154 PHYSICAL GEOGRAPHY. s

 

to open them sufficiently to enable ice-bound vessels to be
pressed forward to the termini of the canals.

The value of the Weather Bureau can scarcely be over-
estimated; the saving of shipping effected by the timely
warning of a single severe storm may more than pay the
entire expenses of the bureau for a large portion of the

year. We append the following résumé of the work of |

the bureau:

(1.) The announcement of probable weather changes by
the publication of “indications.”

(2.) The timely warning of the approach of severe
storms.

(3). The display of signals indicating coming changes in
the weather.

(4). The publication of farmers’ bulletins.

(5.) The river and canal reports.

(6.) The display of symbol-maps, showing the actual
state of the weather throughout the entire country.

(7.) The publication of daily weather maps, monthly
charts, and charts which give the results of the observa-
tions of years.

(8.) The publication of cotton-region reports, embracing
reports of rainfall and maximum and minimum tempera-
ture throughout the cotton districts from April 1 to
October 31.

The International Weather Service ——The suc-
cess of the meteorological observations of the U.S.
Weather Bureau has led to the establishment of
stations for simultaneous observations over a large
portion of the northern hemisphere and some sta-
tions in the southern hemisphere. By simulta-
neous observations of the meteorological conditions
of the whole earth, many things yet unknown as to
weather predictions are likely to be discovered.

Tornadoes resemble cyclones in that they are
whirling motions of the air. The area over

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig, 129, A Tornado,

which they extend is more limited, but the
velocity of the wind is higher than in cyclones,
and, therefore, their destructive power is very

 

 

 

 

great. When they pass over any section of
country they leave devastation and ruin in their
track.

Tornadoes are of frequent occurrence in the
central and western portions of the Mississippi
Valley.

Tornadoes have their origin in a rotary motion
imparted to a mass of warm moist air that is
temporarily imprisoned below a mass of colder
air. The whirling motions begin at the upper ex-
tremity of the column, near the cold air, and
gradually extend downwards. This produces the
characteristic inverted funnel-shaped mass of
dark cloud by which the approach of a tornado
is generally heralded.

The path of the tornado, like that of the
cyclone, is generally eastward.

Weather Maps.—The actual condition of the weather
over the United States, on any day, is represented in
weather maps published by the bureau. Two such maps
are shown on page 152. The upper map shows the meteor-
ological conditions prevailing on a certain day in April.
On that day an area of low barometer existed in Colorado,
Nebraska, and Kansas, within which the barometer was
below 29.5 inches, as shown by the isobar, or line of mean
barometric pressure, of 29.5. The country around this
area had a gradually increasing barometric pressure, as
indicated by the successive isobars 29.6; 29.7, 29.8, 29.9,
etc. At the same time a storm was moving toward the
north-east, as shown by the line of crosses. The rate of
progress of the storm being known, the bureau issued the
following

7 Indications.

For New England, fair weather followed by light rains
to-morrow, north to east winds, slight rise in temperature.

For the Middle Atlantic States, increasing cloudiness and
rain, winds shifting to east and south, slightly warmer
weather, lower barometer.

For the South Atlantic States, local rains, warmer, partly
cloudy weather, south-east to south-west winds, lower ba-
trometer.

For the East Gulf States, threatening weather and rain,
followed by clearing weather, southerly to westerly winds,
slight rise in temperature, followed in west portion by a
slight fall in temperature.

For the West Gulf States, local rains, followed by clear-
ing weather, winds shifting to west and north, nearly sta-
tionary, followed by lower temperature, and rising barom-
eter to-morrow.

For Tennessee and the Ohio Valley, cloudy weather and
rain, southerly to westerly winds, rising temperature, fall-
ing barometer and severe local storms, followed to-mor-
row in west portion by cooler weather and higher barom-
eter.

For the Lower \Lake Region, threatening weather and
rain, east to south winds, lower barometer and rising
temperature. ?

For the Upper Lake Region, threatening weather, with
rain or snow, north-vasterly winds becoming variable, fall-
ing followed by rising barometer, slight rise, followed by
falling temperature.