Year: 1877

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Professor Elias LoomIs, of Yale Col
lege, read q paper on Contributions
OF the National Academy of Sciences
In New York, part of which Is AZ fol-
lows: The published volumes of the
United States signal service (now COW
ering period Of twenty-one months)
show that the great storms of the
United States usually begin near the
Rocky Mountains. the few traced
to the Pacific Ocean none come In
south OF Oregon. A large proportion
OF our storms are developed within our
national limits or near our borders.
Thus we have many chances to observe
the first development and gradual in
crease of storm. The usual circum-
stances attending the first develop. !.
ment are these: There Is an area sew
eral hundred miles i'm diameter over
which the barometer has about It's
mean height. On opposite sides of
this region. generally about east and
west, at distance OF some 1000 miles.
areas OF high, barometer are formed
where the pressure Is from One- fourth
to whole inch above the mean.
From these two oppOsive high pressure
areas the atmosphere moves toward
the central area, the earth's rotation
DETECTING these currents to the right.
There results u diminished pressure
over the central area, causing still
rarefaction Of air increasing the inward
movement of wind. Rain is thus one
circumstance which increases the force
Of storm, but is not regarded as a
cause of its origin. The observations
prove an inward movement of the air
before much precipitation of vapor
takes place. The progress OF an area
of low barometer appears to be mainly
determined by the same causes which
determine the general system Of circu-
lation of the atmosphere. Except in
the southern margin OF the United
States, the average annual progress of
the wind Is from west to east, and Is
determined by causes general in their
operation. Local storms cannot per-
manently, but may temporarily, infiu-
ence this progress, in which case it
reestablished In few days at most,
with force proportioned to the time
greater inward flow of air, thus pro-
ducing q still lower baronetric pres.
sure. Since the air tends to flow In on
all sides toward this low pressure area,
1t assumes all oval shape, sensibly cir
cular If the winds are violent. The
centrifugal force resulting from this
circular movement causes still
further depression of the barometer.
This partial vacuum would soon be
filled, stopping the inward movement
of the air, were it not for the upward
escape of this air, carrying aqueous
vapOr. Tho whole Ia cooled, prOduc-
ing rain. The heat liberated in this
condensation of vapor causes further
1t was interrupted. The atmospheric
disturbance due to storms is mainly
confined to the lower half of the atmos.
phere. The force of this upper current
| from the west added to that of the
lower half Of the atmosphere pressing
in upon the west side Of low -barOme-
trio area, tends to fill up the depression
on that side. On the eastern side of
the storm, the surface wind alone
presses inward, while the upper portion
of the atmosphere Is still moving from
the west. Moreover, the upward mo-
ton of the air takes place chiefly on
the east side of the centre, as Is indi-
Gated by the position of the rain areas.
BY this upward motion the air which
presses in upon the east side of the
centre Is prevented from restoring
the equilibrium of the pressure on that
side, while on the west OF the centre
there is usually little of this upward
movement. Thus the low.pressure
centre tends eastward, or the storm
appears to travel to the east. If there
should be a great precipitation of vapor
OH the west side of the storm centre,
this must be accompanied by ascend
ing currents Of air on that side, which
would oppose the establishment OF the
equilibrium of pressure on that side
and the storm centre may be held sta
tionary or even tend westward. A re.
markable example Of this occurred in
March, 1874. This storm travelled from
Montana to Nova Scotia at the rate ol
twenty-four miles per hour. AT one
time near Nova Scotia its rate was
three miles an hour, then fifteen, then
three, the storm centre oscillatiug tC
and fro. During this period Of slow
rate much snow fell to the west Of the
centre of low pressure. This snowfall
is believed to be the cause of the slow
eastward progress. Other remarkable
instances Of this kind have occurred.