The National tribune. [volume] (Washington, D.C.) 1877-1917, 1890-06-19
THE WEATHER.
Vertical Circulation of the Air in Cyclones Lows)
ny LIEUT.. J. p. FINLEY, U. 8. SIGNAL SERVICE.

L
| correspondence invited suggestions desired,
| and information cheerfniy furnished. Every eff
fort employed to- make this Isnreau of practical
| TO the readers of THE NATIONAL TRIBUNE All
communications should he addressed Feather,
| The NATIONAL TRIBUNE Office. Washington, ID. O.
If the air over any portion of the earth's
| surface is warmer at all attitudes than that OF
| the surrounding parts Mt the Same levels, 1T is
lighter, aud tends to rise up above its original
level and flow out in all directions above. This
| decreases tho pressure at tho earth's surface
over this area, but increases it little over the
surrounding parts Thus there arises gradient
of pressure decreasing from tho exterior to-
| ward the interior below. which causes n flow of
air i'll from all sides to supply the ascending
current.
There is thus n vertical circulation and inter-
change of air between the interior and exterior
parts established, just in the case of the gen.
oral circulation OF the atmosphere, except that
now the motion Is toward the central part be-
low and from it above, while in the general cir
culation it is toward the polar central region
above and from it below The central region
in this case is colder instead of warmer than
the surrounding parts.
1t is not necessary, however, in order to have
such n circulation that all the strata of air
from bottom to top shall BIO warmer in the in
terror than in the surrounding parts, but only
that there shall be such disturbance of the
SVublibg VI temperature tnaU inc pressure Oh
any part of the interior is less than that of tho
exterior part. Where this is tho case tho air
tends to rise up, and that Of greater pressure Mt
the same level to flow in and take its place. and
thus the circulation is established In general
the velocity of tho ascending current in tho
interior part is much greater, but over smaller
area, than that Of the descending current in
tho exterior surrounding part, over n much
greater area, where it is simply very gradual
settling down. Since there is no definite limit
the tendency is for the air above to flow out to
still greater distances, and thus to press down
and bring in the air below from greater dis
tances.
On account Of tho n-hOmogeneity Of tho
earths surface, comprising hills and valleys,
land and water and dry and marshy areas, all
with different radiating and absorbing powers
and also on account of tho frequent irreg-
ular and varying distribution of clouds, it
must often happen that there are consider
able local departures of temperature from
that of the surrounding parts. If 1t should
so happen, rs 1t frequently must, that this
area is of n somewhat circular form, and the
air has higher temperature than that of the
surrounding part of the atmosphere, then We
have tho conditions required to give rise to
vertical circulation, with an ascending cur
rent IN the interior. rs described above But
unless there IS some source of heat by which
this interior higher temperature Is kept up,
this circulation soon ceases for the interchange
of air between the interior and exterior parts
of the air comprised in tho circulation tends to
continually reduce tho difference of tempera-
ture upon which the circulation depends, and
to bring all parts to the Same temperature.
10 Is known that if any portion Of the at
mosphcrc, when 10 10 in the unstable state re-
ceive from u change in temperature, or other
disturbance, an upward motion it becomes
warmer than the surrounding air ut the Same
level. The tendency then Is for it to continue
to rise as long the air is in the unstable state,
and thus 10 give rise to n vertical circulation,
which must continue as long as tho atmosphere
OF tho surrounding parts is In the unstable
state. The more the unstable state differs from
the indifferent state, the greater the energy
which maintains the circulation, aud the
greater tho rapidity of this circulation. In
dry atmosphere there Is probably such an in
torchaBgo and inversion of the air of the up
per aud lower strata that the unstable state is
soon broken up aud the circulation ceases.