Pine Bluff daily graphic. (Pine Bluff, Ark.) 1893-1942, 1914-06-30
HOW Weather
IS FORECAS7ED

Id TUh[UAOILU
Twice Each Day Observa-
tons from 200 Stations are
Wired to Washington.
(From Weekly News Letter of De-
partment of Agricultural
One of our correspondents writes:
"Will you please tell me
through the News Letter, in SIM
pie language, how the weather
forecasts are made?"
To this the shortest and simplest
answer is: By telegraph. The wealth
err forecaster does not look out of the
window and guess. He is not weather.
wise like the old salt who scents the
approach of storm without knowing
how. To the signs by which the ord'-
nary citizen decides whether to take
his umbrella with him or leave it at
home, he pays no attention. Give him
his telegraph wires and he can make
his predictions as accurately in win
dowless cellar as on the top of New
York's highest skyscraper.
The private individual sees the
clouds settle on nearby mountain
or the rain sweep over a field mile
from his house and knows that the
storm will be on him in a few minutes.
With the aid of the telegraph the
weather man sees the same storm
when it is 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 accuracy the ap-
proximate time when it will reach any
place in it's route.
Twice day, at s in the morning and
8 in the evening, reports are tel3-
graphed to Washington from about 200
observers stationed in as many DIFF
ferent localities in the United States
and Canada. In these reports the OBJ
servers do not volunteer their person
al opinions about what is going to
happen. They confine themselves to a
plain statement Of the actual condi-
lions at that particular moment, the
pressure, or weight, Of the atmos-
phere, 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 it is obvious that by comparing
4bo fvaoly nf
the course and progress OF both
forms and periods of clear weather.
From that, the next step is to predict
what sort of weather will prevail for a
day or two in any given locality.
This map is the basis of all scienti-
fic weather forecasting. A glance at
it will show that it is divided into
"high pressure areas" and Glow pres
sure areas These are technical
terms used to describe the regions in
which the weight or pressure of the
atmosphere is great (high) or small
(ow). At sea level the barometer,
which is used to measure this weight,
will register 80 inches under normal
conditions of the atmosphere. When
it registers more than this, say 30.5 or
81, the pressure is "high;" when 29.,'
or less, "low." In this way the height
ter indicates the weight OF the air
just as, in thermometer, it indicates
the temperature.
Low pressures usually mean strong
winds, rain, and rising temperature;
high pressures, cool, clear weather.
For reason to be explained later,
these thighs" and "lows," as they are
called, travel in 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 ar
rival at any specified point. If they
travelled, like a ship steered by COM
pass, an exact course to the east, and
if they moved invariably at the same
speed, then weather forecasting would
be simple sum in arithmetic, like
calculating the time when a railroad
train is 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 do 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 thighs" and
lows" eastward across the country.
This progress is caused by the
shape of the earth and the well-known
fact that hot air rises. The tropical
sun in the regions along the equator
heats great masses of air, which rise
and drift toward the North and South
poles. AS the earth revolves from
west to east these masses are carried
along with it at the same speed. But,
as is also well known, the equator re-
wolves much faster than the poles,
which are practically stationary, in
much the same way as the rim of a
| wheel revolyes faster than the hub.
Therefore these masses of air, revolv-
| ing at the same rate as the equator,
begin, as they approach the pr-:eS, to