Rock Island Argus. (Rock Island, Ill.) 1893-1920, 1914-05-01

| indicates the weight of The air lust as,
| in a thermometer, it indicates the tenn
perature.
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 arriy al
at any specified point. If they travel
ed, like ship steered by compass, 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 caIculat-
ing the time when a railroad train rum
ming miles an hour will arrive at =
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 pTopkesying the complicated
science that il 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 we'll 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 revolves
much faster than the poles, which are
practically stationary, in much the
same way as the rim of wheel re-
wolves faster than the hub. There
fore, these masses of air, revolving at
the same rate as the equator, begin,
as they approach the poles, to move
much faster than the earth beneath
them. There is thus constant move
ment of the atmosphere from west to
east, a movement which becomes more
marked the nearer one gets to the
poles.
This, of course. does not mean that
the wind always blows from the west
The great current flows in that direc-
ton, but surface conditions cause in
eddies which are the winds
we feel. Into a low pressure area, for
instance, the air rushes from every di-
rection. Thus, if the center of the dis
turbance is to the west of us, we will
have an easterly wind as the air rush /
es toward this center; when the storm
has reached and passed us on its west
erly way we will have westerly winds. |
For this reason it is common to say
that west winds mean clear weather.
The air that flows into the low pres |
sure area obviously must go some !
where. Since it is coming in from ev-
cry point of the compass, the only |
available place is up. AS it rises it /,
cools and contracts. In the process |
the moisture is contains is condensed |
and we have rain. That is, we are like |
ly to. 1t can not be stated too en-
phatically that there are many things |
to be considered which may make ex-
ceptions to the most fundamental
rules.
In regard to temperature, everyone |
has noticed that rain in winter means |
warm weather, in summer cool wealth
er. This arises from the fact that /
heat travels more easily through clear
skies than through clouds and moist
ure. IN the daytime heat reaches the |
earth from the sun; at night it leaves |
the earth to be absorbed in the atmos-
micro thPrnfnra W
the days are longer than the nights, |
the earth is being heated for greater /.
part of the "a hours than it is being |
cooled. In consequence the clearer the
weather and the easier it is for heat to |
travel, the hotter it grows. In winter /
the reverse is true. The cooling time |
is longer than the heating, and the |
clearer it is the colder it grows
Thus the pressure of the atmosphere
Is the key to the weather, affecting the |
three vital questions of rain, tempera-
-ure and wind Many things may Gre
ate an area of low pressure and many
-hings may influence its career when
once it has been created. But there
are certain general rules based on the
principles already outlined. The !
weather maps tell the forecaster the
conditions of the last few days. the
tells him of the conditions
1t the moment, and with this informa-
ion he is able to predict the condi-
ions for the immediate future from
! standpoint very different to that of
he amateur observer, however, exper-
enced, who can form an opinion only
rom the signs visible TO his unaided
eye. 1t may. in fact, be said that no
accurate forecasts for more than few
hours in advance are possible unless
he prophet is able to study a series
If observations covering a wide range
# country a few hours after they have