The National tribune., 1890-07-24
THE WEATHER.
Atmospheric Pressure in Cyclones.
BY LIEUT.. J. P. FINLEY, o. g. SIGNAL SERVICE.

BIZ LIEUT.. y. II. FINLEY, u. 3. SIGNAL SERVICE.
Correspondence Invited, suggestions desired
and Information cheerfully furnished. Every Of
Tort employed to make this Bureau of practical use
to the readers of THE NATIONAL TRIBUNE. All
communications should be Addressed Weather,
THE NATIONAL. TRIBUNE Office Washington, ID. O. |
Since the DETECTING forces Of tho cyclonio
(Low) and the anti-cyclOnic High gyratons
OF air are from the center in the former. and
tO'cdrd it in the latter the greatest pressure is
at the distance from the center where they
vanish and change from the one to the other,
at least 80 far as the pressure and pressure
gradients depend upon these forces, The diff
ference of pressure between the lowest Mt tho
center aud tho highest where the gyrations
vanish depends of course upon the summation
OF tho pressure gradients, and as these are COM
paratively steep in the cyclonic part, the differ
ence between the highest pressure aud that at
the center of the cyclone Is generally very
much greater than that in tho anti.cyconio
part between the highest pressure and that of
tho general undisturbed surrounding pressure.
Whatever the nature of tho earths surface
land or water valley or mountain) and the
gyratory wind veocities there tho differences
between these velocities and those Mt any alti-
tude, at the same distance from the cyconic
center, are the same in any case so that where
the gyratory velocites at the earth's surface
are greater for any reason, all those at any
attitudes are increased by the same amount
The pressure at the carth's surface Is scarcely
affected by the upward expansion of air in the
interior from higher temperature, since the
anti-cyclonic gyrations almost entirely prevent
its howing away above, and the consequent
dlminution of the pressure at the earths sur
face. The barOmetric or pressure gradient is
proportional to the horizontal force which
causes IL, and this depends almost entirely
upon the gyratory velocities The gradient,
therefore, diminishes in intensity from the
center of the cyclone outward. What is stated
with regard to pressures at the earths surface
1s true OF those at any altitude above the sur
face For the atmosphere above any given
level can be regarded as IA separate atmosphere,
and the gyratory velOcities of the general at
mosphere at that level as those Of the base of
the atmosphere above that level. At any given
altitude, therefore, the highest pressure in the
plane Of that altitude ID also or very nearly,
where the gyrati0ns vanish and the cyconic
change into the anti-cyclonic.
All that area In a cyclone in which the bgro-
metric pressure is below the usual average, say
80 inches, or 760 millimeters, is called an area
OF low pressure, or simply low, or cyclone. 1t
is evident that the depth of the minimum
pressure depends upon both the extent of the
area aud the gradients ut the different dig
tances from the center.
Where the gyratory velocity Is very great
near tho center of the cyclone the gradient
depends almost entirely upon the centrifugal
force especially in low latitudes, and as this
becomes very great where the radius is small,
such cyclones. oven OF small extent, may have
deeper minimum pressure than large cyclones.