KRONFELD ON GLIDING AND SOARING.pdf - Lakes Gliding Club

300 KRONFELD ON GLIDING & SOARING
The dimensions 1 of the spheres of influence exercised
by mountains upon the movements of wind are largely
determined by observations of cloud formation. On the
other hand there are gaps above the downwind fields in
cloud masses of fairly regular formation ; from the size
of these it is possible to measure the extent of the downwind
area. The sinking air is warmed, the fine particles of
water contained in the cloud dissolve and become invisible ;
the cloud disperses and the weather clears up. This is the
reason why tracts of land lying on the down-draught side of
high mountains usually enjoy cloudless and beautiful
weather. On the other side of the mountain, however,
where we find the up-current field, clouds are likely to become
over saturated owing to the constant accession of fresh
moisture from the plain below. There are then so many
particles of moisture in the air that it cannot sustain them
all ; some begin to sink, carrying others with them on their
way, and fall to the earth as rain. There is always much
rain on the up-current side of mountains. By means of
annual rainfall measurements in certain places it is easy
to ascertain where the influence exercised by the mountains
on the wind begins, for this is the point at which most rain
falls. We may also see in our rivers an excellent point of
comparison with those clouds arising over mountains as
the result of great up*currents. If a rock juts out from
a river bank into the stream, the water will be turned aside
by it, while behind the rock, in the direction of the course of
the stream, a small patch of white foam forms. In the
process of cloud formation the earth would correspond to
the river bank, the mountain to the rock, the air current to
the flowing water and the clouds themselves to the foam
continually forming and dispersing at the same spot.
If we are to believe the latest findings of meteorology,
clouds are formed on a large scale on the boundaries between
warmer and colder bodies of air. There are mighty masses
of cold air in the polar regions and warm air above the
temperate zones and the tropics. Atmospheric conditions
throughout the world are said to be affected by the interpenetration
of these masses of air, which do not flow evenly
1 See p. 295