That Cloud— How Fast Is It Drifting? 



How statistics are gathered upon which meteorologists 

 base their theories concerning the atmospheric circulation 



HOW high 

 are the 

 clouds? 

 How fast do 

 they move? In 

 what directions? 

 Answers to these 

 questions are o.f 

 great interest to 

 the student of 

 the atmosphere, 

 because the 

 clouds furnish 

 the same kind of 

 informa tion 

 about the move- 

 ments of the air 

 at high levels 

 that we ob- 

 tain near 

 r t h 



the 

 by 

 of 



e a 



means 



weather - 



vanes and an- 



emometers. 



Accurate 



measurements 



of the altitude 



of clouds may 



be obtained by 



observations 



made simul- 

 taneously from 



two points with 



theodolites, 



or by a corresponding photographic 

 method. Information on this subject 

 has also been ob- 

 tained by noting 

 the location, with 

 respect to the ob- 

 server, of the 

 shadow of a cloud 

 on the ground, in 

 conjunction with 

 the angular alti- 

 tude of the sun. 

 These data give 

 us the base and 

 the angles of a 

 triangle, of which 

 the altitude may 

 be computed trig- 



A mirror instrument for 

 measuring cloud motions 



onometrically. 

 The altitudes of 

 clouds range all 

 the way from 

 nil, when the 

 cloud rests on 

 the ground and 

 constitutesafog, 

 up to about 

 seven miles, in 

 the case of the 

 feathery clouds 

 known as "cir- 

 rus." 



The instru- 

 ments used in 

 observing the 

 motions of 

 clouds are called 

 nephoscopes, and 

 these are of two 

 principal types, 

 distinguished as 

 reflecting nepho- 

 scopes and direct- 

 vision nepho- 

 scopes. In the 

 former the 

 movements of 

 the cloud are 

 observed in a 

 black mirror, 

 and this is the 

 type commonly 

 employed in the 



United States. In the latter the cloud's 



motion is observed directly, with relation to 



some fixed object. 



The picture on the following page repre- 

 sents an example of the second type; 



viz., the "comb nephoscope," 



invented by Dr. Louis Besson, 



of Paris. This consists of an 



upright brass rod about 9 feet 



long, bearing at its upper end a 



cross piece 33^ 



feet long, to 



which a number 



of equidistant 



vertical spikes 



are attached. 



The rod is 



mounted in a 



The nephometer, a convex mirror, the surface of 

 which is divided by lines into ten sections, is used 

 to determine degrees of cloudiness or relative areas 



The type of measuring in- 

 strument known as the 

 Marvin mirror nephoscope 



674 



