projection, and measuring as before, or by applying the velocity 

 scale to the last minute interval and doubling the scale reading. The 

 latter method is sufficiently accurate when the final portion of the 

 projection is nearly a straight line and the points are evenly spaced, 

 but when the projection is curved the auxiliary point should be used. 

 The same rules should be followed when the balloon enters a cloud 

 layer and the final point represents a fractional part of a minute. 

 When the balloon enters a cloud, however, extreme care should be 

 exercised in extrapolating for the auxiliary point due to the fact that 

 the bases of cloud layers are frequently located at or just above a 

 level where an abrupt shift in wind direction occurs. This can be 

 checked to some extent by noting the change in azimuth angles just 

 prior to the disappearance of the balloon and by eye observation of 

 the cloud movement. 



119. Determining the wind direction. — Measurement of the wind 

 direction is no more difficult than measurement of the wind velocity, 

 though until thoroughly understood is more confusing. The direc- 

 tion to be determined, like the velocity, is the mean or resultant 

 direction from the point or minute in question, for the two-minute 

 interval extending from the beginning of the previous minute to the 

 end of the succeeding minute; thus, the direction of the wind for the 

 first, second, third, etc., minutes will be the resultant direction 

 between the points to 2, 1 to 3, 2 to 4, etc., respectively. 



To measure the resultant direction for point 1, it is only necessary 

 to rotate the protractor until the points to 2 are on the same straight 

 line. For instance, set both of these points over the scale base or 

 index line of scale, and read the direction in degrees from the edge of 

 the protractor over the index of the scale. To determine the direction 

 for succeeding minutes 2, 3, 4, etc., rotate the protractor until the 

 preceding and succeeding points to that being determined are arranged 

 on the same straight line parallel to the scale base; that is, to measure 

 the direction of point 2, arrange the points 1 and 3 so that they are in 

 the same imaginary line parallel to the index of distance scale. For 

 point 3, arrange 2 and 4 on the imaginary parallel line. Note that the 

 later numbered minute is always toward the operator when determin- 

 ing these directions, otherwise the directions will be 180° in error. 

 Read the direction of movement in whole degrees from the edge of the 

 protractor over the same index of distance scale as for the first minute. 



