The Helena independent. [volume] (Helena, Mont.) 1875-1943, 1892-02-11
GUESSING THE WEATHER..
The Making Of Forecasts Based on
Recognized Principles and
A~ntecedent Data.
Movements and Effects OF the Air
Currents Explained by an
Expert.
Causes Which Produces Nain-How Storm
Csnters Are Located-Tho Making
of weather Maps.

MI. W. II. Harmon, chief of the st.
Louis weather bureau, WAS asked by a re-
porter for the Globe Democrat to explain
the methods employed BY the weather bu-
rsau in making q forecast. Mr.. Harmon
excellent explanation of how the forecaster
is able to determine what the condition Of
the weather for the coming twenty-four
hours will be: In the tropical zone the
matter Of forecasts is very simple from the
fact that the wind generally blows in but
one direction. Ih heat Of tho sun at the
equator causes the air to become warm and
to rise. This makes a vacuum and the
winds from the other side blow in. The
earth's revolution from the west to Gust is
completed once every twenty-four hours.
The earth's circumference at the cqua-
tor 10 greater than it is at any
other point, and consequently the
line which is approximately directly under
the SUMS rays have to travel very much
faster than any given point to the north
and south Of the equator. Let US suppose
that the wind III moving toward the
equator has acquired the velocity OF the
earth at the temperate zones. In moving
southwark it is continually passing over
parallels longer than those further north.
or in other words it is continually passing
over petitions of the earth that arc revolv-
ing much faster than the point from which
it started, and consequently faster than the
air. This fact makes it appear to AL OBJ
server on the earth's surface as though the
ia ...o n1zr.;r. Fuo., oou% - waat 0r in n
direction contrary to the sarth's revolu-
ton. This motion, together with the rush
Of airtoward the equator, causesthe north,
east trade winds of the northern and the
southeast trades Of the southern hemis-
phere. 4
Since il can be taken as a general law
that the weather varies as the wind changes,
owing to this almost constant current
within the limits of tho trades there are few
storms in the tropical zone, and forccasting
is hardly necessary. If we follow the course
Of this mass of air that rises at the equator
we find that it probably rises above the sur
rounding atmosphere and over1oWs at the
top. making towards either pole. Az the
mass of air moves northward in the north
ern hemisphere it is continually passing
over parallels of latitude shorter than those
al the equator, and this wave of air, which
Al ~. jaw Ao14B ss eA~
pressed into a much narrower channel, and
consequently depressed or condensed. At
least at a parallel of about 30 degrees the
pressure is greater than at any other part
Of the earth's surface, caused by the I'M
mense weight of the air above. Because on
this extra pressure from above the air on
the sarface is pressed outward in both
northerly and southerly directions. ibis
location forms the northern limit OF the
trade winds.
parallel the earth's revolution affects 1t Ill
exactly an opposite manner from what 1t
did the air moving toward the equator,
causing it to take II northeastsrly course
instead of a southwestern dlrectlon. as IN
the case Of the trades. This current Is by
no means as constant and uniform as are
the trade winds, since it is continually COM
batting with tho normal tendency of air to
move from colder to warmer regions. AS
the air moves out from under the regions
of greater pressure near thirty degrees latl-
tute, it is soon checked ana nnally over
come by the colder current.
AS these two currents, tho polar and the
southwest wind of the temperate zone,
come together the lower temperature OF
the northerly current condenses the large
amount of moisture which tbs south
ern air has collected by evapora.1oh
has passed, This condensation brings
into operation a physical law which is
tho real reason for the storms OF the mid
die latitudss, i. O., making senslble OF the
latent heat Of vapor. In explanation OF
this term, if you take a cup of water in
which there ia a thermometer graduated to
between 200 and 800 degrees and place 1t on
the fire the mercury will rise rapidly to tho
boiling point. 212 degrees; but no matter
how hot the fire is, it will not go above that
point. The water is constantly receiving
the heat, but it il consumed in changing
the water into vapor. Now, if you con-
dense this vapor back into water tho neat
necessary to vaporize it in the first place
will materialize in the condensation.
When the north and south currents COM
mingle the condensing of the vapor gives
off its latent heat. thus warming the air
that contained it and causing 1t to RISE. AS
it rises it passes into colder atmosphere
above and condenses still more OF the
moisture it contains, resulting IN moro la-
tent heat being gwen off and causing II to
process continues until the air is deprived
of its moisture, and it passes off in all direc-
tions. This rise of Aair causes more
air to flow in and take its place,
which in turn rises, causing the moisture
contained in it to be condensed, and so on.
clouds and later rain. The air in rising al
ways takes the form of an eddy, and this
eddy, which was at first slight, continues
to increase in diameter and force, fed, as it
is. by a continuous inflow OF moist air ana
giving off latent heat by its condensation.
The location Of this eddy, or storm center,
as it may now be called, is shown on q
weather map by the greatly diminished
pressure at the center and the inflow OF air
f,om all directions toward it.
The area covered by these storms ia fire.
quently 1900 miles in diameter, and. like
eddies in water, do not remain stationary,
but take up the direction of the prevailing
current. which is towards the north
east. The forscaster, in preparing for
his work. receives Geographic reports
frsn all over the country of the weather
observations taken at the same time.
He records the barometer readings at
various other places, together with other
data. upon q map of the United Stntes.
Upon this map he then draws lines between
ali points where the air pressure is the
same, one limo being drawn for each one
tenth of an inch of pressure. By this means
he is able to discover the censer of low
precanra or tho storm center. for around