Year: 1877




Weather.


In recent essay Prof. Elias Loomis
has traced the genesis of our great
storms. They generally, he finds, begin
In the neighborhood of the Rocky noun
tains. A few can be traced to the Pa
cific ocean, near the northern boundary
Of the United States, but no example is
found of any considerable storm arising
on the Pacific coast south of Oregon. A
large proportion of our great storms
originate and are developed wholly
within our territory. The following is
the usual process in the birth of a storm:
To begin with, there is generally on
area of several hundred miles diameter,
through which the barometer stands at
the mean. On opposite sides of this
area-generally east and west at dis
tance of perhaps 1000 miles apart, are
areas of high barometer. The rise of
the mercury in these side areas is usually
One-quarter of an inch or more above the
mean, sometimes it is full inch above.
The atmosphere in these two areas begins
to move toward the central area. The
currents thus established are DETECTED
toward the right by the earths rotation,
and diminished pressure at the center;
the inflow increases and comes from all
sides. The area of low pressure assumes
an oval form, but if the winds are very
violent, it may be more nearly circular.
| With rotation, centrifugal force is
developed which increases the depress-
ion. An upward movement of the at
mosphere within the depression area is
| nth Il,o ,,,H
restore the equilibrium. This upward
movement carries large amounts of aque-
GUS vapor, which, when cooled at high
er level, condenses as rain. The heat
liberated by condensation increases the
rarefaction of the area and thus rain
increases the force of storm, though
never originating it. The latter conclu-
sion may also be deduced from the fact
that the inflow begins before there is
any precipitation of rain.


AS to the forward progress of the
storm, the following general observa-
tions may be made: The west wind,
which is characteristic of this part of the
world, carries along the storm area. 1t
may, however, meet Interruptions, such
as a local storm. In that case the low
area, thus arrested in its forward pro-
gress, will be the seat of increasing
force during the interruption, and the
storm for few days afterward is likely
to show greater violence. The disturb
ance which constitutes the storm is plain
ly within the lower currents of the at
mospLere, the upper currents being
comparatively unaffected. The pressure
of the general wind from the west tends
to fill up the rarefied area on that side,
both above and below. On the east side
only the lower or surface wind consti-
tutes an inflow, while the upper portion
of the storm is moving toward the east
under the pressure of the west wind. 1t
follows that the upward motion within
the storm area takes place chiefly on the
east side of the area, the motion being
so rapid that the inflow is insufficient to
restore equilibrium. A further result
of this process is that the depression at
the center is constantly transferred to-
ward the east.


But if there should be a great precipi-
tation of vapor on the west side of the
storm area, the center may be prevented
from going eastward may be held sta.
tionary, or may even be quite diverted
toward the west. Several remarkable
instances of this exceptional kind were
cited. In one of the most striking OF
these cases-December 80, 1874, to Jan
uary 1875-the storm oscillated within q
range of about 1000 miles from west to
east. During the most of that time the
winds on the west of that area blew with
the force of hurricanes, accompanied by
snow, sleet and low temperature. The
explanation offered in that instance by
Prof. Loomis is that the extensive pre-
cipitation on the west side of the low
center was probably a cause of the slow
progress, and was itself due to cold
wind from over the continent, underfow-
ing warm currents arising from near
the Gulf Stream.