Montgomery County sentinel. [volume] (Rockville, Md.) 1855-1974, 1914-05-22
How The Weather Forecasts Are
Made.

The weather forecaster does not look on
OF the window and guess. He is not wealth
erwise like the old salt who scents tn
approach of a strom without knowing how
To the signs by which the ordinary citizel
decides whether to take his umbrella will
him or leave it at home, he pays no atten
tion. Give him his telegraph wires and h,
can make his predictions as accurately il
a windowless cellar as on the top of New
York's highest skyscraper.
The private individual sees the cloud,
settle on a nearby mountain or the rair
sweep over a field mile from his house
and knows that the storm will be on him
in 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 approximate time when il
will reach any place in its route.
|
in the evening, reports are telegraphed
to Washington from about 200 observers
stationed in as many different localities
in the United States and Canada. In
these reports the observers do not volun-
teer their personal opinions about what is
going to happen. They confine themselves
to plain statement of the actual conditions
at that particular moment. the pressure,
or weight, of the atmosphere. the temper
allure, the direction and velocity of the
wind, the amount of rain since the last
report, and so forth. From this informa-
ton 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 1t is OBJ
vious that by comparing them the fore
caster can keep track of the course and
progress of both storms and periods of
clear weather. From that. the next step
is to predict what sort of weather will pre- =
vail for a day or two in any given locality
This map is the basis for all scientific
weather fOrecasting A glance at it will
show that it is divided into high pressure
areas" and "low pressure areas" These
are technical terms used to describe the
regions in which the weight or pressure of
the atmosphere is great (high) or small
Qow). At sea level the barometer, which
is used to measure this weight, will register
80 inches under normal conditions of the
atmosphere. When 1t registers more than
this. say sos or 81, the pressureis high :"
when 29.5 or less, low" In this way the
height of column Of mercury in barome-
ter indicates the weight of the air just as
in thermometer ii indicates the tempera-
ture. H
Low pressures generally Indicate strong
winds, rain and rising temperature high
pressures. cool, clear weather. For rea-
son to be explained later, these thighs
and lows" as they are called. travel in a
general direction from west to east The
forecaster notes their progress on the map,
perceives their speed and their route and
then predicts the time of their arrival at
any specified point. II they travelled, like
ship steered by compass. an exact course
to the east. and II they moved invariably
at the same speed, then weather forecast
ing would be simple sum in arithmetic
like calculating the time when 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 do not travel
due east. Their speed is liable to change,
and they are affected by the presence Of
other storms. by the mountain ranges, large
bodies of water. and many other things
which make weather prophesying the COM
plicated science that 1t is. The skeleton
Of the science, however. is the progress Of
those highs" and flows" 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 re-
gions along the Equator heats great masses
OF air, which rise and drift toward the
North and South Poles. As the earth re-
volves from wes, 70 oos, ,boso ,,asaa. aso
car,ied olons With I, a, Tho so,no anaod d..
a. IL olso val Lno,,n ,ha ra.o,n. ss,,o,,'aa
u,,,o, 60. ,,,a,, 49,. -.
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hedin 90 ,hov onn,oach ,ha noIoa - .,a,'.
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a.,
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