stances that attend a local accession of heat over
land and water and the primary effects which it
produces.

Beginning with any area on a small scale.
Let fig. (10) represent a vertical section of the
atmosphere and let the dotted lines represent



56 THE STORY OF THE EARTH'S ATMOSPHERE.

lines of equal barometric pressure beginning with
30 inches at the earth's surface, and let us sup-
pose that the temperature of the central region
is raised by a certain amount.

All the air thus warmed will expand. The
column H.A. will expand to height H.B., and as
each layer will expand all the way up, the sur-
face of the top layer will be most raised. Con-
sequently there will be a flow outwards of the
raised up air down the slopes marked by the
thick lines toward the neighbouring air of the
same pressure, which, not being expanded, lies at
a lower level.

The outflow will be greatest in the highest
layer since it is the most raised (the increase is



denoted by the varying size of the arrows).
Meanwhile the loss of air above will lessen the



THE TEMPERATURE OF THE ATMOSPHERE. 57

pressure on the earth's surface near the centre of
the area. Consequently the surrounding air will
flow in towards this centre chiefly in the lowest
layer, and the action having once started will
continue so long as the central area is more
heated than the neighbourhood.

We have already noticed that where sun heat
falls upon land it heats it up more readily than
water. Therefore particularly in the case of an
island lying in a tropical sea where the sun is
powerful the above action takes place as in fig.
(n) and we have the phenomenon known as the
local sea breeze. When the sun disappears at
night the action is precisely reversed, and the
air near the surface flows outwards as the land
breeze, while above a certain height, which in
local cases is often as low as 1000 feet, the air
streams in over the rapidly cooling land.




FIG. 12.



After the action has once started things ar-
range themselves as in fig. (12) where the lower
curved lines represent the depression caused by
the loss of air which has flowed outwards above,
and where NN 1 represents where the tendency



5 8 THE STORY OF THE EARTH'S ATMOSPHERE.

to flow in and out neutralise each other and there
is a plane of no motion called sometimes the
neutral plane.

The above action involves a certain amount
of upward motion of the air over the central part
of the heated area, and a corresponding down-
ward motion over the surrounding cooler area, but
these movements are evidently much smaller than
the horizontal outflow and inflow. The same
action also explains the origin of the manifest
monsoons of Asia and Australia, where in the
summer season the air blows more or less towards
a heated land area, and in the winter from it
towards the surrounding sea.

It also accounts in part for the annual changes
in the barometric pressure over large areas, espe-
cially the low pressures in the middle of the larger
continents like Asia and America during the sum-
mer, and the corresponding very high pressures
at the opposite season.

Unless some such system of rise and overflow
over the hotter areas and sinking and underflow
over the cooler areas took place, the barometer
would record a steady pressure over both areas,
and if we ascended over the more heated area we
should find the pressure greater than at the same
level over the cooled area, because the air being
more expanded vertically, there would be more
top cover so to speak over our heads.

As a matter of fact, notwithstanding the over-
flow which relieves this state of things, the pressure
at highly elevated stations like Leh (11,800 feet)
north of Kashmir, rises until the beginning of
May, and only falls very slightly in June and
July. Consequently the lowering of pressure
which appears so distinctly over Southern Asia in



THE TEMPERATURE OF THE ATMOSPHERE. 59

the summer is confined to the lower half (by mass)
of the atmosphere, that is to say below 18,000
feet, at which level the pressure is 15 inches.
Above this level there is more or less an outflow
in the summer and an inflow in the winter.

A similar system of land and sea monsoonal
circulation exists everywhere, only in high lati-
tudes it is ordinarily masked by other motions of
the air, introduced by the frequent passage of
cyclones, and large travelling systems or waves
of high and low pressure. Even along Western
Europe the winds blow more towards the land in
summer and from it in the winter.

Where a small area on the land or sea is
heated up above its neighbourhood we have the
initial conditions for the formation of a disturb-
ance of equilibrium. In hot countries where such
a condition is more prevalent, there may arise a
cyclone, tornado, whirlwind, or thunder-storm,
under different conditions, which will be alluded
to later on, but in order that there may be intense
local action and a real " courant ascendant" the air
must not be merely gently lifted up and overflow,
which is the only possible condition when it is
dry, but it must be nearly saturated with vapour,
in which case it will flow upwards so long as the
lowest stratum continues to supply damp air.
The part taken by temperature in causing these
phenomena will be alluded to in a later chapter.

The present account of the temperature of the
atmosphere would be incomplete if it omitted to
notice the transfer of heat from one part of the
earth to another.

So far we have merely examined the heat
which falls locally or generally by means of the
direct solar radiation.



60 THE STORY OF THE EARTH'S ATMOSPHERE.

The temperature over any region is however
largely dependent on the heat brought to it by
winds. When they come from the sea their tem-
perature is modified by the influence of the ocean
currents, warm or cold, over which they have
traveled. When they come from the interior of a
continent, they are usually hotter in the summer
and colder in the winter than the maritime re-
gions towards which they advance.

Thus in summer our hottest wind in England
is the south-east, and the same wind often ac-
companies our most severe frosts in the winter.
The thermal effects of land winds therefore change
with the season.

Sea winds, especially where they are connected
with ocean currents, and blow with some degree
of constancy, exercise a permanent influence
upon the temperature of countries over which
they prevail. The most marked warm sea winds
are felt on Western Europe, the Pacific slope
north of lat. 40, and the eastern coast of South
America.

These winds are not merely warm because
they have accompanied streams of warm water,
such as the Gulf stream of the Atlantic and the
Japan stream of the Pacific, but because their
cooling is retarded by the latent heat set free in
the condensation of the vapour they bring from
the humid tropics.

Several attempts have been made to measure
the heat conveyed by both these streams. Dr.
Haughton of Dublin some years ago estimated
that these two streams together carried one-third
of the total heat received by the northern tropi-
cal zone towards the middle latitudes. Ferrel,
however, has more recently shown that it is more



THE TEMPERATURE OF THE ATMOSPHERE. 6l

probably one-sixth. As we have already seen,
the effect on England is to raise the mean tem-
perature nearly 10 degrees above what it would
otherwise be. Norway is raised as much as 16
degrees, and Spitzbergen 19 degrees. On the
other hand compensating cold currents and the
winds which blow off them depress the tempera-
ture of Eastern Canada, northern China, western
South America, and western South Africa. New-
foundland is thus about 10 degrees colder than
the normal for the latitude. The North China
coast about 7 degrees colder, and even Honolulu,
in the mid Pacific, has its temperature reduced 5
degrees by the return Japan stream cooled after
losing its heat up north.

The general influence of the ocean currents in
reducing the difference which would exist be-
tween the temperature at the equator and the
poles, may be inferred from the fact, that accord-
ing to Ferrel, if the surface of the earth were en-
tirely dry land, and there were consequently no
transfer of heat by oceanic or atmospheric cur-
rents, theoretical considerations shew that the
temperature at the equator would stand at about
131 F., while that at the Pole would be 108 be-
low Zero.

Observations, however, shew that the mean
temperature day and night at the Equator is
about 80 F., while that at the Pole is only o F.
or Zero. Consequently the effect of the circula-
tion of the ocean and the atmosphere together is
to depress the temperature at the Equator about
50 degrees and raise that at the Pole no less than
100 degrees, and in this manner render the earth
generally fit for human habitation, since, if such
extremes as those mentioned prevailed, man



62 THE STORY OF THE EARTH'S ATMOSPHERE.

would have been forced to inhabit a very con-
stricted zone in middle latitudes. In like manner
were the earth deprived of its atmosphere the
mean temperature at the Equator would be 94
degrees below zero F., while that at the poles
would be 328 degrees below zero F., and the
mean temperature of the whole globe 138 degrees
below zero F., a terrible frost. In fact, even if
it were possible to do without air the human
species as at present constituted would in such an
event be quite unable to exist. With the protec-
tion of an atmosphere the average temperature of
the earth, or more correctly of the lowest stratum
of the atmosphere is about 60 F. which is re-
garded as the most delightful that can be enjoyed.
So much do we owe to the invisible envelope of
atmospheric air, which otherwise appears to con-
stitute such a flimsy blanket between us and the
terrible cold of stellar space.

Extreme local temperatures are due to the
concurrence of accidental causes tending to raise
or lower the temperature, such as the passage of
storms, prevalence of winds from north or south,
long continued clear weather, combined with
those of more regular incidence. Extremely
high temperature will generally occur in these
latitudes soon after noon in July and August,
and extremely low ones early in the morning in
January or February. Occasionally, however,
the epochs are considerably displaced.

The highest temperatures on the earth usually
occur in India, the Red Sea, the Persian Gulf, and
Australia. Thus in the centre of the Sahara, 130
degrees has been recorded. At Jacobabad in the
Sind desert, the temperature frequently rises over
120 F. and even in New South Wales, 120 and



THE TEMPERATURE OF THE ATMOSPHERE. 63

121 have been recorded at Bourke and Denili-
quin. In February, 1896, a temperature of 108
degrees was recorded at Sydney, due to a remark-
able prevalence of dry N. W. winds blowing over
it from the interior.

Paris has only once reached 106 degrees and
London has seldom recorded anything over 96.

The coldest temperatures are found not at the
poles themselves, where the water circulation
tends to bring heat from the equator but in the
north-east of Siberia and north-east America.

Werkojansk is the coldest place in the world.
In January the mean temperature there is 55 F.
below zero while all through the year the temper-
ature is only 5 degrees above zero.

During arctic expeditions, the Alert and Dis-
covery experienced 73 below zero, while Capt.
Nares once saw the thermometer descend to 84
F. below zero.

Of recent years, a great extension of our
knowledge of the phenomena of the atmosphere
has been made by the application of what is
known as thermo-dynamics.

Prof. Bezold of Berlin, the late Mr. Ferrel of
Washington, Dr. Hann of Vienna, and others have
cleared away much of the loose and misty reason-
ings which characterize the work of their prede-
cessors, but the subject is too difficult and tech-
nical to be alluded to here. A few of the leading
ideas however will be briefly touched upon when
some of the particular atmospheric phenomena
are being described further on.



64 THE STORY OF THE EARTH'S ATMOSPHERE.



CHAPTER V.

THE GENERAL CIRCULATION OF THE
ATMOSPHERE.

THE " Story of the Winds " is interesting and
important enough to form the subject of a sepa-
rate volume and within the compass of one which
endeavours to cover the varied phenomena of the
atmosphere generally, only the more salient
points in connection with atmospheric motion can
be reviewed. In these latter days, in spite of the
old saying that " the wind bloweth where it list-
eth " and the manifest and apparently capricious
changes which characterize its behaviour in these
midway latitudes, we know that there exists an
independent dominating scheme of general circu-
lation between the poles and the equator. This
scheme results from the action of nearly perma-
nent differences of temperature between these
points in combination with certain mechanical
laws resulting from the shape of the earth and its
rotation on its axis.

In former days many guesses were made more
or less at variance with both facts and theory.
Even Maury's fascinating attempt in 1855 to
weave observation into a connected system, failed
owing to the imperfect knowledge existing at that
time of the winds of the entire globe as well as of
the true laws which operated.

The earliest attempt at any rational scheme of
accounting for the more obvious features of the
general circulation appears to have been made in
1735 b y Hadley.

The regularity of the " trade winds " was then



GENERAL CIRCULATION OF THE ATMOSPHERE. 65

attracting the attention of scientists, and in a
short paper in the Philosophical Transactions,
Hadley advanced a theory to account for this
which sounded so plausible, that for over a cen-
tury it remained unquestioned.

Hadley's theory in brief was, that owing to the
general difference of temperatures between the
polar and equatorial regions, a motion of the air
took place similar to that just described in the
last chapter, in the lower strata towards the zone
of greatest heat, while the easterly * direction of
the trades was attributed to the fact that as the
air continually arrived at parallels where the
earth's surface moved faster eastwards than the
part it had just left, it tended continually to lag
behind in a westward direction, and so appear to
blow partly from the east. Hence it became the
north-east trade on the northern and the south-
east trade on the southern side of the equator.
Carried to its logical conclusions Hadley's theory
would require the trades to blow all the way
from the poles to the equator, the return current
being confined almost entirely to the upper air.

Moreover the highest pressure as measured
by the height of the mercury column in the
barometer air balance, should be found at or near
the poles.

As a matter of fact, however, it was found
that neither of these circumstances took place.

The trades extended no further than latitudes
30 degrees N. and S. of the equator, the pressure
at the poles, especially the south pole was per-
manently lower than at the equator (about ths
of an inch of mercury) while the highest pressure

* From the East.



66 THE STORY OF THE EARTH'S ATMOSPHERE

was found to occupy two belts between 30 and
40 N. and S. of the equator.

Obviously therefore there was something radi-
cally wrong with Hadley's theory.

In 1856 Mr. Ferrel, afterwards Professor in
the United States Weather bureau, tackled the
subject and found out that Hadley had entirely
overlooked the fact that the earth is a sphere.

In consequence his theory contained two seri-
ous errors, one of which was that only air moving
north and south was deflected by the earth's rota-
tion, while that moving in any other direction re-
mained unchanged.

The only circumstances to which Hadley's
theory could possibly apply would involve the
supposition that the earth was a perfectly flat
plane composed of separate planks parallel to a
straight line equator. Also that these planks
moved along with different speeds beginning
with 1000 miles at the equator and gradually
decreasing to about 850 miles at latitude 30,
manifestly a very different affair from a spherical
surface like that of the earth.

Some few years before Ferrel approached the
question, the eminent French mathematician
Poisson in 1837 read a paper before the Paris
Academy, in which he demonstrated that when
a freely moving body passes over the earth's
surface in any direction, the effect of the earth's
rotation is to cause it to deviate (not lag) to the
right of its path in the northern hemisphere, and
to the left in the southern.

Employing the same reasoning as Poisson,
but applying it to masses of air instead of solid
bodies Ferrel gradually built up a satisfactory
explanation of the general circulation, and with



GENERAL CIRCULATION OF THE ATMOSPHERE. 67

the help of suitable modifications, applied the
same principle to explain the leading features of
cyclones and tornadoes.

In general, if a mass of air initially tends to
move on a rotating sphere toward a certain point,
impelled in the first instance by a difference of
density or pressure, it tends to move continually
to the right when looked
at from a point above the
N. pole of rotation and
unless prevented from do-
ing so by any extra force
resisting such motion,
would continue to devi-
ate until it had turned
through a complete circle
thus fig. (13).

Suppose a particle of
air at A starts to move FIG. 13.

towards B. Instead of

moving in the straight line AB, it will tend to
move in the curve AC, and if it is very near the
pole it will eventually complete a circle* as above
in 12 hours, the size depending on its velocity.
Thus for a speed of

Radius of inertia circle

in miles.

20 miles an hour . . . 77

10 " " . . . . 38

5 " . . . . 19

We have the corresponding values of what is
termed the curve of inertia.

Prof. Davis of Harvard has suggested a very

* In other latitudes the inertia curve as it is termed is
more like the series of loops cut by a skater restricted be-
tween certain limits of latitude. At the equator it vanishes.




68 THE STORY OF THE EARTH'S ATMOSPHERE.

pretty experiment which can be performed by
any one who wishes to have visual evidence of
the existence of this inertia curve.

Take a small circular table, and lay a sheet
of white paper over it. Then take a marble and
dip it in ink and lay it near the centre of the
table. Next tilt the table slightly so as to give
the marble a slight motion towards the edge, and
at the same time rotate the table about its centre.
Then the marble will be found to trace out in ink
a curved line on the paper which will fairly rep-
resent the inertia curve or the curve of successive
deviations from a straight line by which the par-
ticle through its inertia (or laziness) is unable to
accommodate itself to the varying motion of the
parts over which it rolls.

In whatever direction the table is tilted the
curve will still be traced out, the curvature sharp-
ening with increased rotation of the table (analo-
gous to increased latitude) and lessening with in-
creased tilt by which the velocity of the particle
is augmented.

This tendency of air to move to the right of
its original direction of motion, is what really ac-
counts for the development of those permanent
or rapidly changing differences of barometric
pressure which accompany the large general or
small particular air motions. A difference of
temperature alone, for example, between the
poles and equator, or between two neighbour-
ing parts of the earth would cause a very slight
alteration in the barometric pressures, but when
the air begins to move in the direction of the
lower pressure its tendency to push to the right,
causes a squeezing and heaping up of air to the
right of its path, and a corresponding stretching




GENERAL CIRCULATION OF THE ATMOSPHERE. 69

apart or lowering of density and pressure to its
left, until the difference of pressures becomes great
enough to prevent its further movement to the
right and it moves in a path regulated by these
joint tendencies, thus

Path (curved

norm i ^f in which, air

area. I



Direction,
in which -
atr starts to
move

jTMtmiiimuiiiinnnin 7 ~~&*

Inertia curve
into which

iff** .....-.,.,-. .,..-. - - - cu'r tends to

Cold area & deflected

force
exerted by
pressure qraaienC
FIG. 14.

Amass of air at the cold area will tend initial-
ly to move towards the less dense warm air, but
once it starts it tends to move along the inertia
curve. Eventually the high pressure (denoted
by the shading) of the heaped up air on this side
exerts a force indicated by the arrow directed
towards the increased low pressure to the left,
and finally the air making a compromise moves
along a line between the two, indicated by the
direction, labelled " Path," etc., so that instead of
moving directly from high to low pressure it only
partly moves towards the latter, keeping the high
pressure to the right and the low pressure to the
left of its path. In the southern hemisphere
owing to the reversed point of view right be-
comes left and the high pressure would be to



70 THE STORY OF THE EARTH'S ATMOSPHERE.

the left of the path and the low pressure to the
right.

This diagram will be found to supply the ex-
planation of the general relations between pres-
sure and wind, especially if it is remembered that
where, as on land and near the surface, the air is
prevented by friction from moving with freedom,
the back thrust in the opposite direction tends to
make the ultimate path point more towards the
low pressure, while at sea and at great altitudes,
where friction is small, it moves almost at right
angles to the line joining the central areas of
high and low pressures, or in the technical lan-
guage borrowed from engineering, at right angles
to the direction of the barometric gradient.

Before alluding to Ferrel's explanation of the
general circulation of air over the globe on these
principles, let us see what this circulation really
is like from observation.

In the two plates adjoining, figs. (15) and (16),
in which the actually observed barometric pres-
sures and winds at two opposite seasons of the
year are represented, it will be noticed that, over-
looking minor features, there is a broad belt over
the equator, over which the barometric pressure
is about 29.80 inches, gradually rising on either
side to two belts of high pressure, in latitude
30 in places reaching 30.2 inches, and generally
.about 0.2 inches higher than over the equator.
Within this area, the trade winds blow through-
out the year on each side of the equator, except
over the North Indian Ocean, where in July they
blow in towards an area of excessively low pres-
sure and high temperature as the south-west
monsoon of the Indian seas, which brings the
rain, that has made India such a much more fer-



GENERAL CIRCULATION OF THE ATMOSPHERE. 71

tile and populous country than the neighbouring-
peninsula of Arabia. In the map fig. (20), p. 81,
the monsoon winds are represented blowing over
India during July. In January, the south-west
winds disappear, and in the general chart it will
be seen that their place is taken by Northerly or
North-easterly winds, blowing down towards the
equator, from the large area of high pressure
which at this season spreads over the whole of
north-eastern Asia.

On the polar sides of these bands or nuclei of
high pressure, it will be observed that the winds
blow more or less towards the poles, especially in
the southern hemisphere.

The lines (isobars) on these maps, by which
the changes in the distribution of the mean
monthly barometric pressure is indicated, are
similar to the contour lines or lines of equal ele-
vation employed to represent the contour of a
hilly country. They do not necessarily represent
real elevations or depressions of the atmosphere,
because increased or decreased pressure is more
due to a greater or less squeezing or density, than
to a piling up of the atmosphere into absolute
heaps and hollows, but since the effective results
would be very much the same in either case, they
may practically be considered as atmospheric
contours. More correctly, they are the lines
along which atmospheric contours intersect the
earth's surface, the pressure over which at sea
level, (about 30 inches), lies half way up the at-
mospheric slope. The accompanying figure will
render this clearer.

The sloping lines marked 30.2, 30, 29.8 etc..