Encyclopaedia Britannica > Volume 3, Athens-BOI

29
ATMOSPHERE
during the whole year. The depression round the north
pole is divided into two distinct centres, at each of which
there is a diminution of pressure greatly lower than the
average north polar depression. These two centres lie in
the north of the Atlantic and Pacific Oceans respectively.
The distribution of pressure in the different months of the
year differs widely from the annual average, particularly in
January and July, the two extreme months. In January
the highest pressures are over the continents^ of the nor¬
thern hemisphere,—and the larger the continental mass
the greater the pressure,—and the lowest pressures are
over the northern portions of the Atlantic and Pacific,
South America and South Africa, and the Antarctic Ocean.
In the centre of Asia the mean pressure of the atmosphere
in this month is fully 30-400 inches, whereas in the North
Atlantic, round Iceland, it is only 29-340 inches, or
upwards of an inch lower than in Central Asia. The area
of high barometer is continued westwards through Central
and Southern Europe, the North Atlantic between •) and
45° N. lat., North America, except the north and north-west,
and the Pacific for some distance on either side of 15°
N. lat. It is thus an exaggerated form of the high belt of
annual mean pressure, spreading, however, over a much
greater breadth in North America, and a still greater
breadth in Asia.
In July, on the other hand, the mean pressure of Central
Asia is only 29"468 inches, or nearly an inch lower than
during January; or, putting this striking result in other
words, about a thirtieth of the pressure of the atmo¬
sphere is removed from this region during the hottest
months of the year as compared with the winter season.
The lowest pressures of the northern hemisphere are now
distributed over the continents, and the larger the con¬
tinental mass the greater is the depression. At the same
time, the highest are over the ocean between 50 N. and
50° S. lat., particularly over the North Atlantic and the
North Pacific between 25° and 40° N. _ lat., and in the
southern hemisphere over the belt of high mean annual
pressure, which in this month reaches its maximum height.
Pressure is high in South Africa and in Australia, just as in
the winter of the northern hemisphere pressures are high
over the continents.
Over the ocean, if we except the higher latitudes,
atmospheric pressure is more regular throughout the
year than over the land. In the ocean to westwards
of each of the continents there occurs at all seasons an area
of high pressure, from 0T0 inch to 0"30 inch higher than
what prevails on the coast westward of which it lies. The
distance of these spaces of high pressure is generally about
30° of longitude ; and their longitudinal axes lie, roughly
speaking, about the zones of the tropics. The maximum
is reached during the winter months, and these areas of
high pressure are most prominently marked west of those
continents which have the greatest breadth in 30° lat.; and
the steepest barometric gradients are on their eastern sides.
It is scarcely possible to over-estimate the importance of
these regions of high and low mean pressures, from their
intimate bearing on atmospheric physics, but more par¬
ticularly from their vital connection with prevailing winds
and the general circulation of the atmosphere. This rela¬
tion will be apprehended when it is considered that winds
are simply the flowing away of the air from regions where
there is a surplus (regions of high pressure) to where there
is a deficiency of air (regions of low pressure). Every¬
where over the globe this transference takes place in strict
accordance with Buys-Ballot’s “Law of the Winds,” which
may be thus expressed:—The wind neither blows round
the space of lowest pressure in circles returning on them¬
selves, nor does it blow directly toward that space ; but it
takes a direction intermediate, approaching, however, more
nearly to the direction and course of circular curves than
of radii to a centre. More exactly, the angle is not a right
angle, but from 45° to 80°. Keeping this relation between
wind and the distribution of pressure in mind, the isobarxc
lines give the proximate causes of the prevailing winds
over the globe, and through these the prominent features
of climates. As regards the ocean, the prevailing winds
indicate the direction of the drift-currents and other sur¬
face-currents, and thereby the anomalous distribution of
the temperature of the sea as seen in the Chili, Guinea, and
other ocean currents, and the peculiarly marked climates of
the coasts past which these currents flow, are explained; for
observations have nowr proved that the prevailing winds and
surface-currents of all oceans are all but absolutely coincident.
As regards the annual march of pressure through the
months of the year, curves representing it for the different
regions of the earth differ from each other in every con¬
ceivable way. It is only when the results are set down
in their proper places on charts of the globe that the
subject can be well understood. When thus dealt with,
many of the results are characterised by great beauty and
simplicity. Thus, of all influences which determine the
barometric fluctuation through the months, the most impor¬
tant are the temperature, and through the temperature the
humidity. Comparing, then, the average pressure in
January with that in July, which two months give the
greatest possible contrasts of temperature, the following is
the broad result:—
The January exceeds the July pressure over the whole
of Asia except Kamtchatka and the extreme north-east,
the greatest excess being near the centre of the continent;
over Europe to south and east of a line drawn from the
White Sea south-westward to the Naze, thence southward
to the mouth of the Weser, then to Tours, Bordeaux, and
after passing through the north of Spain, out to sea at
Coruna ; over North America, except the north-east and
north-west. On the other hand, the July exceeds the
January pressure generally over the whole of the southern
hemisphere, over the northern part of the North Atlantic
and regions immediately adjoining (the excess amounting
in Iceland to 0-397 inch), and over the northern part of
the North Pacific and surrounding regions. Thus the pres¬
sure which is so largely removed from the Old and New
Continents of the northern hemisphere in July is trans¬
ferred, partly to the southern hemisphere, and partly to the
northern portions of the Atlantic and Pacific Oceans.
Atmospheric pressure is more uniformly distributed over
the globe in April and October than in any of the other
months. In May and November, being the months
immediately following, occur the great annual rise and
fall of temperature ; and since these rapid changes tahe
place at very different rates, according to the relative
distribution of land and water in each region, a comparison
of the geographical distribution of May with that for the
year brings out in strong relief the more prominent causes
which influence climate, and some of the more striking
results of these causes. This comparison shows a diminu¬
tion of pressure in May over tropical and sub-tropical
regions, including nearly the whole of Asia, the southern
half of Europe, and the United States. An excess prevails
over North America to the north of the Lakes, over Arctic
America, Greenland, the British Isles, and to the north of
a line passing through the English Channel in a north¬
easterly direction to the Arctic Sea. The excess in the
southern hemisphere includes the southern half of south
America and of Africa, the whole of Australia, and adjacent
parts of the ocean. The influence of the land of the
southern hemisphere, which in this month is colder than
the surrounding seas, brings about an excess of pressure;
on the other hand, the influence of land over those regions