Page:Popular Science Monthly Volume 90.djvu/690

 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

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