Pittsburg dispatch. [volume] (Pittsburg [Pa.]) 1880-1923, 1889-06-09

heated air seeks 10 get upward through the
everything colder layer and now and then
bores hole through 1t, forming a channel
of escape. ANY little accident of the sur
face may determine the point at which this
heated air, tending upward like a balloon,
forces its way through the cold air which
presses down upon it. A tall tree or Chin
ney may make an updraft and create point
of escape.-Fig. 2. As soon as the air begins
to move upward, we perceive that it takes
on a whirling motion, which at first in
wolves a column only few feet in diameter;
but under favorable circumstances it may
extend this diameter until the whirlwind
has width of so feet or more, a great
amount of air then rushing from every side
to find its way upward through the new
won path of escape. In this manner the fa-
miliar dust whirls of our open plains and
streets are formed. They continue until all
the patch OF locally heated air, which occu-
pies given field, is drained away into the
upper levels OF the atmosphere, and so the
overlying cooler air is allowed to settle down
upon the surface. Waterspouts on the seas
are due essentially to the same cause as the
wHIRLwINDs ON THE LAND.
The air next the surface of the ocean, es-
pecially in regions where the sea is warm,
as in the tropics or in higher latitudes over
the surface OF the Gulf Stream, is, on ac-
count of its warmth, struggling to find
way upward through the overlying cooler
air. When the wind is blowing as in the
district of the trades the atmosphere is con-
stantly rolling over and over, and so the
heated air has a chance to escape from the
surface of the sea; but in calms this relief
is not afforded, and so we have exactly the
phenomena of the small, upward setting
streams which make our dust whirls, only
they are on a larger scale. The greater size
of the streams is due to the fact tnat the
ocean, being great plain, larger amount
of air is heated than on the irregular sur
face of the land. Sometimes the whirl-
winds which produce the waterspouts are
strong enough to lift the surface of the water
in the form of spray or upward-bounding
waves to the height of score of feet above the
sea-level.. The dark cloud of the waterspout
is not produced as is commonly supposed by
the drawing down of the clouds, or the up
ward making of the water, but in totally
different manner, viz: The warm air that
sets upward in the spiral column is very
moist AS it rises above the sea level 1t ex-
pands and becomes cooler by that process.
As soon as it is cooled the moisture enters
into the state of mist and so becomes the
dark column which is a conspicuous feat
ure in all waterspouts.
Those who are familiar with high mount
aims have often had chance to observe how
the warm, translucent air of the valleys
driven against the steep mountain side, and
by its slope carried to higher level, gener-
ates a sheet of cloud which sometimes hangs
like banner about the peak. Figure 8
Passing the summit of the moisture-laden
air, being heavier than the atmosphere
about the height, quickly falls again to-
ward the plain. AS it descends it again be-
comes warmer, and the mist returns to the
state Of unseen vapor.