The National tribune. [volume] (Washington, D.C.) 1877-1917, 1891-04-09
THE WEATHER.
The Production OF Precipitation in the AtmOs-
phere.
BY LIEUT.. ). r. FINLEY, U. =. SIGNAL srRvIc

phere.
BY LIEUT. ). r. FINLEY, U. g. SIGNAL SERVICE.
TCorrespondence invited, suggestions desired
and Information cheerfuly furnished Every e.
Tort employed 10 make tins Bureau of practical use
TO the readers Of The NATIOEAL TEIBUNE. All
communications should be Weather
Tn NATIONAL TRIBUNE Office, Washington, ID. g.
Whatever lowers the temperature Of the air
may be considered as a cause Of rain. Various
causes may cOnspiTc 10 effect this object, but it
Is chiefly brought about By the ascent OF air
lnto the higher regions Of the atmosphere
Moist air currents are forced up INTO the higher
parts of the atmosphere by colder dricr and
therefore heavier wiud currents getting be-
meath them. The same result is accomplished
by ranges Of mountains opposing their masses
TO the onward horizontal course OF the winds
90 that the air forced up their slopes is cooled
aud It's yapur condensed into showers OF rain or
snow. Moist air currents are also drawn up
lntO the higher regions II the atmosphere by
cyclOnlc circulation within an area or low Ia rO-
metric pressure. This is the primal cause Of
nil precipitation, but il is modified by tOpog-
rapAy on the earth's surface which gives rise 10
local pecularties, producing heavy falis Of rain
ln some places and little or none in others.
The temperature Of the air Is lowered aud the
amount OF rainfall increased by those winds
which convey the air 10 higher ATTITUDES. The
meeting aud mixing Of air currents of differ
ent temperatures which Is going on constantly
IN cyclOnIc disturbances is prolific cause Of
rain, since the several portions, when COM
blned into one, are Incapable Of holding ia sus-
pension the same quantity Of vapor that each
could hold separately. The rainfall is also
Increased II The prevailing winds arise immodi-
ataly from thc sea, and are therefore moist; but
Diminished a. They have previously passed over
large tracts Of land, particularly mountain
ranges, and are therefore dry. Since be quantity
Of rain is evidently much modified hv tie tenn
peTature Of the earh's surface over which the
rain-prOIucing winds dow, il follows that sandy
deserts, by allowing solar aud nocturnal radia-
ton 10 take immcd date effect Ia raising or de-
pressing tic temperature aud forests. by delay
lng, d not ln many cases c0unteractng the
ELECTS OF radiation, have each a peculiar in
huCHce on rainfall.
The influence Of changes on the surface OF
the earth ln the distribution or rainfall Is con-
siderable, butuut well understood The extent
OF this article will not permit Of a discussion OF
the subject. IL wall be reserved for a sefiarate
presentation. The specific conditions under
which rain is precipilated may be briefly given
as follows: First Two layers OF clouds at
least IN upper layer, tie ciIIus, which being
al a great hight - composed Of minute parti-
cles Of ice NJ a very low temperature, probably
not higher than 40 degrees below zero; and a
lower layer, the cuWulus or cumulo-strafus
which has it, density increased AID It's tom pera-
ture diminished by the descent Of the ice-crys-
tas q the cirrus. Second. The temperature Of
the air 80 tic rgrth's surface as high rs posi-
bic. Third. The atmospheric pressure notably
lower than ln surrounding regions Fourth
Regular horizontal. currents Of air allowing the
atmosphere 10 rcmaia a sufficiently long time
IN a state Of unstable equilibrium Fifth. A
rapid movement Of tie AIR tending 10 rc-cstab.
4 LISA The equilibrium Of temperature aud press
ure by mixing together tho Gllereut layers Of
the atmosphere. The geographical distribution
OF RAIN over the globe is proportional 10 the
/ temperature the humidity, the mean depression
Of the barometer the fluctuations iu the tenn
/ porature, AID the coIiguEat!on Of the earth's
surface.