The Prince George's enquirer and southern Maryland advertiser. (Upper Marlborough, Md.) 1882-1925, 1907-04-19
WHY THE WIND BLOWS
4 Study of the Circulation of the
Worlds Atmosphere.
FORCES THAT MOVE THE AIR.
Contrasts In Temperature, High and
Low Pressure and Law of Nature
Called the DETECTING Force of the
Earth's Rotation.

Earth's Rotation.
1t is a matter of common observa-
lion that when the window Of a warm
room is opened on still winter night
the cold air from without rushes into
the room. Nearer the ceiling the
warmer air is forced out of the win
dow, thus completing general atmos-
pheric circulation on miniature scale.
These currents Of air, which might
| properly be called wind, would not
occur if the air within doors was not
warmer and consequently lighter than
the air without
The range in temperature between
the equator and the north pole amounts
in winter to considerably more than
100 degrees F., and in summer the
contrast is also great. Moreover, in
summer the continents are warmer
than the oceans, but in winter the re-
verse Is true. Three examples will
serve to illustrate how such contrasts
affect the winds Of the world.
At the equator the temperature aver
ages about so degrees throughout the
year. Consequently the lower air flows
| in from regions of high pressure on
each siIe forming what are known as
the trades. These winds cover nearly
| one-half Of the earth's surface and
blow with much steadiness the year
round
The monsoons, or ''seasona. Winds y
of India and the Indian ocean are the
most interesting of their class. In
summer the cooler ocean air pushes In
toward. the land, while the warmer air
over the continents rises to a consid-
erable height and then flows out to
sea, forming a systematic circulation
| between ocean and continent. In VIM
ter the ocean is warmer than the con-
! tinent, and the winds reverse their
| direction.
The "land and sea breezes" occur
with much regularity near large bodies
OF water in some parts Of the world.
The ocean is cooler than the land dur
ing the day and warmer at night, caus-
ing on a small scale a daily inter
change of air similar to that caused
by the monsoons.
A clear knowledge of the term "air
pressure" is very helpful in studying
! the causes Of wind. Air, like stone,
presses against the ground-in other
words, it has weight, amounting to no
less than 2,117 pounds upon every
square foot Of the earth's surface at
sea level-but, unlike a stone, the at
| mosphere is elastic to high degree
and also presses in all other directions.
On account Of this elasticity Of the
air, certain forces which arise from
differences in temperature and the
earth's rotation cause it to become
| dense or heavy in some regions and
rare or light in other regions.
It is the effort Of the atmosphere to
overcome these pressure differences
and resume a state Of equal density
| that causes the winds to blow.
The column of mercury in barome-
ter tube is always just balancing a
column of air Of the same diameter,
reaching from the barometer to the
top of the atmosphere. If the air Is
dense the mercury will Of course stand
high in the tube, and to express this
| condition we use the term high
PRESSURE" but if the air is rare the
mercury will stand low in the tube,
and we then use the term Glow pres
sure"
Over the United States, Canada and
other parts Of the world the pressure
is ascertained each day at numerous
stations, The barometer readings, ex-
pressed in inches of mercury, are tele-
graphed to a central point and there
charted on a map. The exact regions
where the pressure is high or low
may then be seen at a glance. 1t has
been learned from such observations
that these areas are constantly moving
eastward at an average rate Of about
| 600 miles per day.
Technically the low pressure areas
are called cyclones and the high
pressure areas "anticyclones." They
are frequently 1000 or more miles In
diameter The little storms OF great
destructive force so often called cy-
clones are really tornadoes.
The higher the pressure in any par
ticular region relative to some other
region the greater will be the velocity
of the wind. The winds blow much
faster in winter than in summer, be-
cause the greater contrasts of tempera-
ture cause more decided differences in
pressure.
Observations demonstrate, however.
that the wind never blows in straight
lines, because all bodies of air when
in motion are acted upon by a law Of
nature called the ''defiecting force of
the earth's rotation" This force turns
all wind to the right OF its course in
the northern hemisphere and to the
left in the southern.
Thus if wind in our hemisphere
starts north it is soon turned slowly
toward the northeast, or If 1t starts
west it will soon turn toward the
northwest. When it is remem bered
that at the equator the earth Is rotat-
ing at the enormous velocity of 1,O35
miles an hour, one will not wonder that
such a DETECTING force could exist All
areas of high and low pressure, from
whatever cause, therefore become
whirling masses of air, and a little
thought will show that they must turn
in opposite directions. In the north
ern hemisphere the low areas, or
Cows as they are designated on the
weather map, always rotate In di-
IT, ti0n contrary to that Of the hands
4aI, 80 mt