Temperature.
Among the places on the globe where the temperature falls lowest are
some in northern Asia; and among those where it rises highest are some
in southern Asia. The mean temperature of the north coast of eastern
Siberia is but a few degrees above the zero of Fahrenheit; the lowest
mean temperature anywhere observed is about 4 deg. Fahr., at Melville
Island, north of the American continent. The isothermals of mean
annual temperature lie over northern Asia on curves tolerably regular
in their outline, having their western branches in a somewhat higher
latitude than their eastern; a reduction of 1 deg. of latitude
corresponds approximately--and irrespective of modifications due to
elevation--to a rise of 1/2 deg. Fahr., as far say as 30 deg. N, where
the mean temperature is about 75 deg. Fahr. Farther south the increase
is slower, and the highest mean temperature anywhere attained in
southern Asia is not much above 82 deg. Fahr.
The variations of temperature are very great in Siberia, amounting
near the coast to more than 100 deg. Fahr., between the mean of the
hottest and coldest months, and to still more between the extreme
temperatures of those months. In southern Asia, and particularly near
the sea, the variation between the hottest and coldest monthly means
is very much less, and under the equator it is reduced to about 5 deg.
In Siberia the difference between the means of the hottest and coldest
months is hardly anywhere less than 60 deg. Fahr. On the Sea of Aral
it is 80 deg. Fahr., and at Astrakhan, on the Caspian, more than 50
deg. At Tiflis it is 45 deg. In northern China, at Peking, it is 55
deg., reduced to 30 deg. at Canton, and to 20 deg. at Manila. In
northern India the greatest difference does not exceed 40 deg., and it
falls off to about 15 deg. at Calcutta and to about 10 deg. or 12 deg.
at Bombay and Madras. The temperatures at the head of the Persian Gulf
approximate to those of northern India, and those of Aden to Madras.
At Singapore the range is less than 5 deg., and at Batavia in Java,
and Galle in Ceylon, it is about the same. The extreme temperatures in
Siberia may be considered to lie between 80 deg. and 90 deg. Fahr. for
maxima, and between -40 deg. and -70 deg. Fahr. for minima. The
extreme of heat near the Caspian and Aral Seas rises to nearly 100
deg. Fahr., while that of cold falls to -20 deg. Fahr. or lower.
Compared with these figures, we find in southern Asia 110 deg. or 112
deg. Fahr. as a maximum hardly ever exceeded. The absolute minimum in
northern India, in lat. 30 deg., hardly goes below 32 deg.; at
Calcutta it is about 40 deg., though the thermometer seldom falls to
50 deg. At Madras it rarely falls as low as 65 deg., or at Bombay
below 60 deg. At Singapore and Batavia the thermometer very rarely
falls below 70 deg., or rises above 90 deg. At Aden the minimum is a
few degrees below 70 deg., the maximum not much exceeding 90 deg.
These figures sufficiently indicate the main characteristics of the
air temperatures of Asia. Throughout its northern portion the winter
is long and of extreme severity; and even down to the circle of 35
deg. N. lat., the minimum temperature is almost as low as zero of
Fahrenheit. The summers are hot, though short in the northern
latitudes, the maximum of summer heat being comparatively little less
than that observed in the tropical countries farther south. The
moderating effect of the proximity of the ocean is felt in an
important degree along the southern and eastern parts of Asia, where
the land is broken up into islands or peninsulas. The great elevations
above the sea-level of the central part of Asia, and of the
table-lands of Afghanistan and Persia, tend to exaggerate the winter
cold; while the sterility of the surface, due to the small rainfall
over the same region, operates powerfully in the opposite direction in
increasing the summer heat. In the summer a great accumulation of
solar heat takes place on the dry surface soil, from which it cannot
be released upwards by evaporation, as might be the case were the soil
moist or covered with vegetation, nor can it be readily conveyed away
downwards as happens on the ocean. In the winter similar consequences
ensue, in a negative direction, from the prolonged loss of heat by
radiation in the long and clear nights--an effect which is intensified
wherever the surface is covered with snow, or the air little charged
with vapour. In illustration of the very slow diffusion of heat in the
solid crust of the earth, and as affording a further indication of the
climate of northern Asia, reference may here be made to the frozen
soil of Siberia, in the vicinity of Yakutsk. In this region the earth
is frozen permanently to a depth of more than 380 ft. at which the
temperature is still 5 deg. or 6 deg. Fahr. below the freezing point
of water, the summer heat merely thawing the surface to a depth of
about 3 ft. At a depth of 50 ft. the temperature is about 15 Fahr.
below the freezing point. Under such conditions of the soil, the land,
nevertheless, produces crops of wheat and other grain from fifteen to
forty fold.
The very high summer temperatures of the area north of the tropic of
Cancer are sufficiently accounted for, when compared with those
observed south of the tropic, by the increased length of the day in
the higher latitude, which more than compensates for the loss of heat
due to the smaller mid-day altitude of the sun. The difference between
the heating power of the sun's rays at noon on the 21st of June, in
latitude 20 deg. and in latitude 45 deg., is only about 2%; while the
accumulated heat received during the day, which is lengthened to
15-1/2 hours in the higher latitude, is greater by about 11% than in
the lower latitude, where the day consists only of 13-1/4 hours.
Although the foregoing account of the temperatures of Asia supplies
the main outline of the observed phenomena, a very important modifying
cause, of which more will be said hereafter, comes into operation over
the whole of the tropical region, namely, the periodical summer rains.
These tend very greatly to arrest the increase of the summer heat over
the area where they prevail, and otherwise give it altogether peculiar
characteristics.
Pressure and Winds.
The great summer heat, by expanding the air upwards, disturbs the
level of the planes of equal pressure, and causes an outflow of the
upper strata from the heated area. The winter cold produces an effect
of just an opposite nature, and causes an accumulation of air over the
cold area. The diminution of barometric pressure which takes place all
over Asia during the summer months, and the increase in the winter,
are hence, no doubt, the results of the alternate heating and cooling
of the air over the continent.
The necessary and immediate results of such periodical changes of
pressure are winds, which, speaking generally, blow from the area of
greatest to that of least pressure--subject, however, to certain
modifications of direction, arising from the absolute motion of the
whole body of the air due to the revolution of the earth on its axis
from west to east. The south-westerly winds which prevail north of the
equator during the hot half of the year, to which navigators have
given the name of the south-west monsoon (the latter word being a
corruption of the Indian name for season), arise from the great
diminution of atmospheric pressure over Asia, which begins to be
strongly marked with the great rise of temperature in April and May,
and the simultaneous relatively higher pressure over the equator and
the regions south of it. This diminution of pressure, which continues
as the heat increases till it reaches its maximum in July soon after
the solstice, is followed by the corresponding development of the
south-west monsoon; and as the barometric pressure is gradually
restored, and becomes equalized within the tropics soon after the
equinox in October, with the general fall of temperature north of the
equator, the south-west winds fall off, and are succeeded by a
north-east monsoon, which is developed during the winter months by the
relatively greater atmospheric pressure which then occurs over Asia,
as compared with the equatorial region.
Although the succession of the periodical winds follows the progress
of the seasons as just described, the changes in the wind's direction
everywhere take place under the operation of special local influences
which often disguise the more general law, and make it difficult to
trace. Thus the south-west monsoon begins in the Arabian Sea with west
and north-westerly winds, which draw round as the year advances to
south-west and fall back again in the autumn by north-west to north.
In the Bay of Bengal the strength of the south-west monsoon is rather
from the south and south-east, being succeeded by north-east winds
after October, which give place to northerly and north-westerly winds
as the year advances. Among the islands of the Malay Archipelago the
force of the monsoons is much interrupted, and the position of this
region on the equator otherwise modifies the directions of the
prevailing winds. The southerly summer winds of the Asiatic seas
between the equator and the tropic do not extend to the coasts of
Java, and the south-easterly trade winds are there developed in the
usual manner. The China Sea is fully exposed to both monsoons, the
normal directions of which nearly coincide with the centre of the
channel between the continent of Asia and the eastern islands.
The south-west monsoon does not generally extend, in its character of
a south-west wind, over the land. The current of air flowing in from
over the sea is gradually diverted towards the area of least pressure,
and at the same time is dissipated and loses much of its original
force. The winds which pass northward over India blow as
south-easterly and easterly winds over the north-eastern part of the
Gangetic plain, and as south winds up the Indus. They seem almost
entirely to have exhausted their northward velocity by the time they
have reached the northern extremity of the great Indian plain; they
are not felt on the table-lands of Afghanistan, and hardly penetrate
into the Indus basin or the ranges of the Himalaya, by which
mountains, and those which branch off from them into the Malay
peninsula, they are prevented from continuing their progress in the
direction originally imparted to them.
Among the more remarkable phenomena of the hotter seas of Asia must be
noticed the revolving storms or cyclones, which are of frequent
occurrence in the hot months in the Indian Ocean and China Sea, in
which last they are known under the name of typhoon. The cyclones of
the Bay of Bengal appear to originate over the Andaman and Nicobar
islands, and are commonly propagated in a north-westward direction,
striking the east coast of the Indian peninsula at various points, and
then often advancing with an easterly tendency over the land, and
passing with extreme violence across the delta of the Ganges. They
occur in all the hot months, from June to October, and more rarely in
November, and appear to be originated by adverse currents from the
north meeting those of the south-west monsoon. The cyclones of the
China Sea also occur in the hot months of the year, but they advance
from north-east to south-west, though occasionally from east to west;
they originate near the island of Formosa, and extend to about the
10th degree of N. lat. They are thus developed in nearly the same
latitudes and in the same months as those of the Indian Sea, though
their progress is in a different direction. In both cases, however,
the storms appear to advance towards the area of greatest heat. In
these storms the wind invariably circulates from north by west through
south to east.
Rainfall.
The heated body of air carried from the Indian Ocean over southern
Asia by the south-west monsoon comes up highly charged with watery
vapour, and hence in a condition to release a large body of water as
rain upon the land, whenever it is brought into circumstances which
reduce its temperature in a notable degree. Such a reduction of
temperature is brought about along the greater part of the coasts of
India and of the Burmo-Siamese peninsula by the interruption of the
wind current by continuous ranges of mountains, which force the mass
of air to rise over them, whereby the air being rarefied, its specific
capacity for heat is increased and its temperature falls, with a
corresponding condensation of the vapour originally held in
suspension.
This explanation of the principal efficient cause of the summer rains
of south Asia is immediately based on an analysis of the complicated
phenomena actually observed, and it serves to account for many
apparent anomalies. The heaviest falls of rain occur along lines of
mountain of some extent directly facing the vapour-bearing winds, as
on the Western Ghats of India and the west coast of the Malay
peninsula. The same results are found along the mountains at a
distance from the sea, the heaviest rainfall known to occur anywhere
in the world (not less than 600 in. in the year) being recorded on the
Khasi range about 100 m. north-east of Calcutta, which presents an
abrupt front to the progress of the moist winds flowing up from the
Bay of Bengal. The cessation of the rains on the southern border of
Baluchistan, west of Karachi, obviously arises from the projection of
the south-east coast of Arabia, which limits the breadth of the
south-west monsoon air current and the length of the coast-line
directly exposed to it. The very small and irregular rainfall in Sind
and along the Indus is to be accounted for by the want of any obstacle
in the path of the vapour-bearing winds, which, therefore, carry the
uncondensed rain up to the Punjab, where it falls on the outer ranges
of the western Himalaya and of Afghanistan.
The diurnal mountain winds are very strongly marked on the Himalaya,
where they probably are the most active agents in determining the
precipitation of rain along the chain--the monsoon currents, as before
stated, not penetrating among the mountains. The formation of dense
banks of cloud in the afternoon, when the up wind is strongest, along
the southern face of the snowy ranges of the Himalaya, is a regular
daily phenomenon during the hotter months of the year, and heavy rain,
accompanied by electrical discharges, is the frequent result of such
condensation.
Too little is known of the greater part of Asia to admit of any more
being said with reference to this part of the subject, than to
mention a few facts bearing on the rainfall. In northern Asia there
is a generally equal rainfall of 19 to 29 in. between the Volga and
the Lena in Manchuria and northern China, rather more considerable
increase in Korea, Siam and Japan. At Tiflis the yearly fall is 22
in.; on the Caspian about 7 or 8 in.; on the Sea of Aral 5 or 6 in. In
south-western Siberia it is 12 or 14 in., diminishing as we proceed
eastward to 6 or 7 in. at Barnaul, and to 5 or 6 in. at Urga in
northern Mongolia. In eastern Siberia it is about 15 to 20 in. In
China we find about 23 in. to be the fall at Peking; while at Canton,
which lies nearly on the northern tropic and the region of the
south-west monsoon is entered, the quantity is increased to 78 in. At
Batavia in Java the fall is about 78 in.; at Singapore it is nearly
100 in. The quantity increases considerably on that part of the coast
of the Malay peninsula which is not sheltered from the south-west by
Sumatra. On the Tenasserim and Burmese coast falls of more than 200
in. are registered, and the quantity is here nowhere less than 75 or
80 in., which is about the average of the eastern part of the delta of
the Ganges, Calcutta standing at about 64 in. On the hills that flank
Bengal on the east the fall is very great. On the Khasi hills, at an
elevation of about 4500 ft., the average of ten years is more than 550
in. As much as 150 in. has been measured in one month, and 610 in. in
one year. On the west coast of the Indian peninsula the fall at the
sea-level varies from about 75 to 100 in., and at certain elevations
on the mountains more than 250 in. is commonly registered, with
intermediate quantities at intervening localities. On the east coast
the fall is far less, nowhere rising to 50 in., and towards the
southern apex of the peninsula being reduced to 25 or 30 in. Ceylon
shows from 60 to 80 in. As we recede from the coast the fall
diminishes, till it is reduced to about 25 or 30 in. at the head of
the Gangetic plain. The tract along the Indus to within 60 or 80 m. of
the Himalaya is almost rainless, 6 or 8 in. being the fall in the
southern portion of the Punjab. On the outer ranges of the Himalaya
the yearly fall amounts to about 200 in. on the east in Sikkim, and
gradually diminishes on the west, where north of the Punjab it is
about 70 or 80 in. In the interior of the chain the rain is far less,
and the quantity of precipitation is so small in Tibet that it can be
hardly measured. It is to the greatly reduced fall of snow on the
northern faces of the highest ranges of the Himalaya that is to be
attributed the higher level of the snow-line, a phenomenon which was
long a cause of discussion.
In Afghanistan, Persia, Asia Minor and Syria, winter and spring appear
to be the chief seasons of condensation. In other parts of Asia the
principal part of the rain falls between May and September, that is,
in the hottest half of the year. In the islands under the equator the
heaviest fall is between October and February. (R. S.)