One must rise above the ground some distance to get the true air flow,
or what is known as the gradient wind, the flow which balances the
gradient, i.e. a flow along the isobars. The gradient velocity is found
about 300 metres above the ground, and the gradient direction a little
higher. The lower clouds as a rule indicate true wind values very well;
and so, it is desirable in studying winds to use cloud directions and
velocities rather than surface values. In cloud work a nephoscope is
essential. The unaided eye, unless properly shielded, suffers from the
glare of a sunlit sky; and moreover, there are no fixed points or
references. A black mirror, with suitable sighting rods and measuring
devices, enables an observer to follow the cloud, estimate its height
and determine with accuracy the direction from which it is moving. There
is an average difference of 30 degrees between the cloud direction and
the surface wind; the upper direction being more to the right. At times
the directions may be opposite.
[Illustration: FIG. 4. APHELIOTES--THE EAST WIND]
It may seem surprising but few of us, except at sunrise and sunset,
really see what is going on in cloud land.
Some meteorologists hold that the circulation of air 3000 to 5000
metres above the ground controls the path and perhaps the intensity of
storms. It is therefore important to know something of the flow at high
levels if we would improve the forecasts.