The law of the turning of the wind with altitude.
A casual observation of the lower clouds where no means of measuring
small angles is available will not usually show any difference between
the motion of the clouds and the surface wind; but with the upper clouds
the case is different, and one readily detects a difference.
Several thousand observations with various agencies, such as kites and
pilot balloons and more especially measurements made with theodolites
and nephoscopes, show that there is a definite twist to the right with
elevation. The amount of the deflection is shown in Figure 13. Turning
of the Wind with Altitude. Here the average yearly values are given for
directions and velocities. Thus if the mean wind direction at Blue Hill
is from a point a little to the north of west, 306 grads or 275 degrees,
and the mean velocity 7 metres per second; the clouds at 1000 metres
elevation will move from 312 or 280 degrees and at a speed of
approximately 11 metres per second (24 miles an hour).
These however, are average values. In individual cases the difference
between surface winds and stratus clouds may be considerably greater. It
may be as much as 180 degrees; that is, the cloud may move directly
opposite to the wind. In general there will be a difference of 10 to 20
degrees.