METEOROLOGY.

   

 

 

coast and Great Lakes of the approach of danger-
ous storms, and to collect such information as

 

would be of value to shipping and other interests.

In 1890, the direction of the Weather Bureau was, by
Act of Congress, taken from the War Department and
transferred to the Department of Agriculture.

There are about 500: stations established by
the Weather Bureau in- different sections of
the United States. At these stations there are
trained and intelligent observers, who several
times each day are simultaneously required to
make careful observations of the temperature,
humidity, and pressure of the air, the direction
and force of the wind, the clearness or cloudiness
of the sky, and the amount of rain or snow that
has fallen during a given time.

These observations are telegraphed to the Cen-

tral Office at Washington, so that the Bureau is ~

enabled to see the actual meteorological condi-
tions which exist throughcut the country at any
given time, and from such knowledge, guided by
previous experience, to prepare “synopses” of
the weather and “indications,” or forecasts.

For the preparation of the “indications” the officer in
charge prepares a number of graphic charts, based on the
various data telegraphed to the Central Office, as the result
of the simultaneous observations at the different stations.
These charts exhibit the actual meteorological conditions
that then exist, those that-existed during the previous
eight hours, and the previous twenty-four hours, and the
conditions normal for the place at that particular time of
the year. The data shown on these charts include the
temperature, barometric pressure, humidity of the air,
precipitation, condition of sky, force and direction of
wind, etc. The “ indications” are telegraphed to the press
throughout the country. In general about 85 per cent. of
these indications are verified.

Tt should be borne in mind, in considering this very
large percentage of verified forecasts, that the indications
are predicted for extended areas, and, therefore, although
the change may not have occurred in some limited section
of the predicted area, it may have occurred in nearly all

‘the other portions of the region.

Changes in the Weather—Passage of a Great
Storm.—Since nearly all the great storms of the
United States are species of cyclones, that move
over the country in a general easterly direction,
when such a storm is once started it is not a dif
ficult matter to predict its general path, and thus
foretell coming changes in the weather.

The principal elements of uncertainty ‘are the exact path
in which the storm will move over the country, and the
velocity.of such motion. These the b.reau can predict,
approximately, from a comparison o£ all the previous

storms of which it has records. /
The whirling direction of the wind in the Northern

Hemisphere is in the opposite Carection to that of the -

a

hands of a watch. Therefore, as the eastern side of a
storm approaches any section of country, the winds blow
generally from the south toward the north. The approach
of a cyclone is generally attended by a fall of rain or snow.
As the cyclone moves onward, and its western side passes
over any locality, the general direction of the wind is
from the north to the south. The passing of the cyclone
is generally attended by clearing, cooler weather.

Cold Waves.—On the edges of a cyclone the
barometer is nigh. When one storm follows an-
other at a short interval, the area of high ba-
rometer between them causes the wind to blow in
all directions from the centre of high barometer,
and a cyclonic movement of the air is thus es-
tablished, possessing a progressive motion like a
true cyclone. Since the direction of rotation of
such a storm is opposite to that of a cyclone, it
is called an anti-cyclone. Cold waves generally
originate in anti-cyclones.

The Weather Signals consist of signal-flags
designed to indicate the probable weather and
temperature of the coming day.

The temperature signal indicates warmer weather when

placed above the other flags, and colder weather when
placed below them. The cold-wave flag indicates a de- .
cided fall in temperature.
X The Storm Signals are displayed at all ports on
the Great Lakes or the Atlantic seaboard whenever
it is considered probable that within twelve hours
there will be experienced at those ports, or within
one hundred miles thereof, a wind dangerous to
navigation. The information signal is intended
to notify ship-masters that, on application to the
local observer, information will be given them
relative to an approaching storm, which it is
thought will be dangerous to vessels about to
sail to certain ports. The cautionary signal is
displayed on the Lakes only, when the winds
expected will be severe, but not dangerous to
well-equipped vessels.

To reach the different cities, towns, and villages, and the
hamlets of the rural districts, the indications, or forecasts,
are telegraphed every midnight from the Central Office to
centres of distribution, situated in different States. These
reports are at once printed at each of these distributing
stations, enclosed in envelopes, and forwarded to every
post-office which can be reached by the swiftest mail
facilities by 2 P.M. of, the next day. Great benefit is
thus conferred. on agricultural interests.

Warnings of coming floods, movements of river ice,
sudden or unusual change of level in rivers, are also given
as the occasion warrants. The warning is given whenever
the water rises above a certain level, called the danger level.

Another series of reports are for the benefit of internal
navigation. They consist in’ the announcement, from day
to day, of such changes of temperature for different sec-
tions of the country as would be likely either to stop
navigation by the freezing. of the canals, or temporarily