The National tribune. [volume] (Washington, D.C.) 1877-1917, 1890-01-02

Relation Of Baromeirie Pressure to Wind DIrec.
ton.

Relation OF Bnrometrie Pressure 60 Wind DIree.
tIou.
ny LIEUT.. J. p. FTSLEY, U. 8. SIGNAL SERVICE.
correspondence invited. suggestions desired.
aud Information checrfuIly furnished. Every ef-
fost employed 10 make tija Bureau of prHcfeal use
10 the readers Of THE NArIoNAT, TRIBUNE. All
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TIM NATIONAL TRIBUNE Oflicc. Washington d. O. |
Changes of weather arc closely related to
changes of wind, and changes Of wind to
changes in the distribution Of atmospheric
pressure. IL would not indeed, ba safe TO
assert that the weather conditions are ian NIL
casrs absolutely, the effects OF atmospheric cur.
rents or that the atmospheric currents are
absolutely the effects of the distribution of
pressure, For, as we shall see. the nteraeton
between these several elements is not quite so
| simple as would be conveyed by such an asser-
ton. II will nevertheless be- rtble. for the
sake of clearuess 10 speak first Of the distinc-
ton OF pressure IN its relation to wind, much
rs we should do If IL were simply the cause
aud 10 no sort Of sense the effect of wind.
When the atmosphere ia in a state Of equi-
lihrigm no movement of air can Of course
occur; but AS soon tho equilibrium il dis
tufhed current Of wind is generated, the air
tending To move from the district OF greater TO
the district OF less pressnrc-.in other words
the heavier afr tending % drive out the lighter
Absolute equilibrium Is in nature hardly ever
attained, the air being kept in almost perpetual
agitation by changes OF pressure. The prime
canse OF atmospheric disturbance Is found Ia
the unequal distribution OF solar heat over the
eartli's surface IN the changes diufnal aud
seasonal. tn that distribution. and in tho un-
equal effect thus produced on the tension OF
the air itself aud OF the vapor suspended in It,
But as soon rs . current is established the di-
rccton Of ia movement is affected by other
forces besides that Of gravity. The effect of the
carth's rotation on It's axis Is To cause every
current TO deviatc 10 the right Of what would
otherwise be its course in tho Northern Hemt-
sphere and 10 the left in the Southern Henri
sphere. The winds thus blow TO a certain ex.
tent round instead of directly INTO any area in
which the pressure OF the atmosphere Il rela-
lively low, and tho centrifugal force of the
currents themselves helps IN this case 10 n7-siEl-
tain tho inequality OF pressure This effect of
the force Of the earth's rotation varies with the
latitude. In the high latitndes We Cad that If
VIC describe lines along which nj any given
time the pressure OF the atmosphere 40 the level
of the sea IL equal. while il is higher on the
one side and lower ou the other, tho wind
blows Ia direction which, for practical pnr-
poses, may be regarded ms parallel to thnse
lines. In low attitudes on tho other hand, the
winds move nearly at right angles 10 such
lines, aud at the Equator the detecting elect
oil the earths rotation Is zero, aud the air must
consequently there travel in direct lines from
the points at which the pressure Is tho greatest
to those at which IL Is least. The amount Of
deviation-that Is. the degree in which the
movement OF the air differs from what il would
be IL unaffected by the earth's rotation Gothic
right in the Northern and 30 tho left in the
Southern appears 10 be further affected by
the amount Of friction which the currents en-
counter in traversipg the surface of the earth,
being diminished when the friction is In
creased