The true Democrat. [volume] (Bayou Sara [La.]) 1892-1928, 1914-05-09
HOW THEY FORECAST THE
WEATHER.
Government News Letter:


News Letter, in simple language, how
the weather forecasts are nader"
To this the shortest and simplest
answer is: BY telegraph. With the
aid of the telegraph the weather man
sees the storm when it Ia thousand
miles away, and not only this storm
but every other one in the country.
He knows in what direction and at
what speed each is moving, and can
thus calculate with reasonable accu-
racy the approximate time when 1t
will reach any place in Its route.
Twice a day, at in the morning
and in the evening, reports are tele-
graphed to Washington from about 200
observers stationed in as many differ
ent localities in the United States and
Canada. In these reports the observ-
ers do not volunteer their personal
opinions about what Is going to hap
pen. They conflne themselves to a
plain statement of the actual condi-
lions at that particular moment, the
pressure, or weight, Of the atmosphere,
the temperature, the direction and
velocity of the wind, the amount Of
rain since the last report and so forth.
From this information the Weather
Map of the United States is made up,
showing the conditions that prevail In
every part of the country. Since there
are two maps for each day tits obvious
that by comparing them the forecaster
can keep track of the course and
progress of both storms and periods
Iof clear weather From that, the next
step is to predict what sort of weather
will prevail for day or two in any
given locality.
This map is the basis for all scien-
tific weather forecasting. A glance at
lit will show that it is divided into
high pressure areas" and "low pres
sure areas" These are technical terms
fused to describe the regions in which
the weight or pressure of the atmos-
phere is great (high) or small (low).
At sea level the barometer, which liz
used to measure this weight, will reg
| lister 80 inches under normal condi
tions of.the atmosphere. When 1t reg.
listers more than this, say 305 or 81,
the pressure is high" when 29.5 or
less, Cow" In this way the height of
column of mercury in barometer
indicates the weight of the air just as,
in a thermometer, it indicates the tenn
lperature.
Low pressures usually mean strong
winds, rain, and rising temperature;
high pressures, cool, clear weather
For a reason to be explained later,
these highs" and lows" as they are
called, travel in a general direction
from west to east. The forecaster
notes their progress on the map, per-
ceives their speed and their route, and
then predicts the time of their arrival
at any specified point II they travel
ed, like a ship steered by compass, an
exact course to the east, and ii they
moved invariably at the same speed,
then weather forecasting would be a
simple sum in arithmetic, like caIcu-
lating the time when a railroad train
running so miles an hour will arrive at
a station 500 miles away. But storms
are not railroad trains. They travel
in an easterly direction, but they dc
not travel due east. Their speed Is
liable to change, and they are affected
by the presence of other storms, by
mountain ranges, large bodies of water
and many other things which make
weather prophesying the complicated
science that it is. The skeleton of the
science, however, is the progress Of
those highs" and lows" eastward
across the country.
This progress is caused by the shape
of the earth and the well-knoWh fac
that hot air rises. The tropical sun Ii
the regions along the Equator heats
great masses Of air, which rise ant
drift toward the North and Soutl
Poles. AS the earth revolves from
west to east these masses are carried
along with 1t at the same speed But
the Equator revolves much faster thar
the poles, which are practically sta
) tionary, In much the same way as tn,
rim of wheel revolves faster than tn
hub. Therefore these masses Of ali
J revolving at the same rate as tn
. Equator, begin, as they approach tn
poles, to move much faster than tn
} earth beneath them. There Is thus
constant movement Of the atmospher
from west to east, movement whici
. becomes more marked the nearer on
gets to the poles.