The National tribune. [volume] (Washington, D.C.) 1877-1917, 1891-07-09
THE WEATHER.
A Talk About the Causes Of Contrary Wind Cur
rents


EDITOR NATIONAL tribune: I wish 20
make an inquiry. What becomes Of the air
where two currents blow in opposite directions
for days and the meeting point Cor linel moves
slowly toward one Of the currents? Also what
is the average weight Of water in the air, pound
for pound, and what arc the extremes Of the
weight Of water in the air2-JOsEPH MYERS,
Kidder, No.
Air blowing from opposite directions over a
large area Of country, toward q common center,
indicates a large disturbance in the atmosphere,
called by meteorOlOgists a cyclone. it has a
diameter of from 300 to 1000 miles. The air
within this area moves in directions about the
center, which shows tendency to q whirling
motion, only, however, when the whole dis
turbance is plotted. At any point Of observa-
ton within the storm area the air is found to
move in direct lines, except in the case Of
tornado which always develops in the south
west quadrant Of a cyclone. Within a cyclonie
area the air moves toward the center from all
parts Of it, except at the very center, or eye
Of the Storm" having diameter Of 10 to 20
miles where the air rises TO the upper regions
Of the atmosphere aud flows over and descends
at some other point on the earth. This pecu-
liar cyclonic circulation gives rise TO westerly
winds south Of tho center, sOuthCrly winds on
the east side, eastcrly winds on the north side
and northerly winds on the west side.
All storms move from west TO east under the
influence Of the ayial rotation Of the earth.
Most Of our cyclonic disturbances come from
the north Pacific Ocean over British Columbia,
entering the United States in Montana or Da-
kOtA When one of these storms approaches
the Mississippi Valley the wind shifts 10 points
between southeast and southwest over that
territory lying to the south and east of the
cyclonic center. The progressive velocity of
storms is considerably less between the Pacific
and the Mississippi Valley than between the
latter and the Atlantic owing to the fact that
the former region is exceedingly dry. Moist
ure in the form Of vapOr is absolutely essential
to the development Of storms. Heat and
moisture constitute tho food Of cyclOnic dis
turbances, without which they cannot exist.
In passing over n dry country these disturb
ances are very much reduced IN energy, and
therefore their progressive velocity is greatly
lessened In the Mississippi Valley and to the
east ward the storms are fed by theinexbaustibe
sources of moisture carried To them by the
vapor- laden winds from the Gulf and Gulf.
Stream. Their energy is greatly augmented,
violent local storms develop, the winds in
crease in force, rain falls in great quantities,
and the disturbance rushes forward with high
velocity. West of the Mississippi Valley the
movement is slow, az above explained, and
therefore az places south and east of the cy-
c.onic center the sOuthorly winds prevail for
several days The slower the progressive
movement Oh INC dbl4it' ifil-A4t hot uAiiCGo LU
longer will the winds from any direction con-
time.
The direction of air-cuTents, the formation
Of clouds aud their movements, aud the distri-
button Of heat and moisture in the atmosphere
cannot be clearly understood without adequate
knowledge OF the development and movement
Of cyclouic disturbances. The eastern half of
a cyclone brings with il increase of heat, moist
IRE, heavy clouds, rain, and high winds. The
western half is atteuded with clearing weather,
cool dry air, and winds shifting to north and
west wIth rising barometer
10 is important t0 observe that dry air is
heavier than aqueous yapor in the proportion
Of to ;, and hence a cubic foot of moist air
weighs less than cubic foot of dry air at the
same temperature. About IS cubic feet of dry
air weighs one pound avoirdupOis. An in
crease in temperature Of one degree Fahrenheit
above 80 will provide for the accumulation of
about 1900 tons Of water in u cubic mile Of
air. The average weight Of aqueous vapor in
the air over any place or region will depend
upon the average temperature, other things
being equal. At temperature OF 90 Fahren-
heit the air 40 saturatiou will hold about 5000
tons Of water in the form Of vaPOr. This cor
responds 20 a percentage Of relative humidity
expressed by 100. The vapor tension of the
air is extremely variable and difficult to COM
pute with accuracy At temperature Of zero
Fahrenheit the air will contain ON the average
about one quarter of grain of water per cubic
foot