[1] The original writ of summons (1299) was addressed in Latin,
_Roberto domino de Clifford_, i.e. Robert, lord of Clifford, and
subsequently the barons styled themselves indifferently Lords
Clifford or de Clifford, until in 1777 the 11th lord definitively
adopted the latter form. The "De" henceforth became part of the name,
having quite lost its earliest significance, and with unconscious
tautology the barony is commonly referred to as that of De Clifford.
CLIFFORD, JOHN (1836- ), British Nonconformist minister and politician,
son of a warp-machinist at Sawley, Derbyshire, was born on the 16th of
October 1836. As a boy he worked in a lace factory, where he attracted
the notice of the leaders of the Baptist community, who sent him to the
academy at Leicester and the Baptist college at Nottingham to be
educated for the ministry. In 1858 he was called to Praed Street chapel,
Paddington (London), and while officiating there he attended University
College and pursued his education by working at the British Museum. He
matriculated at London University (1859), and took its B.A. degree
(1861), B.Sc. (1862), M.A. (1864), and LL.B. (1866), and in 1883 he was
given the honorary degree of D.D. by Bates College, U.S.A., being known
therefrom as Dr Clifford. This degree, from an American college of minor
academic status, afterwards led to sarcastic allusions, but Dr Clifford
had not courted it, and his London University achievements were evidence
enough of his intellectual equipment. At Praed Street chapel he
gradually obtained a large following, and in 1877 Westbourne Park
chapel was opened for him. As a preacher, writer, propagandist and
ardent Liberal politician, he became a power in the Nonconformist body.
He was president of the London Baptist Association in 1879, of the
Baptist Union in 1888 and 1899, and of the National Council of
Evangelical Churches in 1898. His chief prominence in politics, however,
dates from 1903 onwards in consequence of his advocacy of "passive
resistance" to the Education Act of 1902. Into this movement he threw
himself with militant ardour, his own goods being distrained upon, with
those of numerous other Nonconformists, rather than that any
contribution should be made by them in taxation for the purpose of an
Education Act which in their opinion was calculated to support
denominational religious teaching in the schools. The "passive
resistance" movement, with Dr Clifford as its chief leader, had a large
share in the defeat of the Unionist government in January 1906, and his
efforts were then directed to getting a new act passed which should be
undenominational in character. The rejection of Mr Birrell's bill in
1906 by the House of Lords was accordingly accompanied by denunciations
of that body from Dr Clifford and his followers; but as year by year
went by, up to 1909, with nothing but failure on the part of the Liberal
ministry to arrive at any solution of the education problem,--failure
due now not to the House of Lords but to the inherent difficulties of
the subject (see EDUCATION),--it became increasingly clear to the public
generally that the easy denunciations of the act of 1902, which had
played so large a part in the elections of 1906, were not so simple to
carry into practice, and that a compromise in which the
denominationalists would have their say would have to be the result.
Meanwhile "passive resistance" lost its interest, though Dr Clifford and
his followers continued to protest against their treatment.
CLIFFORD, WILLIAM KINGDON (1845-1879), English mathematician and
philosopher, was born on the 4th of May 1845 at Exeter, where his father
was a prominent citizen. He was educated at a private school in his
native town, at King's College, London, and at Trinity College,
Cambridge, where he was elected fellow in 1868, after being second
wrangler in 1867 and second Smith's prizeman. In 1871 he was appointed
professor of mathematics at University College, London, and in 1874
became fellow of the Royal Society. In 1875 he married Lucy, daughter of
John Lane of Barbados. In 1876 Clifford, a man of high-strung and
athletic, but not robust, physique, began to fall into ill-health, and
after two voyages to the South, died during the third of pulmonary
consumption at Madeira, on the 3rd of March 1879, leaving his widow with
two daughters. Mrs W. K. Clifford soon earned for herself a prominent
place in English literary life as a novelist, and later as a dramatist.
Her best-known story, _Mrs Keith's Crime_ (1885), was followed by
several other volumes, the best of which is _Aunt Anne_ (1893); and the
literary talent in the family was inherited by her daughter Ethel (Mrs
Fisher Dilke), a writer of some charming verse.
Owing to his early death, Professor Clifford's abilities and
achievements cannot be fairly judged without reference to the opinion
formed of him by his contemporaries. He impressed every one as a man of
extraordinary acuteness and originality; and these solid gifts were set
off to the highest advantage by quickness of thought and speech, a lucid
style, wit and poetic fancy, and a social warmth which made him
delightful as a friend and companion. His powers as a mathematician were
of the highest order. It harmonizes with the concrete visualizing turn
of his mind that, to quote Professor Henry Smith, "Clifford was above
all and before all a geometer." In this he was an innovator against the
excessively analytic tendency of Cambridge mathematicians. In his theory
of graphs, or geometrical representations of algebraic functions, there
are valuable suggestions which have been worked out by others. He was
much interested, too, in universal algebra, non-Euclidean geometry and
elliptic functions, his papers "Preliminary Sketch of Bi-quaternions"
(1873) and "On the Canonical Form and Dissection of a Riemann's Surface"
(1877) ranking as classics. Another important paper is his
"Classification of Loci" (1878). He also published several papers on
algebraic forms and projective geometry.
As a philosopher Clifford's name is chiefly associated with two phrases
of his coining, "mind-stuff" and the "tribal self." The former
symbolizes his metaphysical conception, which was suggested to him by
his reading of Spinoza. "Briefly put," says Sir F. Pollock, "the
conception is that mind is the one ultimate reality; not mind as we know
it in the complex forms of conscious feeling and thought, but the
simpler elements out of which thought and feeling are built up. The
hypothetical ultimate element of mind, or atom of mind-stuff, precisely
corresponds to the hypothetical atom of matter, being the ultimate fact
of which the material atom is the phenomenon. Matter and the sensible
universe are the relations between particular organisms, that is, mind
organized into consciousness, and the rest of the world. This leads to
results which would in a loose and popular sense be called materialist.
But the theory must, as a metaphysical theory, be reckoned on the
idealist side. To speak technically, it is an idealist monism." The
other phrase, "tribal self," gives the key to Clifford's ethical view,
which explains conscience and the moral law by the development in each
individual of a "self," which prescribes the conduct conducive to the
welfare of the "tribe." Much of Clifford's contemporary prominence was
due to his attitude towards religion. Animated by an intense love of
truth and devotion to public duty, he waged war on such ecclesiastical
systems as seemed to him to favour obscurantism, and to put the claims
of sect above those of human society. The alarm was greater, as theology
was still unreconciled with the Darwinian theory; and Clifford was
regarded as a dangerous champion of the anti-spiritual tendencies then
imputed to modern science.
His works, published wholly or in part since his death, are _Elements
of Dynamic_ (1879-1887); _Seeing and Thinking_, popular science
lectures (1879); _Lectures and Essays_, with an introduction by Sir F.
Pollock (1879); _Mathematical Papers_, edited by R. Tucker, with an
introduction by Henry J. S. Smith (1882); and _The Common Sense of the
Exact Sciences_, completed by Professor Karl Pearson (1885).
CLIFFORD OF CHUDLEIGH, THOMAS CLIFFORD, 1ST BARON (1630-1673), English
lord treasurer, a member of the ancient family of Clifford, descended
from Walter de Clifford of Clifford Castle in Herefordshire, was the son
of Hugh Clifford of Ugbrook near Exeter, and of Mary, daughter of Sir
George Chudleigh of Ashton, Devonshire. He was born on the 1st of August
1630, matriculated in 1647 at Exeter College, Oxford, where he showed
distinguished ability, supplicated for the B.A. degree in 1650, and
entered the Middle Temple in 1648. He represented Totnes in the
convention parliament and in that of 1661; and he joined the faction of
young men who spoke "confidently and often," and who sought to rise to
power by attacking Clarendon. The chancellor, according to Burnet, had
repulsed his advances on account of his Romanism, and Clifford
accordingly offered his services to Arlington, whose steady supporter he
now became.
On the 16th of February 1663 Clifford obtained the reversion of a
tellership in the exchequer, and in 1664, on the outbreak of the Dutch
war, was appointed commissioner for the care of the sick, wounded and
prisoners, with a salary of L1200. He was knighted, and was present with
James at the victory off Lowestoft over the Dutch on the 3rd of June
1665, was rewarded with the prize-ship "Patriarch Isaac," and in August,
under the earl of Sandwich, took a prominent part in the unsuccessful
attempt to capture the Dutch East India fleet in Bergen harbour. In
August he was appointed by Arlington's influence ambassador with Henry
Coventry to the north of Europe. Subsequently he served again with the
fleet, was present with Albemarle at the indecisive fight on the 1st to
the 4th of June 1666, and at the victory on the 25th of July. In October
1667 he was one of those selected by the Commons to prepare papers
concerning the naval operations. He showed great zeal and energy in
naval affairs, and he is described by Pepys as "a very fine gentleman,
and much set by at court for his activity in going to sea and stoutness
everywhere and stirring up and down." He became the same year controller
of the household and a privy councillor, in 1667 a commissioner for the
treasury, and in 1668 treasurer of the household. In the Commons he
supported the court, opposing the bill for frequent parliaments in 1668
and the Coventry Act (see COVENTRY, SIR JOHN) in 1670.
Clifford was an ardent Roman Catholic, a supporter of the royal
prerogative and of the French alliance. He regarded with favour the plan
of seeking French assistance in order to force Romanism and absolute
government upon the country, and his complete failure to understand the
real political position and the interests of the nation is reflected in
the advice he was said to have given to Charles, to accept the pension
from Louis, and "be the slave of one man rather than of 500." As one of
the Cabal ministry, therefore, he co-operated very zealously with the
king in breaking through the Triple Alliance and in effecting the
understanding with France. He was the only minister besides Arlington
entrusted with the secret treaty of Dover of 1670, signing both this
agreement and also the ostensible treaty imparted to all the members of
the Cabal, and did his utmost to urge Charles to join France in the
attack upon the Dutch, whom he detested as republicans and Protestants.
In 1672, during the absence of Arlington and Coventry abroad, Clifford
acted as principal secretary of state, and was chiefly responsible for
the "stop of the exchequer," and probably also for the attack upon the
Dutch Smyrna fleet. He was appointed this year a commissioner to inquire
into the settlement of Ireland. On the 22nd of April he was raised to
the peerage as Baron Clifford of Chudleigh, and on the 28th of November,
by the duke of York's interest, he was made lord treasurer; his conduct
to Arlington, whose claims to the office he had pretended to press, was,
according to Evelyn, the only act of "real ingratitude" in his career.
Arlington, however, quickly discovered a means of securing Clifford's
fall. The latter was strongly in favour of Charles's policy of
indulgence, and supported the declaration of this year, urging the king
to overcome the resistance of parliament by a dissolution. Arlington
advocated the contrary policy of concession, and after Charles's
withdrawal of the declaration gave his support to the Test Act of 1673.
Clifford spoke with great vehemence against the measure, describing it
as "monstrum horrendum ingens," but his speech only increased the
anti-Roman Catholic feeling in parliament and ensured the passing of the
bill. In consequence Clifford, as a Roman Catholic, followed the duke of
York into retirement. His resignation caused considerable astonishment,
since he had never publicly professed his religion, and in 1671 had even
built a new Protestant chapel at his home at Ugbrook. According to
Evelyn, however, his conduct was governed by a promise previously given
to James. He gave up the treasuryship and his seat in the privy council
in June. On the 3rd of July 1673 he received a general pardon from the
king. In August he said a last farewell to Evelyn, and in less than a
month he died at Ugbrook. In Evelyn's opinion the cause of death was
suicide, but his suspicions do not appear to have received any
contemporary support. Clifford was one of the worst advisers of Charles
II., but a sincere and consistent one. Evelyn declares him "a valiant,
uncorrupt gentleman, ambitious, not covetous, generous, passionate, a
most constant, sincere friend." He married Elizabeth, daughter of
William Martin of Lindridge, Devonshire, by whom he had fifteen
children, four sons and seven daughters surviving him. He was succeeded
as 2nd baron by Hugh, his fifth, but eldest surviving son, the ancestor
of the present Lord Clifford of Chudleigh. (P. C. Y.)
CLIFTON, a suburb and residential district of Bristol, England,
adjoining it on the west; 122 m. W. of London by the Great Western
railway. The river Avon (q.v.) here runs in a gorge, followed closely by
a railway on either side, and having several quarries, which have in a
measure spoiled the beauty of its hanging woods. At a height of 245 ft.
above high water Isambard Brunel's famous suspension bridge bestrides
this gorge. It was begun in 1832 and completed in 1864. It has a span of
702 ft., and its total weight is 1500 tons, and it is calculated to bear
a burden of 9 tons per sq. in. The long famous hot springs of Clifton,
to which, in fact, the town was indebted for its rise, issue from an
aperture at the foot of St Vincent's Rock, in the portion of Clifton
known as Hotwells. The water has a temperature of about 76 deg. F. A
hydropathic establishment is attached to them. Immediately above the
suspension bridge the Clifton Rocks railway ascends from the quays by
the river-side to the heights above. The Clifton and Durdham Downs (both
on the Gloucestershire side of the river), form the principal
pleasure-grounds of Bristol. They lie high above the river, extend for
some 5000 acres, and command a beautiful prospect over the city, with
its picturesque irregular site and many towers, and over the surrounding
well-wooded country.
Three ancient British earthworks bear witness to an early settlement on
the spot, and a church was in existence as far back as the time of Henry
II., when it was bestowed by William de Clyfton on the abbot of the
Austin canons in Bristol; but there are no longer any architectural
vestiges of an earlier date than the 18th century. Clifton gives name to
a Roman Catholic bishopric. Of the churches the most important are St
Andrew's parish church; All Saints, erected in 1863 after the designs of
G. E. Street, and remarkable for the width of its nave and the
narrowness of its aisles; and the Roman Catholic pro-cathedral church of
the Holy Apostles, with a convent and schools attached. Clifton College,
a cluster of buildings in Gothic style, was founded in 1862 by a limited
liability company, and takes rank among the principal modern English
public schools. Down the river from Clifton is Shirehampton, a favourite
resort from Bristol.
CLIM (or CLYM) OF THE CLOUGH, a legendary English archer, a supposed
companion of the Robin Hood band. He is commemorated in the ballad _Adam
Bell, Clym of the Cloughe and Wyllyam of Cloudeslee_. The three were
outlaws who had many adventures of the Robin Hood type. The oldest
printed copy of this ballad is dated 1550.
CLIMACTERIC (from the Gr. [Greek: klimakter], the rung or step of a
[Greek: klimax] or ladder), a critical period in human life; in a
medical sense, the period known as the "change of life," marked in women
by the menopause. Certain ages, especially those which are multiples of
seven or nine, have been superstitiously regarded as particularly
critical; thus the sixty-third and the eighty-first year of life have
been called the "grand climacteric." The word is also used, generally,
of any turning-point in the history of a nation, a career or the like.
CLIMATE AND CLIMATOLOGY. The word _clima_ (from Gr. [Greek: klinein], to
lean or incline; whence also the English "clime," now a poetical term
for this or that region of the earth, regarded as characterized by
climate), as used by the Greeks, probably referred originally either to
the supposed slope of the earth towards the pole, or to the inclination
of the earth's axis. It was an astronomical or a mathematical term, not
associated with any idea of physical climate. A change of _clima_ then
meant a change of latitude. The latter was gradually seen to mean a
change in atmospheric conditions as well as in length of day, and
_clima_ thus came to have its present meaning. "Climate" is the average
condition of the atmosphere. "Weather" denotes a single occurrence, or
event, in the series of conditions which make up climate. The climate of
a place is thus in a sense its average weather. Climatology is the study
or science of climates.
_Relation of Meteorology and Climatology._--Meteorology and climatology
are interdependent. It is impossible to distinguish sharply between
them. In a strict sense, meteorology deals with the physics of the
atmosphere. It considers the various atmospheric phenomena individually,
and seeks to determine their physical causes and relations. Its view is
largely theoretical. When meteorology (q.v.) is considered in its
broadest meaning, climatology is a subdivision of it. Climatology is
largely descriptive. It aims at giving a clear picture of the
interaction of the various atmospheric phenomena at any place on the
earth's surface. Climatology may almost be defined as geographical
meteorology. Its main object is to be of practical service to man. Its
method of treatment lays most emphasis on the elements which are most
important to life. Climate and crops, climate and industry, climate and
health, are subjects of vital interest to man.
_The Climatic Elements and their Treatment._--Climatology has to deal
with the same groups of atmospheric conditions as those with which
meteorology is concerned, viz. temperature (including radiation);
moisture (including humidity, precipitation and cloudiness); wind
(including storms); pressure; evaporation, and also, but of less
importance, the composition and chemical, optical and electrical
phenomena of the atmosphere. The characteristics of each of these
so-called _climatic elements_ are set forth in a standard series of
numerical values, based on careful, systematic, and long-continued
meteorological records, corrected and compared by well-known methods.
Various forms of graphic presentation are employed to emphasize and
simplify the numerical results. In Hann's _Handbuch der Klimatologie_,
vol i., will be found a general discussion of the methods of presenting
the different climatic elements. The most complete guide in the
numerical, mathematical and graphic treatment of meteorological data for
climatological purposes is Hugo Meyer's _Anleitung zur Bearbeitung
meteorologischer Beobachtungen fur die Klimatologie_ (Berlin, 1891).
Climate deals first of all with _average_ conditions, but a satisfactory
presentation of a climate must include more than mere averages. It must
take account, also, of regular and irregular daily, monthly and annual
changes, and of the departures, mean and extreme, from the average
conditions which may occur at the same place in the course of time. The
mean minimum and maximum temperatures or rainfalls of a month or a
season are important data. Further, a determination of the frequency of
occurrence of a given condition, or of certain values of that condition,
is important, for periods of a day, month or year, as for example the
frequency of winds according to direction or velocity; or of different
amounts of cloudiness; or of temperature changes of a certain number of
degrees; the number of days with and without rain or snow in any month,
or year, or with rain of a certain amount, &c. The probability of
occurrence of any condition, as of rain in a certain month; or of a
temperature of 32 deg., for example, is also a useful thing to know.
_Solar Climate._--Climate, in so far as it is controlled solely by the
amount of solar radiation which any place receives by reason of its
latitude, is called _solar climate_. Solar climate alone would prevail
if the earth had a homogeneous land surface, and if there were no
atmosphere. For under these conditions, without air or ocean currents,
the distribution of temperature at any place would depend solely on the
amount of energy received from the sun and upon the loss of heat by
radiation. And these two factors would have the same value at all points
on the same latitude circle.
The relative amounts of insolation received at different latitudes and
at different times have been carefully determined. The values all refer
to conditions at the upper limit of the earth's atmosphere, i.e. without
the effect of absorption by the atmosphere. The accompanying figure
(fig. 1), after Davis, shows the distribution of insolation in both
hemispheres at different latitudes and at different times in the year.
The latitudes are given at the left margin and the time of year at the
right margin. The values of insolation are shown by the vertical
distance above the plane of the two margins.
At the equator, where the day is always twelve hours long, there are two
maxima of insolation at the equinoxes, when the sun is vertical at noon,
and two minima at the solstices when the sun is farthest off the
equator. The values do not vary much through the year because the sun is
never very far from the zenith, and day and night are always equal. As
latitude increases, the angle of insolation becomes more oblique and the
intensity decreases, but at the same time the length of day rapidly
increases during the summer, and towards the pole of the hemisphere
which is having its summer the gain in insolation from the latter cause
more than compensates for the loss by the former. The double period of
insolation above noted for the equator prevails as far as about lat. 12
deg. N. and S.; at lat. 15 deg. the two maxima have united in one, and
the same is true of the minima. At the pole there is one maximum at the
summer solstice, and no insolation at all while the sun is below the
horizon. On the 21st of June the equator has a day twelve hours long,
but the sun does not reach the zenith, and the amount of insolation is
therefore less than at the equinox. On the northern tropic, however, the
sun is vertical at noon, and the day is more than twelve hours long.
Hence the amount of insolation received at this latitude is greater than
that received on the equinox at the equator. From the tropic to the pole
the sun stands lower and lower at noon, and the value of insolation
would steadily decrease with latitude if it were not for the increase in
the length of day. Going polewards from the northern tropic on the 21st
of June, the value of insolation increases for a time, because, although
the sun is lower, the number of hours during which it shines is greater.
A maximum value is reached at about lat. 43-1/2 deg. N. The decreasing
altitude of the sun then more than compensates for the increasing length
of day, and the value of insolation diminishes, a minimum being reached
at about lat. 62 deg.. Then the rapidly increasing length of day towards
the pole again brings about an increase in the value of insolation,
until a maximum is reached at the pole which is greater than the value
received at the equator at any time. The length of day is the same on
the Arctic circle as at the pole itself, but while the altitude of the
sun varies during the day on the former, the altitude at the pole
remains 23-1/2 deg. throughout the 24 hours. The result is to give the
pole a maximum. On the 21st of June there are therefore two maxima of
insolation, one at lat. 43-1/2 deg. and one at the north pole. From lat.
43-1/2 deg. N., insolation decreases to zero on the Antarctic circle,
for sunshine falls more and more obliquely, and the day becomes shorter
and shorter. Beyond lat. 66-1/2 deg. S. the night lasts 24 hours. On the
21st of December the conditions in southern latitudes are similar to
those in the northern hemisphere on the 21st of June, but the southern
latitudes have higher values of insolation because the earth is then
nearer the sun.
[Illustration: From Davis's _Elementary Meteorology_.
FIG. 1.--Distribution of Insolation over the Earth's Surface.]
At the equinox the days are equal everywhere, but the noon sun is lower
and lower with increasing latitude in both hemispheres until the rays
are tangent to the earth's surface at the poles (except for the effect
of refraction). Therefore, the values of insolation diminish from a
maximum at the equator to a minimum at both poles.
The effect of the earth's atmosphere is to weaken the sun's rays. The
more nearly vertical the sun, the less the thickness of atmosphere
traversed by the rays. The values of insolation at the earth's surface,
after passage through the atmosphere, have been calculated. They vary
much with the condition of the air as to dust, clouds, water vapour, &c.
As a rule, even when the sky is clear, about one-half of the solar
radiation is lost during the day by atmospheric absorption. The great
weakening of insolation at the pole, where the sun is very low, is
especially noticeable. The following table (after Angot) shows the
effect of the earth's atmosphere (coefficient of transmission 0.7) upon
the value of insolation received at sea-level.
_Values of Daily Insolation at the Upper Limit of the Earth's
Atmosphere and at Sea-Level_.
+-----------------+----------------------------+--------------------------+
| | Upper Limit of Atmosphere. | Earth's Surface. |
| Lat. +----------+-------+---------+---------+-------+--------+
| | Equator. |40 deg.| N. Pole.| Equator.|40 deg.|N. Pole.|
+-----------------+----------+-------+---------+---------+-------+--------+
| Winter solstice | 948 | 360 | 0 | 552 | 124 | 0 |
| Equinoxes | 1000 | 773 | 0 | 612 | 411 | 0 |
| Summer solstice | 888 | 1115 | 1210 | 517 | 660 | 494 |
+-----------------+----------+-------+---------+---------+-------+--------+
The following table gives, according to W. Zenker, the relative
thickness of the atmosphere at different altitudes of the sun, and also
the amount of transmitted insolation:
_Relative Distances traversed by Solar Rays through the Atmosphere,
and Intensities of Radiation per Unit Areas_.
+------------------+----+----+----+----+----+----+----+----+----+---------+
|Altitude of sun | O | 5 | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 |
| |deg.|deg.|deg.|deg.|deg.|deg.|deg.|deg.|deg.|deg.|deg.|
| | | | | | | | | | | | |
|Relative lengths | | | | | | | | | | | |
| of path through | | | | | | | | | | | |
| the atmosphere |44.7|10.8|5.7 |2.92|2.00|1.56|1.31|1.15|1.06|1.02|1.00|
| | | | | | | | | | | | |
|Intensity of | | | | | | | | | | | |
| radiation on a | | | | | | | | | | | |
| surface normal | | | | | | | | | | | |
| to the rays | 0.0|0.15|0.31|0.51|0.62|0.68|0.72|0.75|0.76|0.77|0.78|
| | | | | | | | | | | | |
|Intensity of | | | | | | | | | | | |
| radiation on | | | | | | | | | | | |
| a horizontal | | | | | | | | | | | |
| surface | 0.0|0.01|0.05|0.17|0.31|0.44|0.55|0.65|0.72|0.76|0.78|
+------------------+----+----+----+----+----+----+----+----+----+----+----+
_Physical Climate._--The distribution of insolation explains many of the
large facts of temperature distribution, for example, the decrease of
temperature from equator to poles; the double maximum of temperature on
and near the equator; the increasing seasonal contrasts with increasing
latitude, &c. But the regular distribution of solar climate between
equator and poles which would exist on a homogeneous earth, whereby
similar conditions prevail along each latitude circle, is very much
modified by the unequal distribution of land and water; by differences
of altitude; by air and ocean currents, by varying conditions of
cloudiness, and so on. Hence the climates met with along the same
latitude circle are no longer alike. Solar climate is greatly modified
by atmospheric conditions and by the surface features of the earth. The
uniform arrangement of solar climatic belts, arranged latitudinally, is
interfered with, and what is known as _physical climate_ results.
According to the dominant control we have solar, continental and marine,
and mountain climates. In the first-named, latitude is the essential; in
the second and third, the influence of land or water; in the fourth, the
effect of altitude.
_Classification of the Zones by Latitude Circles._--It is customary to
classify climates roughly into certain broad belts. These are the
climatic zones. The five zones with which we are most familiar are the
so-called torrid, the two temperate, and the two frigid zones. The
torrid, or better, the tropical zone, naming it by its boundaries, is
limited on the north and south by the two tropics of Cancer and
Capricorn, the equator dividing the zone into two equal parts. The
temperate zones are limited towards the equator by the tropics, and
towards the poles by the Arctic and Antarctic circles. The two polar
zones are caps covering both polar regions, and bounded on the side
towards the equator by the Arctic and Antarctic circles.
These five zones are classified on purely astronomical grounds. They are
really zones of solar climate. The tropical zone has the least annual
variation of insolation. It has the maximum annual amount of insolation.
Its annual range of temperature is very slight. It is the summer zone.
Beyond the tropics the contrasts between the seasons rapidly become more
marked. The polar zones have the greatest variation in insolation
between summer and winter. They also have the minimum amount of
insolation for the whole year. They may well be called the winter zones,
for their summer is so short and cool that the heat is insufficient for
most forms of vegetation, especially for trees. The temperate zones are
intermediate between the tropical and the polar in the matter of annual
amount and of annual variation of insolation. Temperate conditions do
not characterize these zones as a whole. They are rather the seasonal
belts of the world.
[Illustration: From _Grundzuge der physischen Erdkunde_, by permission
of Veit & Co.
FIG. 2.--Supan's Temperature Zones.]
_Temperature Zones._--The classification of the zones on the basis of
the distribution of sunshine serves very well for purposes of simple
description, but a glance at any isothermal chart shows that the
isotherms do not coincide with the latitude lines. In fact, in the
higher latitudes, the former sometimes follow the meridians more closely
than they do the parallels of latitude. Hence it has been suggested that
the zones be limited by isotherms rather than by parallels of latitude,
and that a closer approach be thus made to the actual conditions of
climate. Supan[1] (see fig. 2) has suggested limiting the hot belt,
which corresponds to, but is slightly greater than, the old torrid zone,
by the two mean annual isotherms of 68 deg.--a temperature which
approximately coincides with the polar limit of the trade-winds and with
the polar limit of palms. The hot belt widens somewhat over the
continents, chiefly because of the mobility of the ocean waters, whereby
there is a tendency towards an equalization of the temperature between
equator and poles in the oceans, while the stable lands acquire a
temperature suitable to their own latitude. Furthermore, the
unsymmetrical distribution of land in the low latitudes of the northern
and southern hemispheres results in an unsymmetrical position of the hot
belt with reference to the equator, the belt extending farther north
than south of the equator. The polar limits of the temperate zones are
fixed by the isotherm of 50 deg. for the warmest month. Summer heat is
more important for vegetation than winter cold, and where the warmest
month has a temperature below 50 deg., cereals and forest trees do not
grow, and man has to adjust himself to the peculiar climatic conditions
in a very special way. The two polar caps are not symmetrical as regards
the latitudes which they occupy. The presence of extended land masses in
the high northern latitudes carries the temperature of 50 deg. in the
warmest month farther poleward there than is the case in the
corresponding latitudes occupied by the oceans of the southern
hemisphere, which warm less easily and are constantly in motion. Hence
the southern cold cap, which has its equatorial limits at about lat. 50
deg. S., is of much greater extent than the northern polar cap. The
northern temperate belt, in which the great land areas lie, is much
broader than the southern, especially over the continents. These
temperature zones emphasize the natural conditions of climate more than
is the case in any subdivision by latitude circles, and they bear a
fairly close resemblance to the old zonal classification of the Greeks.
_Classification of the Zones by Wind Belts._--The heat zones however,
emphasize the temperature to the exclusion of such important elements
as wind and rainfall. So distinctive are the larger climatic features of
the great wind belts of the world, that a classification of climates
according to wind systems has been suggested.[2] As the rain-belts of
the world are closely associated with these wind systems, a
classification of the zones by winds also emphasizes the conditions of
rainfall. In such a scheme the tropical zone is bounded on the north and
south by the margins of the trade-wind belts, and is therefore larger
than the classic torrid zone. This trade-wind zone is somewhat wider on
the eastern side of the oceans, and properly includes within its limits
the equable marine climates of the eastern margins of the ocean basins,
even as far north as latitude 30 deg. or 35 deg. Most of the eastern
coasts of China and of the United States are thus left in the more
rigorous and more variable conditions of the north temperate zone.
Through the middle of the trade-wind zone extends the sub-equatorial
belt, with its migrating calms, rains and monsoons. On the polar margins
of the trade-wind zone lie the sub-tropical belts, of alternating trades
and westerlies. The temperate zones embrace the latitudes of the stormy
westerly winds, having on their equator-ward margins the sub-tropical
belts, and being somewhat narrower than the classic temperate zones.
Towards the poles there is no obvious limit to the temperate zones, for
the prevailing westerlies extend beyond the polar circles. These circles
may, however, serve fairly well as boundaries, because of their
importance from the point of view of insolation. The polar zones in the
wind classification, therefore, remain just as in the older scheme.
_Need of a Classification of Climates._--A broad division of the earth's
surface into zones is necessary as a first step in any systematic study
of climate, but it is not satisfactory when a more detailed discussion
is undertaken. The reaction of the physical features of the earth's
surface upon the atmosphere complicates the climatic conditions found in
each of the zones, and makes further subdivision desirable. The usual
method is to separate the _continental_ (near sea-level) and the
_marine_. An extreme variety of the continental is the _desert_; a
modified form, the _littoral_; while altitude is so important a control
that _mountain_ and _plateau_ climates are always grouped by themselves.
_Marine or Oceanic Climate._--Land and water differ greatly in their
behaviour regarding absorption and radiation. The former warms and cools
readily, and to a considerable degree; the latter, slowly and but
little. The slow changes in temperature of the ocean waters involve a
retardation in the times of occurrence of the maxima and minima, and a
marine climate, therefore, has a cool spring and a warm autumn, the
seasonal changes being but slight. Characteristic, also, of marine
climates is a prevailingly higher relative humidity, a larger amount of
cloudiness, and a heavier rainfall than is found over continental
interiors. All of these features have their explanation in the abundant
evaporation from the ocean surfaces. In the middle latitudes the oceans
have distinctly rainy winters, while over the continental interiors the
colder months have a minimum of precipitation. Ocean air is cleaner and
purer than land air, and is generally in more active motion.
_Continental Climate._--Continental climate is severe. The annual
temperature ranges increase, as a whole, with increasing distance from
the oceans. The coldest and warmest months are usually January and July,
the times of maximum and minimum temperatures being less retarded than
in the case of marine climates. The greater seasonal contrasts in
temperature over the continents than over the oceans are furthered by
the less cloudiness over the former. Diurnal and annual changes of
nearly all the elements of climate are greater over continents than over
oceans; and this holds true of irregular as well as of regular
variations. Fig. 3 illustrates the annual march of temperature in marine
and continental climates. Bagdad, in Asia Minor (Bd.), and Funchal on
the island of Madeira (M.) are representative continental and marine
stations for a low latitude. Nerchinsk in eastern Siberia (N.) and
Valentia in south-western Ireland (V.) are good examples of continental
and marine climates of higher latitudes in the northern hemisphere. The
data for these and the following curves were taken from Hann's _Lehrbuch
der Meteorologie_ (1901).
Owing to the distance from the chief source of supply of water
vapour--the oceans--the air over the larger land areas is naturally
drier and dustier than that over the oceans. Yet even in the arid
continental interiors in summer the absolute vapour content is
surprisingly large, and in the hottest months the percentages of
relative humidity may reach 20% or 30%. At the low temperatures which
prevail in the winter of the higher latitudes the absolute humidity is
very low, but, owing to the cold, the air is often damp. Cloudiness, as
a rule, decreases inland, and with this lower relative humidity, more
abundant sunshine and higher temperature, the evaporating power of a
continental climate is much greater than that of the more humid,
cloudier and cooler marine climate. Both amount and frequency of
rainfall, as a rule, decrease inland, but the conditions are very
largely controlled by local topography and by the prevailing winds.
Winds average somewhat lower in velocity, and calms are more frequent,
over continents than over oceans. The seasonal changes of pressure over
the former give rise to systems of inflowing and outflowing, so-called
continental, winds, sometimes so well developed as to become true
monsoons. The extreme termperature changes which occur over the
continents are the more easily borne because of the dryness of the air;
because the minimum temperatures of winter occur when there is little or
no wind, and because during the warmer hours of the summer there is the
most air-movement.
[Illustration: FIG. 3.--Annual March of Air Temperature. Influence of
Land and Water. (After Angot.)
M, Madeira.
Bd, Bagdad.
V, Valentia.
N, Nerchinsk.]
_Desert Climate._--An extreme type of continental climate is found in
deserts. Desert air is notably free from micro-organisms. The large
diurnal temperature ranges of inland regions, which are most marked
where there is little or no vegetation, give rise to active convectional
currents during the warmer hours of the day. Hence high winds are common
by day, while the nights are apt to be calm and relatively cool.
Travelling by day is unpleasant under such conditions. Diurnal cumulus
clouds, often absent because of the excessive dryness of the air, are
replaced by clouds of blowing dust and sand. Many geological phenomena,
and special physiographic types of varied kinds, are associated with the
peculiar conditions of desert climate. The excessive diurnal ranges of
temperature cause rocks to split and break up. Wind-driven sand erodes
and polishes the rocks. When the separate fragments become small enough
they, in their turn, are transported by the winds and further eroded by
friction during their journey. Curious conditions of drainage result
from the deficiency in rainfall. Rivers "wither" away, or end in sinks
or brackish lakes.
Desert plants protect themselves against the attacks of animals by
means of thorns, and against evaporation by means of hard surfaces and
by a diminished leaf surface. The life of man in the desert is likewise
strikingly controlled by the climatic peculiarities of strong sunshine,
of heat, and of dust.
[Illustration: PLATE I. ANNUAL DISTRIBUTION OF TEMPERATURE AND
PRESSURE.]
[Illustration: PLATE I. SEASONAL DISTRIBUTION OF TEMPERATURE AND
PRESSURE.
TEMPERATURE JANUARY
TEMPERATURE JULY
PRESSURE & WINDS JANUARY
PRESSURE & WINDS JULY]
_Coast or Littoral Climate._--Between the pure marine and the pure
continental types the coasts furnish almost every grade of transition.
Prevailing winds are here important controls. When these blow from the
ocean, the climates are marine in character, but when they are
off-shore, a somewhat modified type of continental climate prevails,
even up to the immediate sea-coast. Hence the former have a smaller
range of temperature; their summers are more moderate and their winters
milder; extreme temperatures are rare; the air is damp, and there is
much cloud. All these marine features diminish with increasing distance
from the ocean, especially when there are mountain ranges near the
coast. In the tropics, windward coasts are usually well supplied with
rainfall, and the temperatures are modified by sea breezes. Leeward
coasts in the trade-wind belts offer special conditions. Here the
deserts often reach the sea, as on the western coasts of South America,
Africa and Australia. Cold ocean currents, with prevailing winds
along-shore rather than on-shore, are here hostile to rainfall, although
the lower air is often damp, and fog and cloud are not uncommon.
_Monsoon Climate._--Exceptions to the general rule of rainier eastern
coasts in trade-wind latitudes are found in the monsoon regions, as in
India, for example, where the western coast of the peninsula is
abundantly watered by the wet south-west monsoon. As monsoons often
sweep over large districts, not only coast but interior, a separate
group of monsoon climates is desirable. In India there are really three
seasons--one cold, during the winter monsoon; one hot, in the transition
season; and one wet, during the summer monsoon. Little precipitation
occurs in winter, and that chiefly in the northern provinces. In low
latitudes, monsoon and non-monsoon climates differ but little, for
summer monsoons and regular trade-winds may both give rains, and wind
direction has slight effect upon temperature.
The winter monsoon is off-shore and the summer monsoon on-shore under
typical conditions, as in India. But exceptional cases are found where
the opposite is true. In higher latitudes the seasonal changes of the
winds, although not truly monsoonal, involve differences in temperature
and in other climatic elements. The only well-developed monsoons on the
coast of the continents of higher latitudes are those of eastern Asia.
These are off-shore during the winter, giving dry, clear and cold
weather; while the on-shore movement in summer gives cool, damp and
cloudy weather.
_Mountain and Plateau Climate._--Both by reason of their actual height
and because of their obstructive effects, mountains influence climate
similarly in all the zones. Mountains as contrasted with lowlands are
characterized by a decrease in pressure, temperature and absolute
humidity; an increased intensity of insolation and radiation; usually a
greater frequency of, and up to a certain altitude more, precipitation.
At an altitude of 16,000 ft., more or less, pressure is reduced to about
one-half of its sea-level value. The highest human habitations are found
under these conditions. On high mountains and plateaus the pressure is
lower in winter than in summer, owing to the fact that the atmosphere is
compressed to lower levels in the winter and is expanded upwards in
summer.
The intensity of insolation and of radiation both increase aloft in the
cleaner, purer, drier and thinner air of mountain climates. The great
intensity of the sun's rays attracts the attention of mountain-climbers
at great altitudes. The vertical decrease of temperature, which is also
much affected by local conditions, is especially rapid during the warmer
months and hours; mountains are then cooler than lowlands. The
inversions of temperature characteristic of the colder months, and of
the night, give mountains the advantage of a higher temperature then--a
fact of importance in connexion with the use of mountains as winter
resorts. At such times the cold air flows down the mountain sides and
collects in the valleys below, being replaced by warmer air aloft.
Hence diurnal and annual ranges of temperature on the mountain tops of
middle and higher latitudes are lessened, and the climate in this
respect resembles a marine condition. The times of occurrence of the
maximum and minimum temperature are also much influenced by local
conditions. Elevated enclosed valleys, with strong sunshine, often
resemble continental conditions of large temperature range, and
plateaus, as compared with mountains at the same altitude, have
relatively higher temperatures and larger temperature ranges. Altitude
tempers the heat of the low latitudes. High mountain peaks, even on the
equator, can remain snow-covered all the year round.
No general law governs the variations of relative humidity with
altitude, but on the mountains of Europe the winter is the driest
season, and the summer the dampest. At well-exposed stations there is a
rapid increase in the vapour content soon after noon, especially in
summer. The same is true of cloudiness, which is often greater on
mountains than at lower levels, and is usually at a maximum in summer,
while the opposite is true of the lowlands in the temperate latitudes.
One of the great advantages of the higher Alpine valleys in winter is
their small amount of cloud. This, combined with their low wind velocity
and strong insolation, makes them desirable winter health resorts.
Latitude, altitude, topography and winds are the determining factors in
controlling the cloudiness on mountains. In the rare, often dry, air of
mountains and plateaus evaporation is rapid, the skin dries and cracks,
and thirst is increased.
Rainfall usually increases with increasing altitude up to a certain
point, beyond which, owing to the loss of water vapour, this increase
stops. The zone of maximum rainfall averages about 6000 to 7000 ft. in
altitude, more or less, in intermediate latitudes, being lower in winter
and higher in summer. Mountains usually have a rainy and a drier side;
the contrast between the two is greatest when a prevailing damp wind
crosses the mountain, or when one slope faces seaward and the other
landward. Mountains often provoke rainfall, and local "islands," or
better, "lakes," of heavier precipitation result.
Mountains resemble marine climates in having higher wind velocities than
continental lowlands. Mountain summits have a nocturnal maximum of wind
velocity, while plateaus usually have a diurnal maximum. Mountains both
modify the general, and give rise to local winds. Among the latter the
well-known mountain and valley winds are often of considerable hygienic
importance in their control of the diurnal period of humidity,
cloudiness and rainfall, the ascending wind of daytime tending to give
clouds and rain aloft, while the opposite conditions prevail at night.
_Supan's Climatic Provinces._--The broad classification of climates into
the three general groups of marine, continental and mountain, with the
subordinate divisions of desert, littoral and monsoon, is convenient for
purposes of summarizing the interaction of the climatic elements under
the controls of land, water and altitude. But in any detailed study some
scheme of classification is needed in which similar climates in
different parts of the world are grouped together, and in which their
geographic distribution receives particular consideration. An almost
infinite number of classifications might be proposed; or we may take as
the basis of subdivision either the special conditions of one climatic
element, or similar conditions of a combination of two or more elements.
Or we may take a botanical or a zoological basis. Of the various
classifications which have been suggested, that of Supan gives a very
rational, simple and satisfactory scheme of grouping. In this scheme
there are thirty-five so-called climatic provinces.[3] It emphasizes the
essentials of each climate, and serves to impress these essentials upon
the mind by means of a compact, well-considered verbal summary in the
case of each province described. Obviously, no classification of
climates which is at all complete can approach the simplicity of the
ordinary classification of the zones.
_The Characteristics of the Torrid Zone._
_General: Climate and Weather._--The dominant characteristic of the
torrid zone is the simplicity and uniformity of its climatic features.
The tropics lack the proverbial uncertainty and changeableness of the
weather of higher latitudes. Weather and climate are essentially
synonymous terms. Periodic phenomena, depending upon the daily and
annual march of the sun, are dominant. Non-periodic weather changes are
wholly subordinate. In special regions only, and at special seasons, is
the regular sequence of weather temporarily interrupted by an occasional
tropical cyclone. These cyclones, although comparatively infrequent, are
notable features of the climate of the areas in which they occur,
generally bringing very heavy rains. The devastation produced by one of
these storms often affects the economic condition of the people in the
district of its occurrence for many years.
_Temperature._--The mean temperature is high, and very uniform over the
whole zone. There is little variation during the year. The mean annual
isotherm of 68 deg. is a rational limit at the polar margins of the zone,
and the mean annual isotherm of 80 deg. encloses the greater portion of the
land areas, as well as much of the tropical oceans. The warmest latitude
circle for the year is not the equator, but latitude 10 deg. N. The highest
mean annual temperatures, shown by the isotherm of 85 deg., are in Central
Africa, in India, the north of Australia and Central America, but, with
the exception of the first, these areas are small. The temperatures
average highest where there is little rain. In June, July and August
there are large districts in the south of Asia and north of Africa with
temperatures over 90 deg.
Over nearly all of the zone the mean annual range of temperature is less
than 10 deg., and over much of it, especially on the oceans, it is less
than 5 deg. Even near the margins of the zone the ranges are less than
25 deg., as at Calcutta, Hong-Kong, Rio de Janeiro and Khartum. The mean
daily range is usually larger than the mean annual. It has been well
said that "night is the winter of the tropics." Over an area covering
parts of the Pacific and Indian Oceans from Arabia to the Caroline
Islands and from Zanzibar to New Guinea, as well as on the Guiana coast,
the minimum temperatures do not normally fall below 68 deg. Towards the
margins of the zone, however, the minima on the continents fall to or
even below 32 deg. Maxima of 115 deg. and even over 120 deg. occur over
the deserts of northern Africa. A district where the mean maxima exceed
113 deg. extends from the western Sahara to north-western India, and
over Central Australia. Near the equator the maxima are therefore not as
high as those in many so-called "temperate" climates. The tropical
oceans show remarkably small variations in temperature. The "Challenger"
results on the equator showed a daily range of hardly 0.7 deg. in the
surface water temperature, and P. G. Schott determined the annual range
as 4.1 deg. on the equator, 4.3 deg. at latitude 10 deg., and 6.5 deg.
at latitude 20 deg.
_The Seasons._--In a true tropical climate the seasons are not
classified according to temperature, but depend on rainfall and the
prevailing winds. The life of animals and plants in the tropics, and of
man himself, is regulated very largely, in some cases almost wholly, by
rainfall. Although the tropical rainy season is characteristically
associated with a vertical sun, that season is not necessarily the
hottest time of the year. It often goes by the name of winter for this
reason. Towards the margins of the zone, with increasing annual ranges
of temperature, seasons in the extra-tropical sense gradually appear.
_Physiological Effects of Heat and Humidity._--Tropical heat is
associated with high relative humidity except over deserts and in dry
seasons. The air is therefore muggy and oppressive. The high
temperatures are disagreeable and hard to bear. The "hot-house air" has
an enervating effect. Energetic physical and mental action are often
difficult or even impossible. The tonic effect of a cold winter is
lacking. The most humid districts in the tropics are the least desirable
for persons from higher latitudes; the driest are the healthiest. The
most energetic natives are the desert-dwellers. The monotonously
enervating heat of the humid tropics makes man sensitive to slight
temperature changes. The intensity of direct insolation, as well as of
radiation from the earth's surface, may produce heat prostration and
sunstroke. "Beware of the sun" is a good rule in the tropics.
_Pressure._--The uniform temperature distribution in the tropics
involves uniform pressure distribution. Pressure gradients are weak. The
annual fluctuations are slight, even on the continents. The diurnal
variation of the barometer is so regular and so marked that, as von
Humboldt said, the time of day can be told within about twenty minutes
if the reading of the barometer be known.
_Winds and Rainfall._--Along the barometric equator, where the pressure
gradients are weakest, is the equatorial belt of calms, variable winds
and rains--the doldrums. This belt offers exceptionally favourable
conditions for abundant rainfall, and is one of the rainiest regions of
the world, averaging probably about 100 in. Here the sky is prevailingly
cloudy; the air is hot and oppressive; heavy showers and thunderstorms
are frequent, chiefly in the afternoon and evening. Here are the dense
tropical forests of the Amazon and of equatorial Africa. This belt of
calms and rains shifts north and south of the equator after the sun. In
striking contrast are the easterly trade winds, blowing between the
tropical high pressure belts and the equatorial belt of low pressure. Of
great regularity, and contributing largely to the uniformity of tropical
climates, the trades have long been favourite sailing routes because of
the steadiness of the wind, the infrequency of storms, the brightness of
the skies and the freshness of the air. The trades are subject to many
variations. Their northern and southern margins shift north and south
after the sun; at certain seasons they are interrupted, often over wide
areas near their equatorward margins, by the migrating belt of
equatorial rains and by monsoons; near lands they are often interfered
with by land and sea breezes; in certain regions they are invaded by
violent cyclonic storms. The trades, except where they blow on to
windward coasts or over mountains, are drying winds. They cause the
deserts of northern Africa and of the adjacent portions of Asia; of
Australia, South Africa and southern South America. The monsoons on the
southern and eastern coasts of Asia are the best known winds of their
class. In the northern summer the south-west monsoon, warm and sultry,
blows over the latitudes from about 10 deg. N. to and beyond the
northern tropic, between Africa and the Philippines, giving rains over
India, the East Indian archipelago and the eastern coasts of China. In
winter, the north-east monsoon, the normal cold-season outflow from Asia
combined with the north-east trade, and generally cool and dry, covers
the same district, extending as far north as latitude 30 deg. Crossing
the equator, these winds reach northern Australia and the western
islands of the South Pacific as a north-west rainy monsoon, while this
region in the opposite season has the normal south-east trade. Other
monsoons are found in the Gulf of Guinea and in equatorial Africa.
Wherever they occur, they control the seasonal changes.
Tropical rains are in the main summer rains, coming when the normal
trade gives way to the equatorial belt of rains, or when the summer
monsoon sets in. There are, however, many cases of a rainy season when
the sun is low, expecially on windward coasts in the trades. Tropical
rains come usually in the form of heavy downpours and with a well-marked
diurnal period, the maximum varying with the locality between noon and
midnight. Local influences are, however, very important, and in many
places night rainfall maxima are found.
_Land and Sea Breezes._--The sea breeze is an important climatic feature
on many tropical coasts. With its regular occurrence, and its cool,
clean air, it serves to make many districts habitable for white
settlers, and has deservedly won the name of "the doctor." On not a few
coasts, the sea breeze is a true prevailing wind. The location of
dwellings is often determined by the exposure of a site to the sea
breeze.
_Thunderstorms._--Local thunderstorms are frequent in the humid portions
of the tropics. They have a marked diurnal periodicity, find their best
opportunity in the equatorial belt of weak pressure gradients and high
temperature, and are commonly associated with the rainy season, being
most common at the beginning and end of the regular rains. In many
places, thunderstorms occur daily throughout their season, with
extraordinary regularity and great intensity.
_Cloudiness._--Taken as a whole, the tropics are not favoured with such
clear skies as is often supposed. Cloudiness varies about as does the
rainfall. The maximum is in the equatorial belt of calms and rains,
where the sky is always more or less cloudy. The minimum is in the trade
latitudes, where fair skies as a whole prevail. The equatorial cloud
belt moves north and south after the sun. Wholly clear days are very
rare in the tropics generally, especially near the equator, and during
the rainy season heavy clouds usually cover the sky. Wholly overcast,
dull days, such as are common in the winter of the temperate zone, occur
frequently only on tropical coasts in the vicinity of cold ocean
currents, as on the coast of Peru and on parts of the west coast of
Africa.
_Intensity of Sky-Light and Twilight._--The light from tropical skies by
day is trying, and the intense insolation, together with the reflection
from the ground, increases the general dazzling glare under a tropical
sun. During much of the time smoke from forest and prairie fires (in the
dry season), dust (in deserts), and water-vapour give the sky a pale
whitish appearance. In the heart of the trade-wind belts at sea the sky
is of a deeper blue. Twilight within the tropics is shorter than in
higher latitudes, but the coming on of night is less sudden than is
generally assumed.
_Climatic Subdivisions._--The rational basis for a classification of the
larger climatic provinces of the torrid zone is found in the general
wind systems, and in their control over rainfall. Following this scheme
there are: (1) the equatorial belt; (2) the trade-wind belts; (3) the
monsoon belts. In each of these subdivisions there are modifications due
to marine and continental influences. In general, both seasonal and
diurnal phenomena are more marked in continental interiors than on the
oceans, islands and windward coasts. Further, the effect of altitude is
so important that another group should be added to include (4) mountain
climates.
[Illustration: FIG. 4.--Annual march of temperature: equatorial type. A,
Africa, interior; B, Batavia; J, Jaluit, Marshall Islands.]
1. _The Equatorial Belt._--Within a few degrees of the equator, and when
not interfered with by other controls, the annual curve of temperature
has two maxima following the two zenithal positions of the sun, and two
minima at about the time of the solstices. This _equatorial_ type of
annual march of temperature is illustrated in the three curves for the
interior of Africa, Batavia and Jaluit (fig. 4). The greatest range is
shown in the curve for the interior of Africa; the curve for Batavia
illustrates insular conditions with less range, and the oceanic type for
Jaluit, Marshall Islands, gives the least range. This double maximum is
not a universal phenomenon, there being many cases where but a single
maximum occurs.
[Illustration: FIG. 5.--Annual march of rainfall in the tropics.
S.A, South Africa.
Q, Quito.
S.P., Sao Paulo.
M, Mexico.
H, Hilo.
P.D., Port Darwin.]
As the belt of rains swings back and forth across the equator after the
sun, there should be two rainy seasons with the sun vertical, and two
dry seasons when the sun is farthest from the zenith, and while the
trades blow. These conditions prevail on the equator, and as far north
and south of the equator (about 10 deg.-12 deg.) as sufficient time
elapses between the two zenithal positions of the sun for the two rainy
seasons to be distinguished from one another. In this belt, under normal
conditions, there is therefore no dry season of any considerable
duration. The double rainy season is clearly seen in equatorial Africa
and in parts of equatorial South America. The maxima lag somewhat behind
the vertical sun, coming in April and November, and are unsymmetrically
developed, the first maximum being the principal one. The minima are
also unsymmetrically developed, and the so-called "dry seasons" are
seldom wholly rainless. This rainfall type with double maxima and minima
has been called the _equatorial_ type, and is illustrated in the
following curves for South Africa and Quito (fig. 5). The monthly
rainfalls are given in thousandths of the annual mean. The mean annual
rainfall at Quito is 42.12 in. These double rainy and dry seasons are
easily modified by other conditions, as by the monsoons of the
Indo-Australian area, so that there is no rigid belt of _equatorial
rains_ extending around the world. In South America, east of the Andes,
the distinction between rainy and dry seasons is often much confused. In
this equatorial belt the cloudiness is high throughout the year,
averaging .7 to .8, with a relatively small annual period. The curve
following, E (fig. 6), is fairly typical, but the annual period varies
greatly under local controls.
[Illustration: FIG. 6.--Annual march of cloudiness in the tropics. E,
Equatorial type; M, Monsoon type.]
At greater distances from the equator than about 10 deg. or 12 deg. the
sun is still vertical twice a year within the tropics, but the interval
between these two dates is so short that the two rainy seasons merge
into one, in summer, and there is also but one dry season, in winter.
This is the so-called _tropical type_ of rainfall, and is found where
the trade belts are encroached upon by the equatorial rains during the
migration of these rains into each hemisphere. It is illustrated in the
curves for Sao Paulo, Brazil, and for the city of Mexico (fig. 5). The
mean annual rainfall at Sao Paulo is 54.13 in. and at Mexico 22.99 in.
The districts of tropical rains of this type lie along the equatorial
margins of the torrid zone, outside of the latitudes of the _equatorial_
type of rainfall. The rainy season becomes shorter with increasing
distance from the equator. The weather of the opposite seasons is
strongly contrasted. The single dry season lasts longer than either dry
season in the equatorial belt, reaching eight months in typical cases,
with the wet season lasting four months. The lowlands often become dry
and parched during the long dry trade-wind season (winter) and
vegetation withers away, while grass and flowers grow in great abundance
and all life takes on new activity during the time when the equatorial
rainy belt with its calms, variable winds and heavy rains is over them
(summer). The Sudan lies between the Sahara and the equatorial forests
of Africa. It receives rains, and its vegetation grows actively, when
the doldrum belt is north of the equator (May-August). But when the
trades blow (December-March) the ground is parched and dusty. The
Venezuelan _llanos_ have a dry season in the northern winter, when the
trade blows. The rains come in May-October. The _campos_ of Brazil,
south of the equator, have their rains in October-April, and are dry the
remainder of the year. The Nile overflow results from the rainfall on
the mountains of Abyssinia during the northward migration of the belt of
equatorial rains.
[Illustration: FIG. 7.--Annual march of temperature: tropical type. W,
Wadi Halfa; A, Alice Springs; H, Honolulu; J, Jamestown, St Helena; N,
Nagpur.]
The so-called _tropical_ type of temperature variation, with one maximum
and one minimum, is illustrated in the accompanying curves for Wadi
Halfa, in upper Egypt; Alice Springs, Australia; Nagpur, India;
Honolulu, Hawaii; and Jamestown, St Helena (fig. 7). The effect of the
rainy season is often shown in a displacement of the time of maximum
temperature to an earlier month than the usual one.
2. _Trade-Wind Belts._--The trade belts near sea-level are characterized
by fair weather, steady winds, infrequent light rains or even an almost
complete absence of rain, very regular, although slight, annual and
diurnal ranges of temperature, and a constancy and regularity of
weather. The climate of the ocean areas in the trade-wind belts is
indeed the simplest and most equable in the world, the greatest extremes
over these oceans being found to leeward of the larger lands. On the
lowlands swept over by the trades, beyond the polar limits of the
equatorial rain belt (roughly between lats. 20 deg. and 30 deg.), are
most of the great deserts of the world. These deserts extend directly to
the water's edge on the leeward western coasts of Australia, South
Africa and South America.
The ranges and extremes of temperature are much greater over the
continental interiors than over the oceans of the trade-wind belts.
Minima of 32 deg. or less occur during clear, quiet nights, and daily
ranges of over 50 deg. are common. The midsummer mean temperature rises
above 90 deg., with noon maxima of 110 deg. or more in the non-cloudy,
dry air of a desert day. The days, with high, dry winds, carrying dust
and sand, with extreme heat, accentuated by the absence of vegetation,
are disagreeable, but the calmer nights, with active radiation under
clear skies, are much more comfortable. The nocturnal temperatures are
even not seldom too low for comfort in the cooler season, when thin
sheets of ice may form.
While the trades are drying winds as long as they blow strongly over the
oceans, or over lowlands, they readily become rainy if they are cooled
by ascent over a mountain or highland. Hence the windward (eastern)
sides of mountains or bold coasts in the trade-wind belts are well
watered, while the leeward sides, or interiors, are dry. Mountainous
islands in the trades, like the Hawaiian islands, many of the East and
West Indies, the Philippines, Borneo, Ceylon, Madagascar, Teneriffe,
&c., show marked differences of this sort. The eastern coasts of Guiana,
Central America, south-eastern Brazil, south-eastern Africa, and eastern
Australia are well watered, while the interiors are dry. The eastern
highland of Australia constitutes a more effective barrier than that in
South Africa; hence the Australian interior has a more extended desert.
South America in the south-east trade belt is not well enclosed on the
east, and the most arid portion is an interior district close to the
eastern base of the Andes where the land is low. Even far inland the
Andes again provoke precipitation along their eastern base, and the
narrow Pacific coastal strip, to leeward of the Andes, is a very
pronounced desert from near the equator to about lat. 30 deg. S. The cold
ocean waters, with prevailing southerly (drying) winds alongshore, are
additional factors causing this aridity. Highlands in the trade belts
are therefore moist on their windward slopes, and become oases of
luxuriant plant growth, while close at hand, on the leeward sides, dry
savannas or deserts may be found. The damp, rainy and forested windward
side of Central America was from the earliest days of European
occupation left to the natives, while the centre of civilization was
naturally established on the more open and sunny south-western side.
The rainfall associated with the conditions just described is known as
the _trade type_. These rains have a maximum in winter, when the trades
are most active. In cases where the trade blows steadily throughout the
year against mountains or bold coasts, as on the Atlantic coast of
Central America, there is no real dry season. The curve for Hilo (mean
annual rainfall 145.24 in.) on the windward side of the Hawaiian
Islands, shows typical conditions (see fig. 5). The trade type of
rainfall is often much complicated by the combination with it of the
_tropical_ type and of the _monsoon_ type. In the Malay archipelago
there are also complications of equatorial and trade rains; likewise in
the West Indies.
3. _Monsoon Belts._--In a typical monsoon region the rains follow the
vertical sun, and therefore have a simple annual period much like that
of the tropical type above described. This monsoon type of rainfall is
well illustrated in the curve for Port Darwin (mean annual rainfall
62.72 in.), in Australia (see fig. 5). This summer monsoon rainfall
results from the inflow of a body of warm, moist air from the sea upon a
land area; there is a consequent retardation of the velocity of the air
currents, as the result of friction, and an ascent of the air, the
rainfall being particularly heavy where the winds have to climb over
high lands. In India, the precipitation is heaviest at the head of the
Bay of Bengal (where Cherrapunji, at the height of 4455 ft. in the Khasi
Hills, has a mean annual rainfall of between 400 and 500 in.), along the
southern base of the Himalayas (60 to 160 in.), on the bold western
coast of the peninsula (80 to 120 in. and over), and on the mountains of
Burma, (up to 160 in.). In the rain-shadow of the Western Ghats, the
Deccan often suffers from drought and famine unless the monsoon rains
are abundant and well distributed. The prevailing direction of the rainy
monsoon wind in India is south-west; on the Pacific coast of Asia, it is
south-east. This monsoon district is very large, including the Indian
Ocean, Arabian Sea, Bay of Bengal, and adjoining continental areas; the
Pacific coast of China, the Yellow and Japan seas, and numerous islands
from Borneo to Sakhalin on the north and to the Ladrone Islands on the
east. A typical temperature curve for a monsoon district is that for
Nagpur, in the Indian Deccan (fig. 7), and a typical monsoon cloudiness
curve is given in fig. 6, the maximum coming near the time of the
vertical sun, in the rainy season, and the minimum in the dry season.
In the Australian monsoon region, which reaches across New Guinea and
the Sunda Islands, and west of Australia, in the Indian Ocean, over
latitudes 0 deg.-10 deg. S., the monsoon rains come with north-west
winds in the period between November and March or April.
The general rule that eastern coasts in the tropics are the rainiest
finds exceptions in the case of the rainy western coasts in India and
other districts with similar monsoon rains. On the coast of the Gulf of
Guinea, for example, there is a small rainy monsoon area during the
summer; heavy rains fall on the seaward slopes of the Cameroon
Mountains. Goree, lat. 15 deg. N., on the coast of Senegambia, gives a
fine example of a rainy (summer) and a dry (winter) monsoon. Numerous
combinations of equatorial, trade and monsoon rainfalls are found, often
creating great complexity. The islands of the East Indian archipelago
furnish many examples of such curious complications.
4. _Mountain Climate._--In the torrid zone altitude is chiefly important
because of its effect in tempering the heat of the lowlands, especially
at night. If tropical mountains are high enough, they carry snow all the
year round, even on the equator, and the zones of vegetation may range
from the densest tropical forest at their base to the snow on their
summits. The highlands and mountains within the tropics are thus often
sharply contrasted with the lowlands, and offer more agreeable and more
healthy conditions for white settlement. They are thus often sought by
residents from colder latitudes as the most attractive resorts. In
India, the hill stations are crowded during the hot months by civilian
and military officials. The climate of many tropical plateaus and
mountains has the reputation of being a "perpetual spring." Thus on the
interior plateau of the tropical Cordilleras of South America, and on
the plateaus of tropical Africa, the heat is tempered by the altitude,
while the lowlands and coasts are very hot. The rainfall on tropical
mountains and highlands often differs considerably in amount from that
on the lowlands, and other features common to mountain climates the
world over are also noted.
_The Characteristics of the Temperate Zones._
_General._--As a whole, the "temperate zones" are _temperate_ only in
that their mean temperatures and their physiological effects are
intermediate between those of the tropics and those of the polar zones.
A marked changeableness of the weather is a striking characteristic of
these zones. Apparently irregular and haphazard, these continual weather
changes, although they are essentially non-periodic, nevertheless run
through a fairly systematic series. Climate and weather are by no means
synonymous over most of the extra-tropical latitudes.
_Temperature._--The mean annual temperatures at the margins of the north
temperate zone differ by more than 70 deg.. The ranges between the mean
temperatures of hottest and coldest months reach 120 deg. at their
maximum in north-eastern Siberia, and 80 deg. in North America. A
January mean of -60 deg. and a July mean of 95 deg., and maxima of over
120 deg. and minima of -90 deg., occur in the same zone. Such great
ranges characterize the extreme land climates. Under the influence of
the oceans, the windward coasts have much smaller ranges. The annual
ranges in middle and higher latitudes exceed the diurnal, the conditions
of much of the torrid zone thus being exactly reversed. Over much of the
oceans of the temperate zones the annual range is less than 10 deg.. In
the south temperate zone there are no extreme ranges, the maxima,
slightly over 30 deg., being near the margin of the zone in the interior
of South America, South Africa and Australia. In these same localities
the diurnal ranges rival those of the north temperate zone.
The north-eastern Atlantic and north-western Europe are about 35 deg.
too warm for their latitude in January, while north-eastern Siberia is
30 deg. too cold. The lands north of Hudson Bay are 25 deg. too cold,
and the waters of the Alaskan Bay 20 deg. too warm. In July, and in the
southern hemisphere, the anomalies are small. The lands which are the
centre of civilization in Europe average too warm for their latitudes.
The diurnal variability of temperature is greater in the north temperate
zone than elsewhere in the world, and the same month may differ greatly
in its character in different years. The annual temperature curve has
one maximum and one minimum. In the continental type the times of
maximum and minimum are about one month behind the dates of maximum and
minimum insolation. In the marine type the retardation may amount to
nearly two months. Coasts and islands have a tendency to a cool spring
and warm autumn; continents, to similar temperatures in both spring and
fall.