The National tribune. [volume] (Washington, D.C.) 1877-1917, 1890-05-15
THE WEATHER.
Conditions Of summer Rainfall.
by LIEUT.. J. p. FINLEY, U. s. SIGNAL SERVICE

correspondence invited suggestions desired
and Information ohccrfuiiy furnished. Every ef
fort employed To make ths Bureau OF practical use
10 the readers Of TtE NkTfosAL TRIBUNE All
communications should ba addressed Weather
TIM NAfiosAL TRIBUNE Office. Washington. ID. q. |
Whom portion Of the vapor which exists in
the atmosphere is condensed mist or cloud is
formed. Generally this condensation proceeds
slowly, aud the clouds which result DO not fur
nish rain. But when this condemnation takes
place with sufficient rapidity, tho small par
ticlca Of mist which increase in diameter by the
condensation OF more vapor, and forming drops
of considerable size, they descend 20 the earth
in shower of rain.
Rain. therefore. il but the condensed vapor
of the mir and this condemnation can only be
caused by cooling the air below the tempera-
ture Of the dew point. This reduction of tenn
perature may be effected by radiation or by
the contact of warm air with the cold surface
of the ground, especially tho surface of an cle-
waited mountain or by tho mingling of warm
air with colder air. But these processes are SO
gradual or limited IN extent that they geuer-
ally result Ia simply the formation of fog or
cloud. In order to produce an abundant rain
the air must be suddenly cooled below the dew
point tthe temperature at which condensation
OF vapOr takes place in the air. and there is no
modes in which this can be- 80 readily accom-
plished us by forcing the air up To nn elevation
The temperature of the air falls about 35o in
two miles Of elevation, aud IL the air at the
earth's surface should be forced up TO this
hight, a largo portion Of the yapor which is
carried up with tho air must bo condensed
Such effect may result from an interposed
mountain. or from the opposition of two cur
rents Of air. Examples of precipitation under
both of these conditions are OF daily occurrence
When the vnpor OF tho air Is prccip4ated at a
very low temperature the vapor il condensed
In the solid state, without passing through the
condition of liquid and then generally as-
sumes tho crystalline form These minute
crystals of ice attach themselves % each other
aud form fakes of snow, which descend very
slowly TO the earth.
We have now presented the general condi-
tions under which precipitation, both rain and
snow, take place. The conditions which give
rise to fog and cloud are only intensified t0
produce rain and snow. Snow differs from rain
| only in tho degree OF the cold that results In
the crystalli ne state of tho condensed vapor We
now reach the particular subject OF this discus.
sion, viz: Summer rainfall as DISTINGUISHED
from Winter precipitation The d Inference lies
only in the conditions which bring about the
sudden cooling of tho temperature of tho air
| below the doW-p0nt.
| In tho Winter tho anti.cyclones (highs) and
| cyclones (iOws) arc developed with great def.
| niteness on tho weather-map. The laws OF
| Huid inotion arc BGrlCtly adhered 10 in the cir
cnation of tho air about these cyclonic centers
| The areas of warm and cold air progress regu.
larly from west 10 east, and the areas of rain OF
snow and fair weather alternate, and move
eastward ia conformity with the distribution
Of pressure aud temperature. The various
quadrants of the anti-olycone and cyclone are
| distinctly maintained and tho results which
are theoretically probable in each occur with
| regularity. In the Summer all is changed
Tho weather.map is devoid of well.defned
| highs and lows. Regular cyclonic circulation
Is replaced by great variability in wind dlrec.
| ton Steep gradients in barometric pressure
are rarely observed. Precipitation, instead OF
| being confined to the usual quadrants of the
cyclone, as in Winter, occurs here and there
with no general sign of its coming. BY closely
examining the weather-map we find that the
rain has fallen not 80 much from cycl0nic crcu
lation consequent upon the distribution of at
mospheric pressure, as from local circulation
consequent upon the distribution of atmos
pheric temperature. Therefore precipitation
III Winter is gradual and general, but In Sum
| mer il is sudden and local.