The sun. [volume] (New York [N.Y.]) 1833-1916, 1888-01-15

Now, We know. a. q matter Of ordinary oh
servatfon. that heated air will rise, the reasci
being that the heat has driven the particles o
air apart, 80 that, bulk for bulk. II is lighter
than cold air. Near the equator, where We ma,
say It is always summer. the air must, general
ly speaking, be hotter than anywhere else. A
the equator, then, the air will rise. Imme
diately the cooler air on either side flows II
from the direction OF tho poles to take its place
Tho heated air, still being held by the earth's
attraction, cannot rise indefinitely, but, havinl
reached a certain height. It low OF In the d.
rection Of the poles to supply the deficiency
resulting from the indrauGht toward the edua'
tor which its OWE rise has caused.
, Thus through the more heating OF one part
OF the atmosphere more than another, we see
that u system OF currents from the equator u
the poles and from tho poles to the equator has
been established. There is nj interchange Of
position by these currents, resulting in the
production OF calms on each side OF the equator
about one-third Of the way to the poles, which
we need not consider Just now. It Is enough to
remember that the general circulation Of the
air is such as we have described. Now. II the
Garth stood still and were perfectly smooth, the
air would blow straight from the equator to
the poles and vice versa Metsorology would
then be almost an exact science. But the earth
rotates rapidly on It's axis, and part Of It's sur
face is broken by mountain chains and vary
ing elevations of land. These elevations are
nconsidsrable in comparison with the diam
eter Of the earth. but they are considerable in
comparison with the depth Of the atmosphere.
which is a mere Aim enveloping the globe.
First let us think what the effect Of the
earth's rotation is. Recollsct, to begin with,
that the atmosphere, generally speaking, TO
tates with the earth, that Is as Q part Of 1t. II
the air stood still while the Garth rotated. man
and all his works would 10 tumbling around be-
fore a hurricane whose velocity at the equator
would be 1000 miles an hour. and at New York
more than 800 miles. But while the stmos-
phere. us a whole, rotates with tho earth. yet
when currents Of air Tow over the surface to or
from the equator the rate Of rotation Of the
moving air, through tho principle Of inertia,
will be slightly different from that Of the Garth
beneath 1t. Consider. for instance, a current
OF air flowing toward the equator. While every
point on the surface Of the earth makes q
complete rotation in twcnty-four hours. the alb
soluts velocity with which any point moves
will depend upon its distance from the axis. or
we may sny from the pole. At one of the poles,
a point one inch from the true pole, will simply
turn round that pole in twenty-four hours; q
mile from the pole a point will likewise
make a revolution in twenty-four hours, but
1t is plain that tho velocity Of the latter
point must be greater than that of the
point only an inch from the pole. In the
same way the velocity increases up to the
equator. whore 1t attains its greatest value.
Our current Of air approaching the equator
comes then from a part Of the earth whore the
velocity is comparatively slow. and moves over
a surface whose velocity 10 continually increas-
ing as 1t approaches the equator. Owing to its
trnrtfo tha az AM - -. >Itok>I- -.I. -
same velocity as that Of the surface beneath It,
and so. since tho earth rotates from west to
east. as the air draws nearer the equator it
will, by dropping behind the surface in its rate
OF rotation. appear In the northern hemisphere
to blow over that surface. not directly from tho
north, but from the northeast. By similar
reasoning it can be shown that In the southern
hemisphere the direction OF the wind caused
by such a current will be from tho southeast.
These are the trade winds which are found
on either side OF the equator up to about So
Of latitude. and are the most constant winds of
which we have any experience.
Now lot US attend for o moment to the inter-
change OF position between the current from
the equator and that from the pole which We
mentioned above. At about latitude 80 the
heated air from tho equator. having risen to a
great elevation and moved forward. has become
SSoiobt uhbAi Ao SSh4pCAnbuko 10 Sudu tlot
OF the air flowing oward the equator beneath
it. The two currents then gradually inter-
change places, and from this point on the cur
rent from the equator flows next to the earth's
surface. while that from the pole flows over 1t
in the upper air. In tho region Of interchange
the opposing currents result In q belt Of Gains
surrounding the globe. From latitude So north,
then, we have the surface air tending toward
the pole. and, as 1t Is moving over a surface
whose velocity progressively decreases. 1t will
gain upon the earth's rotation, and consequent
ly appear to flow not from the south. but from
the southwest. Thus We 1have the so-called
counter-trade winds Of the temperate zone.
which, however, are by no means as constant
as the trade winds themselves. Nearer the pole
another interchange OF currents, and conse-
quent change OF direction in the surface winds,
takes place, producing another system of
west and northeast winds are more or less In
termingled In the middle ATTITUDES.
The effect Of the inequalities Of the sarth's
surface Ia to defect and break up tho general
atmospheric currents into winds more or less
local In their characteristics. 1t Is easily seen
that u great mountain range. broad and )ons
valleys. and other accidents OF the surface may
have IN important influence upon the direction
OF the winds. The juxtaposition Of land and
sea also powerfully affects the motions OF the
atmosphere, both through difference in heating
power and difference in the amount Of mols-
ture supplied to the air. Upon the whole the
effect OF the varied nature OF the earth's sur
face is to introduce great complexity into the
air currents over 1t, and whose broader features
are due. as we have explained. FIRST to the con-
trast AS to'heat and cold between the equator
and the poles, and second to the earth's rotation
These general principles OF atmospheric cir
must be- kept in mind In order to I'M
modern weather science. and we have
stated them here as preliminary to IN account
OF the present condition OF meteorology as it
Is set forth In an excellent book on Weather. by
RALPH ABERcRoMBY I'd. Appleton Il Co... We
the author OF this book has fairly kept
the promise made In his preface, where he says:
ITO those who have only known meteorology
rs q dull branch of statistics. the perusal of
these pages may, perhaps. open a new prospect
In science and q new vision to the mind"