are much less regular than the trades. They vary greatly in velocity in
different regions and in different seasons, and are stronger in winter
than in summer. They are much interfered with, especially in the higher
northern latitudes, by seasonal changes of temperature and pressure over
the continents, whereby the latter establish, more or less successfully,
a system of obliquely outflowing winds in winter and of obliquely
inflowing winds in summer. In summer, when the lands have low pressure,
the northern oceans are dominated by great oval areas of high pressure,
with outflowing spiral eddies, while in winter, when the northern lands
have high pressure, the northern portions of the oceans develop cyclonic
systems of inflowing winds over their warm waters. All these great
continental and oceanic systems of spiralling winds are important
climatic controls.
The westerlies are also much confused and interrupted by storms, whence
their designation of _stormy westerlies_. So common are such
interruptions that the prevailing westerly wind direction is often
difficult to discern without careful observation. Cyclonic storms are
most numerous and best developed in winter. Although greatly interfered
with near sea-level by continental changes of pressure, by cyclonic and
anticyclonic whirls, and by local inequalities of the surface, the
eastward movement of the atmosphere remains very constant aloft. The
south temperate zone being chiefly water, the westerlies are but little
disturbed there by continental effects. Between latitudes 40 deg. and 60
deg. S. the "brave west winds" blow with a constancy and velocity found
in the northern hemisphere only on the oceans, and then in a modified
form. Storms, frequent and severe, characterize these southern
hemisphere westerlies, and easterly wind directions are temporarily
noted during their passage. Voyages to the west around Cape Horn against
head gales, and in cold wet weather, are much dreaded. South of Africa
and Australia, also, the westerlies are remarkably steady and strong.
The winter in these latitudes is stormier than the summer, but the
seasonal difference is less than north of the equator.
_Rainfall._--Rainfall is fairly abundant over the oceans and also over a
considerable part of the lands (30-80 in. and more). It comes chiefly in
connexion with the usual cyclonic storms, or in thunderstorms. So great
are the differences, geographic and periodic, in rainfall produced by
differences in temperature, topography, cyclonic conditions, &c., that
only the most general rules can be laid down. The equatorward margin of
the temperate zone rains is clearly defined on the west coasts, at the
points where the coast deserts are replaced by belts of light or
moderate rainfall. Bold west coasts, on the polar side of lat. 40 deg.,
are very rainy (100 in. and more a year in the most favourable
situations). The hearts of the continents, far from the sea, and
especially when well enclosed by mountains, or when blown over by cool
ocean winds which warm while crossing the land, have light rainfall
(less than 10-20 in.). East coasts are wetter than interiors, but drier
than west coasts. Winter is the season of maximum rainfall over oceans,
islands and west coasts, for the westerlies are then most active,
cyclonic storms are most numerous and best developed, and the cold lands
chill the inflowing damp air. At this season, however, the low
temperatures, high pressures, and tendency to outflowing winds over the
continents are unfavourable to rainfall, and the interior land areas as
a rule then have their minimum. The warmer months bring the maximum
rainfall over the continents. Conditions are then favourable for
inflowing damp winds from the adjacent oceans; there is the best
opportunity for convection; thunder-showers readily develop on the hot
afternoons; the capacity of the air for water vapour is greatest. The
marine type of rainfall, with a winter maximum, extends in over the
western borders of the continents, and is also found in the winter
rainfall of the sub-tropical belts. Rainfalls are heaviest along the
tracks of most frequent cyclonic storms.
For continental stations the typical daily march of rainfall shows a
chief maximum in the afternoon, and a secondary maximum in the night or
early morning. The chief minimum comes between 10 A.M. and 2 P.M. Coast
stations generally have a night maximum and a minimum between 10 A.M.
and 4 P.M.
_Humidity and Cloudiness._--S.A. Arrhenius gives the mean cloudiness for
different latitudes as follows:--
+-----------+----+
| 70 deg. N.| 59 |
| 60 deg. | 61 |
| 50 deg. | 48 |
| 40 deg. | 49 |
| 30 deg. | 42 |
| 20 deg. | 40 |
| 10 deg. | 50 |
| Eq. | 58 |
| 10 deg. | 57 |
| 20 deg. | 48 |
| 30 deg. | 46 |
| 40 deg. | 56 |
| 50 deg. | 66 |
| 60 deg. S.| 75 |
+-----------+----+
The higher latitudes of the temperate zones thus have a mean cloudiness
which equals and even exceeds that of the equatorial belt. The amounts
are greater over the oceans and coasts than inland. The belts of minimum
cloudiness are at about lat. 30 deg. N. and S. Over the continental
interiors the cloudiest season is summer, but the amount is never very
large. Otherwise, winter is generally the cloudiest season and with a
fairly high mean annual amount.
The absolute humidity as a whole decreases as the temperature falls. The
relative humidity averages 90%, more or less, over the oceans, and is
high under the clouds and rain of cyclonic storms, but depends, on land,
upon the wind direction, winds from an ocean or from a lower latitude
being damper, and those from a continent or from a colder latitude being
drier.
_Seasons._--Seasons in the temperate zones are classified according to
temperature; not, as in the tropics, by rainfall. The four seasons are
important characteristics, especially of the middle latitudes of the
north temperate zone. Towards the equatorial margins of the zones the
difference in temperature between summer and winter becomes smaller, and
the transition seasons weaken and even disappear. At the polar margins
the change from winter to summer, and vice versa, is so sudden that
there also the transition seasons disappear.
These seasonal changes are of the greatest importance in the life of
man. The monotonous heat of the tropics and the continued cold of the
polar zones are both depressing. Their tendency is to operate against
man's highest development. The seasonal changes of the temperate zones
stimulate man to activity. They develop him, physically and mentally.
They encourage higher civilization. A cold, stormy winter necessitates
forethought in the preparation during the summer of clothing, food and
shelter. Development must result from such conditions. In the warm,
moist tropics life is too easy; in the cold polar zones it is too hard.
Near the poles, the growing season is too short; in the moist tropics it
is so long that there is little inducement to labour at any special
time. The regularity, and the need, of outdoor work during a part of the
year are important factors in the development of man in the temperate
zones.
_Weather._--An extreme changeableness of the weather, depending on the
succession of cyclones and anticyclones, is another characteristic. For
most of the year, and most of the zone, settled weather is unknown. The
changes are most rapid in the northern portion of the north temperate
zone, especially on the continents, where the cyclones travel fastest.
The nature of these changes depends on the degree of development, the
velocity of progression, the track, and other conditions of the
disturbance which produces them. The particular weather types resulting
from this control give the climates their distinctive character.
The types vary with the season and with the geographical position. They
result from a combination, more or less irregular, of periodic diurnal
elements, under the regular control of the sun; and of non-periodic
cyclonic and anticyclonic elements. In summer, on land, when the
Cyclonic element is weakest and the solar control is the strongest, the
dominant types are associated with the regular changes from day to
night. Daytime cumulus clouds; diurnal variation in wind velocity;
afternoon thunderstorms, with considerable regularity, characterize the
warmest months over the continents and present an analogy with tropical
conditions. Cyclonic and anticyclonic spells of hotter or cooler, rainy
or dry, weather, with varying winds differing in the temperatures and
the moisture which they bring, serve to break the regularity of the
diurnal types. In winter the non-periodic, cyclonic control is
strongest. The irregular changes from clear to cloudy, from warmer to
colder, from dry air to snow or rain, extend over large areas, and show
little diurnal control. Spring and fall are transition seasons, and have
transition weather types. The south temperate zone oceans have a
constancy of non-periodic cyclonic weather changes through the year
which is only faintly imitated over the oceans of the northern
hemisphere. Winter types differ little from summer. The diurnal control
is never very strong. Stormy weather prevails throughout the year
although the weather changes are more frequent and stronger in the
colder months.
_Climatic Subdivisions._--There are fundamental differences between the
north and south temperate zones. The latter zone is sufficiently
individual to be given a place by itself. The marginal sub-tropical
belts must also be considered as a separate group by themselves. The
north temperate zone as a whole includes large areas of land, stretching
over many degrees of latitude, as well as of water. Hence it embraces so
remarkable a diversity of climates that no single district can be taken
as typical of the whole. The simplest and most rational scheme for a
classification of these climates is based on the fundamental differences
which depend upon land and water, upon the prevailing winds, and upon
altitude. Thus there are the ocean areas and the land areas. The latter
are then subdivided into western (windward) and eastern (leeward)
coasts, and interiors. Mountain climates remain as a separate group.
_South Temperate Zone._--Because of the large ocean surface, the whole
meteorological regime in the south temperate zone is more uniform than
in the northern. The south temperate zone may properly be called
"temperate." Its temperature changes are small; its prevailing winds are
stronger and steadier than in the northern hemisphere; its seasons are
more uniform; its weather is prevailingly stormier, more changeable, and
more under cyclonic control. The uniformity of the climatic conditions
over the far southern oceans is monotonously unattractive. The
continental areas are small, and develop to a limited degree only the
more marked seasonal and diurnal changes which are characteristic of
lands in general. The summers are less stormy than the winters, but even
the summer temperatures are not high. Such an area as that of New
Zealand, with its mild climate and fairly regular rains, is really at
the margins of the zone, and has much more favourable conditions than
the islands farther south. These islands, in the heart of this zone,
have dull, cheerless and inhospitable climates. The zone enjoys a good
reputation for healthfulness, which fact has been ascribed chiefly to
the strong and active air movement, the relatively drier air than in
corresponding northern latitudes, and the cool summers. It must be
remembered, also, that the lands are mostly in the sub-tropical belt,
which possesses peculiar climatic advantages, as will be seen.
_Sub-tropical Belts: Mediterranean Climates._--At the tropical margins
of the temperate zones are the so-called sub-tropical belts. Their
rainfall regime is alternately that of the westerlies and of the trades.
They are thus associated, now with the temperate and now with the torrid
zones. In winter the equatorward migration of the great pressure and
wind systems brings these latitudes under the control of the westerlies,
whose frequent irregular storms give a moderate winter precipitation.
These winter rains are not steady and continuous, but are separated by
spells of fine sunny weather. The amounts vary greatly.[4] In summer,
when the trades are extended polewards by the outflowing equatorward
winds on the eastern side of the ocean anticyclones, mild, dry and
nearly continuous fair weather prevails, with general northerly winds.
The sub-tropical belts of winter rains and dry summers are not very
clearly defined. They are mainly limited to the western coasts of the
continents, and to the islands off these coasts in latitudes between
about 28 deg. and 40 deg.. The sub-tropical belt is exceptionally wide
in the old world, and reaches far inland there, embracing the countries
bordering on the Mediterranean in southern Europe and northern Africa,
and then extending eastward across the Dalmatian coast and the southern
part of the Balkan peninsula into Syria, Mesopotamia, Arabia north of
the tropic, Persia and the adjacent lands. The fact that the
Mediterranean countries are so generally included has led to the use of
the name "Mediterranean climate." Owing to the great irregularity of
topography and outline, the Mediterranean province embraces many
varieties of climate, but the dominant characteristics are the mild
temperatures, except on the higher elevations, and the sub-tropical
rains.
On the west coasts of the two Americas the sub-tropical belt of winter
rains is clearly seen in California and in northern Chile, on the west
of the coast mountain ranges. Between the region which has rain
throughout the year from the stormy westerlies, and the districts which
are permanently arid under the trades, there is an indefinite belt over
which rains fall in winter. In southern Africa, which is controlled by
the high pressure areas of the South Atlantic and south Indian oceans,
the south-western coastal belt has winter rains, decreasing to the
north, while the east coast and adjoining interior have summer rains,
from the south-east trade. Southern Australia is climatically similar to
South Africa. In summer the trades give rainfall on the eastern coast,
decreasing inland. In winter the westerlies give moderate rains, chiefly
on the south-western coast.
The sub-tropical climates follow the tropical high pressure belts across
the oceans, but they do not retain their distinctive character far
inland from the west coasts of the continents (except in the
Mediterranean case), nor on the east coasts. On the latter, summer
monsoons and the occurrence of general summer rains interfere, as in
eastern Asia and in Florida.
[Illustration: FIG. 8.--Annual March of Rainfall: Sub-tropical Type.
W.A, Western Australia: M, Malta.]
Strictly winter rains are typical of the coasts and islands of this
belt. The more continental areas have a tendency to spring and autumn
rains. The rainy and dry seasons are most marked at the equatorward
margins of the belt. With increasing latitude, the rain is more evenly
distributed through the year, the summer becoming more and more rainy
until, in the continental interiors of the higher latitudes, the summer
becomes the season of maximum rainfall. The monthly distribution of
rainfall in two sub-tropical regions is shown in the accompanying curves
for Malta and for Western Australia (fig. 8). In Alexandria the dry
season lasts nearly eight months; in Palestine, from six to seven
months; in Greece, about four months. The sub-tropical rains are
peculiarly well developed on the eastern coast of the Atlantic Ocean.
The winter rains which migrate equatorward are separated by the Sahara
from the equatorial rains which migrate poleward. An unusually extended
migration of either of these rain belts may bring them close together,
leaving but a small part, if any, of the intervening desert actually
rainless. The Arabian desert occupies a somewhat similar position. Large
variations in the annual rainfall may be expected towards the equatorial
margins of the sub-tropical belts.
The main features of the sub-tropical rains east of the Atlantic are
repeated on the Pacific coasts of the two Americas. In North America the
rainfall decreases from Alaska, Washington and northern Oregon
southwards to lower California, and the length of the summer dry season
increases. At San Diego, six months (May-October) have each less than 5%
of the annual precipitation, and four of these have 1%. The southern
extremity of Chile, from about latitude 38 deg. S. southward, has heavy
rainfall throughout the year from the westerlies, with a winter maximum.
Northern Chile is persistently dry. Between these two there are winter
rains and dry summers. Neither Africa nor Australia extends far enough
south to show the different members of this system well. New Zealand is
almost wholly in the prevailing westerly belt. Northern India is unique
in having summer monsoon rains and also winter rains, the latter from
weak cyclonic storms which correspond with the sub-tropical winter
rains.
[Illustration: FIG. 9.--Annual March of Temperature for selected
Sub-tropical Stations. C, Cordoba; A, Auckland; Ba, Bermuda; Bd,
Bagdad.]
From the position of the sub-tropical belts to leeward of the oceans,
and at the equatorial margins of the temperate zones, it follows that
their temperatures are not extreme. Further, the protection afforded by
mountain ranges, as by the Alps in Europe and the Sierra Nevada in the
United States, is an important factor in keeping out extremes of winter
cold. The annual march and ranges of temperature depend upon position
with reference to continental or marine influences. This is seen in the
accompanying data and curves for Bagdad, Cordoba (Argentina), Bermuda
and Auckland (fig. 9). The Mediterranean basin is particularly favoured
in winter, not only in the protection against cold afforded by the
mountains but also in the high temperature of the sea itself. The
southern Alpine valleys and the Riviera are well situated, having good
protection and a southern exposure. The coldest month usually has a mean
temperature well above 32 deg.. Mean minimum temperatures of about, and
somewhat below, freezing occur in the northern portion of the district,
and in the more continental localities minima a good deal lower have
been observed. Mean maximum temperatures of about 95 deg. occur in
northern Italy, and of still higher degrees in the southern portions.
Somewhat similar conditions obtain in the sub-tropical district of North
America. Under the control of passing cyclonic storm areas, hot or cold
winds, which often owe some of their special characteristics to the
topography, bring into the sub-tropical belts, from higher or lower
latitudes, unseasonably high or low temperatures. These winds have been
given special names (mistral, sirocco, bora, &c.).
These belts are among the least cloudy districts in the world. The
accompanying curve, giving an average for ten stations, shows the small
annual amount of cloud, the winter maximum and the marked summer
minimum, in a typical sub-tropical climate (fig. 10). The winter rains
do not bring continuously overcast skies, and a summer month with a mean
cloudiness of 10% is not exceptional in the drier parts of the
sub-tropics.
[Illustration: FIG. 10.--Annual March of Cloudiness in a Sub-tropical
Climate (Eastern Mediterranean).]
With prevailing fair skies, even temperatures, and moderate rainfall,
the sub-tropical belts possess many climatic advantages which fit them
for health resorts. The long list of well-known resorts on the
Mediterranean coast, and the shorter list for California, bear witness
to this fact.
_North Temperate Zone: West Coasts._--Marine climatic types are carried
by the prevailing westerlies on to the western coasts of the continents,
giving them mild winters and cool summers, abundant rainfall, and a high
degree of cloudiness and relative humidity. North-western Europe is
particularly favoured because of the remarkably high temperatures of the
North Atlantic Ocean. January means of 40 deg. to 50 deg. in the British
Isles and on the northern French coast occur in the same latitudes as
those of 0 deg. and 10 deg. in the far interior of Asia. In July means
60 deg. to 70 deg. in the former contrast with 70 deg. to 80 deg. in the
latter districts. The conditions are somewhat similar in North America.
Along the western coasts of North America and of Europe the mean annual
ranges are under 25 deg.--actually no greater than some of those within
the tropics. Irregular cyclonic temperature changes are, however, marked
in the temperate zone, while absent in the tropics. The curves for the
Scilly Isles and for Thorshavn, Faroe Islands, illustrate the insular
type of temperature on the west coasts (fig. 11). The annual march of
rainfall, with the marked maximum in the fall and winter which is
characteristic of the marine regime, is illustrated in the curve for
north-western Europe (fig. 12). On the northern Pacific coast of North
America the distribution is similar, and in the southern hemisphere the
western coasts of southern South America, Tasmania and New Zealand show
the same type. The cloudiness and relative humidity average high on
western coasts, with the maximum in the colder season.
[Illustration: FIG. 11.--Annual March of Temperature for Selected
Stations in the Temperate Zones.
S. I., Scilly Isles.
P, Prague.
C, Charkow.
S, Semipalatinsk.
K, Kiakta.
B, Blagovyeshchensk.
Sa, Sakhalin.
T, Thorshavn.
Y, Yakutsk.]
[Illustration: FIG. 12.--Annual March of Rainfall: Temperate Zone. C.E.
Central Europe; A. Northern Asia; N.A. Atlantic coast of North
America; N.W.E. North-west Europe.]
The west coasts therefore, including the important climatic province of
western Europe, and the coast provinces of north-western North America,
New Zealand and southern Chile, have as a whole mild winters, equable
temperatures, small ranges, and abundant rainfall, fairly well
distributed through the year. The summers are relatively cool.
_Continental Interiors._--The equable climate of the western coasts
changes, gradually or suddenly, into the more extreme climates of the
interiors. In Europe, where no high mountain ranges intervene, the
transition is gradual, and broad stretches of country have the benefits
of the tempering influence of the Atlantic. In North America the change
is abrupt, and comes on crossing the lofty western mountain barrier. The
curves in fig. 11 illustrate well the gradually increasing
continentality of the climate with increasing distance inland in
Eurasia.
The continental interiors of the north temperate zone have the greatest
extremes in the world. Towards the Arctic circle the winters are
extremely severe, and January mean temperatures of -10 deg. and -20 deg.
occur over considerable areas. At the cold pole of north-eastern Siberia
a January mean of -60 deg. is found. Mean minimum temperatures of -40
deg. occur in the area from eastern Russia, over Siberia and down to
about latitude 50 deg. N. Over no small part of Siberia minimum
temperatures below -70 deg. may be looked for every winter. Thorshavn
and Yakutsk are excellent examples of the temperature differences along
the same latitude line (see fig. 11). The winter in this interior region
is dominated by a marked high pressure. The weather is prevailingly
clear and calm. The ground is frozen all the year round below a slight
depth over wide areas. The extremely low temperatures are most trying
when the steppes are swept by icy storm winds (_buran_, _purga_),
carrying loose snow, and often resulting in loss of life. In the North
American interior the winter cold is somewhat less severe. North
American winter weather in middle latitudes is often interrupted by
cyclones, which, under the steep poleward temperature gradient then
prevailing, cause frequent, marked and sudden changes in wind direction
and temperature over the central and eastern United States. Cold waves
and warm waves are common, and blizzards resemble the buran or purga of
Russia and Siberia. With cold northerly winds, temperatures below
freezing are carried far south towards the tropic.
The January mean temperatures in the southern portions of the
continental interiors average about 50 deg. or 60 deg. In summer the
northern continental interiors are warm, with July means of 60 deg. and
thereabouts. These temperatures are not much higher than those on the
west coasts, but as the northern interior winters are much colder than
those on the coasts, the interior ranges are very large. Mean maximum
temperatures of 86 deg. occur beyond the Arctic circle in north-eastern
Siberia, and beyond latitude 60 deg. in North America. In spite of the
extreme winter cold, agriculture extends remarkably far north in these
regions, because of the warm, though short, summers, with favourable
rainfall distribution. The summer heat is sufficient to thaw the upper
surface of the frozen ground, and vegetation prospers for its short
season. At this time great stretches of flat surface become swamps. The
southern interiors have torrid heat in summer, temperatures of over 90
deg. being recorded in the south-western United States and in southern
Asia. In these districts the diurnal ranges of temperature are very
large, often exceeding 40 deg., and the mean maxima exceed 110 deg.
The winter maximum rainfall of the west coasts becomes a summer maximum
in the interiors. The change is gradual in Europe, as was the change in
temperature, but more sudden in North America. The curves for central
Europe and for northern Asia illustrate these continental summer rains
(see fig. 12). The summer maximum becomes more marked with the
increasing continental character of the climate. There is also a
well-marked decrease in the amount of rainfall inland. In western Europe
the rainfall averages 20 to 30 in., with much larger amounts (reaching
80-100 in. and even more) on the bold west coasts, as in the British
Isles and Scandinavia, where the moist Atlantic winds are deflected
upwards, and also locally on mountain ranges, as on the Alps. There are
small rainfalls (below 20 in.) in eastern Scandinavia and on the Iberian
peninsula. Eastern Europe has generally less than 20 in., western
Siberia about 15 in., and eastern Siberia about 10 in. In the southern
part of the great overgrown continent of Asia an extended region of
steppes and deserts, too far from the sea to receive sufficient
precipitation, shut in, furthermore, by mountains, controlled in summer
by drying northerly winds, receives less than 10 in. a year, and in
places less than 5 in. In this interior district of Asia population is
inevitably small and suffers under a condition of hopeless aridity.
The North American interior has more favourable rainfall conditions than
Asia, because the former continent is not overgrown. The heavy rainfalls
on the western slopes of the Pacific coast mountains correspond, in a
general way, with those on the west coast of Europe, although they are
heavier (over 100 in. at a maximum). The close proximity of the
mountains to the Pacific, however, involves a much more rapid decrease
of rainfall inland than is the case in Europe, as may be seen by
comparing the isohyetal lines[5] in the two cases. A considerable
interior region is left with deficient rainfall (less than 10 in.) in
the south-west. The eastern portion of the continent is freely open to
the Atlantic and the Gulf of Mexico, so that moist cyclonic winds have
access, and rainfalls of over 20 in. are found everywhere east of the
100th meridian. These conditions are much more favourable than those in
eastern Asia. The greater part of the interior of North America has the
usual warm-season rains. In the interior basin, between the Rocky and
Sierra Nevada mountains, the higher plateaus and mountains receive much
more rain than the desert lowlands. Forests grow on the higher
elevations, while irrigation is necessary for agriculture on the
lowlands. The rainfall here comes largely from thunderstorms.
In South America the narrow Pacific slope has heavy rainfall (over 80
in.). East of the Andes the plains are dry (mostly less than 10 in.).
The southern part of the continent is very narrow, and is open to the
east, as well as more open to the west owing to the decreasing height of
the mountains. Hence the rainfall increases somewhat to the south,
coming in connexion with passing cyclones. Tasmania and New Zealand have
most rain on their western slopes.
[Illustration: FIG. 13.--Annual March of Cloudiness: Temperate Zones. E,
Central Europe; A, Eastern Asia; M, mountain.]
In a typical continental climate the winter, except for radiation fogs,
is very clear, and the summer the cloudiest season, as is well shown in
the accompanying curve for eastern Asia (A, fig. 13). In a more moderate
continental climate, such as that of central Europe (E, fig. 13), and
much of the United States, the winter is the cloudiest season. In the
first case the mean cloudiness is small; in the second there is a good
deal of cloud all the year round.
_East Coasts._--The prevailing winds carry the continental climates of
the interiors off over the eastern coasts of the temperate zone lands,
and even for some distance on to the adjacent oceans. The east coasts
therefore have continental climates, with modifications resulting from
the presence of the oceans to leeward, and are necessarily separated
from the west coasts, with which they have little in common. On the west
coasts of the north temperate lands the isotherms are far apart. On the
east coasts they are crowded together. The east coasts share with the
interiors large annual and cyclonic ranges of temperature. A glance at
the isothermal maps of the world will show at once how favoured, because
of its position to leeward of the warm North Atlantic waters, is western
Europe as compared with eastern North America. A similar contrast, less
marked, is seen in eastern Asia and western North America. In eastern
Asia there is some protection, by the coast mountains, against the
extreme cold of the interior, but in North America there is no such
barrier, and severe cold winds sweep across the Atlantic coast states,
even far to the south. Owing to the prevailing offshore winds, the
oceans to leeward have relatively little effect.
As already noted, the rainfall increases from the interiors towards the
east coasts. In North America the distribution through the year is very
uniform, with some tendency to a summer maximum, as in the interior
(N.A, fig. 12).
In eastern Asia the winters are relatively dry and clear, under the
influence of the cold offshore monsoon, and the summers are warm and
rainy. Rainfalls of 40 in. are found on the east coasts of Korea,
Kamchatka and Japan, while in North America, which is more open, they
reach farther inland. Japan, although occupying an insular position, has
a modified continental rather than a marine climate. The winter monsoon,
after crossing the water, gives abundant rain on the western coast,
while the winter is relatively dry on the lee of the mountains, on the
east. Japan has smaller temperature ranges than the mainland.
_Mountain Climates._--The mountain climates of the temperate zone have
the usual characteristics which are associated with altitude everywhere.
If the altitude is sufficiently great the decreased temperature gives
mountains a polar climate, with the difference that the summers are
relatively cool while the winters are mild owing to inversions of
temperature in anticyclonic weather. Hence the annual ranges are smaller
than over lowlands. At such times of inversion the mountain-tops often
appear as local areas of higher temperature in a general region of
colder air over the valleys and lowlands. The increased intensity of
insolation aloft is an important factor in giving certain mountain
resorts their deserved popularity in winter (_e.g._ Davos and Meran). Of
Meran it has been well said that from December to March the nights are
winter, but the days are mild spring. The diurnal ascending air currents
of summer usually give mountains their maximum cloudiness and highest
relative humidity in the warmer months, while winter is the drier and
clearer season. This is shown in curve M, fig. 13. The clouds of winter
are low, those of summer are higher. Hence the annual march of
cloudiness on mountains is usually the opposite of that on lowlands.
_Characteristics of the Polar Zones._
_General._--The temperate zones merge into the polar zones at the Arctic
and Antarctic circles, or, if temperature be used as the basis of
classification, at the isotherms of 50 deg. for the warmest month, as
suggested by Supan. The longer or shorter absence of the sun gives the
climate a peculiar character, not found elsewhere.
Beyond the isotherm of 50 deg. for the warmest month forest trees and
cereals do not grow. In the northern hemisphere this line is well north
of the Arctic circle in the continental climate of Asia, and north of it
also in north-western North America and in northern Scandinavia, but
falls well south in eastern British America, Labrador and Greenland, and
also in the North Pacific Ocean. In the southern hemisphere this
isotherm crosses the southern extremity of South America, and runs
fairly east and west around the globe there. The conditions of life are
necessarily very specialized for the peculiar climatic features which
are met with in these zones. There is a minimum of life, but more in the
north polar than the south polar zone. Plants are few and lowly. Land
animals which depend upon plant food must therefore likewise be few in
number. Farming and cattle-raising cease. Population is small and
scattered. There are no permanent settlements at all within the
Antarctic circle. Life is a constant struggle for existence. Man seeks
his food by the chase on land, but chiefly in the sea. He lives along,
or near, the sea-coast. The interior lands, away from the sea, are
deserted. Gales and snow and cold cause many deaths on land, and,
especially during fishing expeditions, at sea. Under such hard
conditions of securing food, famine is a likely occurrence.
In the arctic climate vegetation must make rapid growth in the short,
cool summer. In the highest latitudes the summer temperatures are not
high enough to melt snow on a level. Exposure is therefore of the
greatest importance. Arctic plants grow and blossom with great rapidity
and luxuriance where the exposure is favourable, and where the water
from the melting snow can run off. The soil then dries quickly, and can
be effectively warmed. Protection against cold winds is another
important factor in the growth of vegetation. Over great stretches of
the northern plains the surface only is thawed out in the warmer months,
and swamps, mosses and lichens are found above eternally frozen ground.
Direct insolation is very effective in high latitudes. Where the
exposure is favourable, snow melts in the sun when the temperature of
the air in the shade is far below freezing.
Arctic and antarctic zones differ a good deal in the distribution and
arrangement of land and water around and in them. The southern zone is
surrounded by a wide belt of open sea; the northern, by land areas. The
northern is therefore much affected by the conditions of adjacent
continental masses. Nevertheless, the general characteristics are
apparently much the same over both, so far as is now known, the
antarctic differing from the arctic chiefly in having colder summers and
in the regularity of its pressure and winds. Both zones have the lowest
mean annual temperatures in their respective hemispheres, and hence may
properly be called the _cold zones_.
_Temperature._--At the solstices the two poles receive the largest
amounts of insolation which any part of the earth's surface ever
receives. It would seem, therefore, that the temperatures at the poles
should then be the highest in the world, but as a matter of fact they
are nearly or quite the lowest. Temperatures do not follow insolation in
this case because much of the latter never reaches the earth's surface;
because most of the energy which does reach the surface is expended in
melting the snow and ice of the polar areas; and because the water areas
are large, and the duration of insolation is short.
A set of monthly isothermal charts of the north polar area, based on all
available observations, has been prepared by H. Mohn and published in
the volume on Meteorology of the Nansen expedition. In the winter months
there are three cold poles, in Siberia, in Greenland and at the pole
itself. In January the mean temperatures at these three cold poles are
-49 deg., -40 deg. and -40 deg. respectively. The Siberian cold pole
becomes a maximum of temperature during the summer, but the Greenland
and polar minima remain throughout the year. In July the temperature
distribution shows considerable uniformity; the gradients are relatively
weak. A large area in the interior of Greenland, and one of about equal
extent around the pole, are within the isotherm of 32 deg.. For the year
a large area around the pole is enclosed by the isotherm of -4 deg.,
with an isotherm of the same value in the interior of Greenland, but a
local area of -7.6 deg. is noted in Greenland, and one of -11.2 deg. is
centred at lat. 80 deg. N. and long. 170 deg. E.
The north polar chart of annual range of temperature shows a maximum
range of about 120 deg. in Siberia; of 80 deg. in North America; of 75.6
deg. at the North Pole, and of 72 deg. in Greenland. The North Pole
obviously has a continental climate. The minimum ranges are on the
Atlantic and Pacific Oceans. The mean annual isanomalies show that the
interior of Greenland has a negative anomaly in all months. The
Norwegian sea area is 45 deg. too warm in January and February. Siberia
has +10.8 deg. in summer, and -45 deg. in January. Between Bering Strait
and the pole there is a negative anomaly in all months. The influence of
the Gulf Stream drift is clearly seen on the chart, as it is also on
that of mean annual ranges.
For the North Pole Mohn gives the following results, obtained by graphic
methods:--
_Mean Temperatures at the North Pole._
+--------+--------+--------+--------+--------+--------+
| Jan. | Feb. | Mar. | Apr. | May. | June. |
+--------+--------+--------+--------+--------+--------+
| -41.8 | -41.8 | -31.0 | -18.4 | 8.6 | 28.4 |
| deg. | deg. | deg. | deg. | deg. | deg. |
+--------+--------+--------+--------+--------+--------+
+--------+--------+--------+--------+--------+--------+-------+
| July. | Aug. | Sept. | Oct. | Nov. | Dec. | Year. |
+--------+--------+--------+--------+--------+--------+-------+
| 30.2 | 26.6 | 8.6 | -11.2 | -27.4 | -36.4 | -8.9 |
| deg. | deg. | deg. | deg. | deg. | deg. | deg. |
+--------+--------+--------+--------+--------+--------+-------+
It appears that the region about the North Pole is the coldest place in
the northern hemisphere for the mean of the year, and that the interior
ice desert of Greenland, together with the inner polar area, are
together the coldest parts of the northern hemisphere in July. In
January, however, Verkhoyansk, in north-eastern Siberia, just within the
Arctic circle, has a mean temperature of about -60 deg., while the inner
polar area and the northern interior of Greenland have only -40 deg.. Thus
far no minima as low as those of north-eastern Siberia have been
recorded in the Arctic.
For the Antarctic our knowledge is still very fragmentary, and relates
chiefly to the summer months. Hann has determined the mean temperatures
of the higher southern latitudes as follows:--[6]
_Mean Temperatures of High Southern Latitudes._
S. Lat. 50 deg. 60 deg. 70 deg. 80 deg.
Mean Annual 41.9 28.4 11.3 -3.6
January 46.9 37.8 30.6 20.3
July 37.2 18.3 -8.0 -24.7
From lat. 70 deg. S. polewards, J. Hann finds that the southern hemisphere
is colder than the northern. Antarctic summers are decidedly cold. The
mean annual temperatures experienced have been in the vicinity of 10 deg.,
and the minima of an ordinary antarctic winter go down to -40 deg. and
below, but so far no minima of the severest Siberian intensity have been
noted. The maxima have varied between 35 deg. and 50 deg.
The temperatures at the South Pole itself furnish an interesting subject
for speculation. It is likely that near the South Pole will prove to be
the coldest point on the earth's surface for the year, as the
distribution of insolation would imply, and as the conditions of land
and ice and snow there would suggest. The lowest winter and summer
temperatures in the southern hemisphere will almost certainly be found
in the immediate vicinity of the pole. It must not be supposed that the
isotherms in the antarctic region run parallel with the latitude lines.
They bend polewards and equatorwards at different meridians, although
much less so than in the Arctic.
The annual march of temperature in the north polar zone, for which we
have the best comparable data, is peculiar in having a much-retarded
minimum in February or even in March--the result of the long, cold
winter. The temperature rises rapidly towards summer, and reaches a
maximum in July. Autumn is warmer than spring.
The continents do not penetrate far enough into the arctic zone to
develop a pure continental climate in the highest latitudes.
Verkhoyansk, in lat. 67 deg. 6' N., furnishes an excellent example of an
exaggerated continental type for the margin of the zone, with an annual
range of 120 deg.. One-third as large a range is found on Novaya Zemlya.
Polar climate as a whole has large annual and small diurnal ranges, but
sudden changes of wind may cause marked irregular temperature changes
within twenty-four hours, especially in winter. The smaller ranges are
associated with greater cloudiness, and vice versa. The mean diurnal
variability is very small in summer, and reaches its maximum in winter,
about 7 deg. in February, according to Mohn.
_Pressure and Winds._--Owing to the more symmetrical distribution of
land and water in the southern than in the northern polar area, the
pressures and winds have a simpler arrangement in the former, and may be
first considered. The rapid southward decrease of pressure, which is so
marked a feature of the higher latitudes of the southern hemisphere on
the isobaric charts of the world, does not continue all the way to the
South Pole. Nor do the prevailing westerly winds, constituting the
"circumpolar whirl," which are so well developed over the southern
portions of the southern hemisphere oceans, blow all the way home to the
South Pole. The steep poleward pressure gradients of these southern
oceans end in a trough of low pressure, girdling the earth at about the
Antarctic circle. From here the pressure increases again towards the
South Pole, where a permanent inner polar anticyclonic area is found,
with outflowing winds deflected by the earth's rotation into easterly
and south-easterly directions. These easterly winds have been observed
by the recent expeditions which have penetrated far enough south to
cross the low-pressure trough. The limits between the prevailing
westerlies and the outflowing winds from the pole ("easterlies") vary
with the longitude and migrate with the seasons. The change in passing
from one wind system to the other is easily observed. This south polar
anticyclone, with its surrounding low-pressure girdle, migrates with the
season, the centre apparently shifting polewards in summer and towards
the eastern hemisphere in winter. The outflowing winds from the polar
anticyclones sweep down across the inland ice. Under certain topographic
conditions, descending across mountain ranges, as in the case of the
Admiralty Range in Victoria Land, these winds may develop high velocity
and take on typical _fohn_ characteristics, raising the temperature to
an unusually high degree. _Fohn_ winds are also known on both coasts of
Greenland, when a passing cyclonic depression draws the air down from
the icy interior. These Greenland _fohn_ winds are important climatic
elements, for they blow down warm and dry, raising the temperature even
30 deg. or 40 deg. above the winter mean, and melting the snow.
In the Arctic area the wind systems are less clearly defined and the
pressure distribution is much less regular, on account of the irregular
distribution of land and water. The isobaric charts published in the
report of the Nansen expedition show that the North Atlantic
low-pressure area is more or less well developed in all months. Except
in June, when it lies over southern Greenland, this tongue-shaped trough
of low pressure lies in Davis strait, to the south-west or west of
Iceland, and over the Norwegian Sea. In winter it greatly extends its
limits farther east into the inner Arctic Ocean, to the north of Russia
and Siberia. The Pacific minimum of pressure is found south of Bering
Strait and in Alaska. Between these two regions of lower pressure the
divide extends from North America to eastern Siberia. This divide has
been called by Supan the "Arktische Wind-scheide." The pressure
gradients are steepest in winter. At the pole itself pressure seems to
be highest in April and lowest from June to September. The annual range
is only about 0.20 in.
The prevailing westerlies, which in the high southern latitudes are so
symmetrically developed, are interfered with to such an extent by the
varying pressure controls over the northern continents and oceans in
summer and winter that they are often hardly recognizable on the wind
maps. The isobaric and wind charts show that on the whole the winds blow
out from the inner polar basin, especially in winter and spring.
_Rain and Snow._--Rainfall on the whole decreases steadily from equator
to poles. The amount of precipitation must of necessity be comparatively
slight in the polar zones, chiefly because of the small capacity of the
air for water vapour at the low temperatures there prevailing; partly
also because of the decrease, or absence, of local convectional storms
and thunder-showers. Locally, under exceptional conditions, as in the
case of the western coast of Norway, the rainfall is a good deal
heavier. Even cyclonic storms cannot yield much precipitation. The
extended snow and ice fields tend to give an exaggerated idea of the
actual amount of precipitation. It must be remembered, however, that
evaporation is slow at low temperatures, and melting is not excessive.
Hence the polar store of fallen snow is well preserved: interior
snowfields, ice sheets and glaciers are produced.
The commonest form of precipitation is naturally snow, the summer limit
of which, in the northern hemisphere, is near the Arctic circle, with
the exception of Norway. So far as exploration has yet gone into the
highest latitudes, rain falls in summer, and it is doubtful whether
there are places where _all_ the precipitation falls as snow. The snow
of the polar regions is characteristically fine and dry. At low polar
temperatures flakes of snow are not found, but precipitation is in the
form of ice spicules. The finest glittering ice needles often fill the
air, even on clear days, and in calm weather, and gradually descending
to the surface, slowly add to the depth of snow on the ground. Dry snow
is also blown from the snowfields on windy days, interfering with the
transparency of the air.
_Humidity, Cloudiness and Fog._--The absolute humidity must be low in
polar latitudes, especially in winter, on account of the low
temperatures. Relative humidity varies greatly, and very low readings
have often been recorded. Cloudiness seems to decrease somewhat towards
the inner polar areas, after passing the belt of high cloudiness in the
higher latitudes of the temperate zones. In the marine climates of high
latitudes the summer, which is the calmest season, has the maximum
cloudiness; the winter, with more active wind movement, is clearer. The
curve here given illustrates these conditions (fig. 14). The summer
maximum is largely due to fogs, which are produced where warm, damp air
is chilled by coming in contact with ice. They are also formed over open
waters, as among the Faeroe Islands, for example, and open water spaces,
in the midst of an ice-covered sea, are commonly detected at a distance
by means of the "steam fog" which rises from them. Fogs are less common
in winter, when they occur as radiation fogs, of no great thickness. The
small winter cloudiness, which is reported also from the antarctic zone,
corresponds with the low absolute humidity and small precipitation. The
coasts and islands bathed by the warm waters of the Gulf Stream drift
usually have a higher cloudiness in winter than in summer. The place of
fog is in winter taken by the fine snow crystals, which often darken the
air like fog when strong winds raise the dry snow from the surfaces on
which it is lying. Cumulus cloud forms are rare, even in summer, and it
is doubtful whether the cloud occurs at all in its typical development.
Stratus is probably the commonest cloud of high latitudes, often
covering the sky for days without a break. Cirrus cloud forms probably
decrease polewards.
[Illustration: FIG. 14.--Annual March of Cloudiness in Polar Latitudes
(marine type).]
_Cyclones and Weather._--The prevailing westerlies continue up into the
margins of the polar zones. Many of their cyclonic storms also continue
on to the polar zones, giving sudden and irregular pressure and weather
changes. The inner polar areas seem to be beyond the reach of frequent
and violent cyclonic disturbance. Calms are more common; the weather is
quieter and fairer; precipitation is less. Most of the observations thus
far obtained from the Antarctic come from this marginal zone of great
cyclonic activity, violent winds, and wet, disagreeable, inhospitable
weather, and therefore do not show the features of the actual south
polar climate.
During the three years of the "Fram's" drift depressions passed on all
sides of her, with a preponderance on the west. The direction of
progression averaged nearly due east, and the hourly velocity 27 to 34
m., which is about that in the United States. For the higher latitudes,
most of the cyclones must pass by on the equatorial side of the
observer, giving "backing" winds in the northern hemisphere. The main
cyclonic tracks are such that the wind characteristically backs in
Iceland, and still more so in Jan Mayen and on the eastern coast of
Greenland, these districts lying on the north and west of the path of
progression. Frightful winter storms occasionally occur along the east
coast of Greenland and off Spitzbergen.
For much of the year in the polar zones the diurnal control is weak or
absent. The successive spells of stormy or of fine weather are wholly
cyclonically controlled. Extraordinary records of storm and gale have
been brought back from the far south and the far north. Wind direction
and temperature vary in relation to the position of the cyclone. During
the long dreary winter night the temperature falls to very low readings.
Snowstorms and gales alternate at irregular short intervals with calmer
spells of more extreme cold and clearer skies. The periods of greatest
cold in winter are calm. A wind from any direction will bring a rise in
temperature. This probably results from the fact that the cold is the
result of local radiation, and a wind interferes with these conditions
by importing higher temperatures, or by mixing upper and lower strata.
During the long summer days the temperature rises well above the winter
mean, and under favourable conditions certain phenomena, such as the
diurnal variation in wind velocity, for example, give evidence of the
diurnal control. But the irregular cyclonic weather changes continue, in
a modified form. There is no really warm season. Snow still falls
frequently. The summer is essentially only a modified winter, especially
in the Antarctic. In summer clear spells are relatively warm, and winds
bring lower temperatures. In spite of its lack of high temperatures,
the northern polar summer, near the margins of the zone, has many
attractive qualities in its clean, pure, crisp, dry air, free from dust
and impurities; its strong insolation; its slight precipitation.
_Twilight and Optical Phenomena._--The monotony and darkness of the
polar night are decreased a good deal by the long twilight. Light from
moon and stars, and from the aurora, also relieves the darkness. Optical
phenomena of great variety, beauty and complexity are common. Solar and
lunar haloes, and coronae, and mock suns and moons are often seen.
Auroras seem to be less common and less brilliant in the Antarctic than
in the Arctic. Sunset and sunrise colours within the polar zones are
described as being extraordinarily brilliant and impressive.
_Physiological Effects._--The north polar summer, as has been pointed
out, in spite of its drawbacks, is in some respects a pleasant and
healthful season. But the polar night is monotonous, depressing,
repelling. Sir W. E. Parry said that it would be difficult to conceive
of two things which are more alike than two polar winters. An
everlasting uniform snow covering; rigidity; lifelessness;
silence--except for the howl of the gale or the cracking of the ice.
Small wonder that the polar night has sometimes unbalanced men's minds.
The first effects are often a strong desire for sleep, and indifference.
Later effects have been sleeplessness and nervousness, tending in
extreme cases to insanity; anaemia, digestive troubles. Extraordinarily
low winter temperatures are easily borne if the air be dry and still.
Zero weather seems pleasantly refreshing if clear and calm. But high
relative humidity and wind--even a light breeze--give the same degree of
cold a penetrating feeling of chill which may be unbearable. Large
temperature ranges are endured without danger in the polar winter when
the air is dry. When exposed to direct insolation the skin burns and
blisters; the lips swell and crack. Thirst has been much complained of
by polar explorers, and is due to the active evaporation from the warm
body into the dry, relatively cold air. There is no doubt that polar air
is singularly free from micro-organisms--a fact which is due chiefly to
lack of communication with other parts of the world. Hence many diseases
which are common in temperate zones, "colds" among them, are rare.
_Changes of Climate._
_Popular Belief in Climatic Change._--Belief in a change in the climate
of one's place of residence, within a few generations, and even within
the memory of living men, is widespread. Evidence is constantly being
brought forward of apparent climatic variations of greater or less
amount which are now taking place. Thus we have many accounts of a
gradual desiccation which seems to have been going on over a large
region in Central Asia during historical times. In northern Africa
certain ancient historical records have been taken by different writers
to indicate a general decrease of rainfall during the last 3000 or more
years. In his crossing of the Sahara between Algeria and the Niger, E.
F. Gautier found evidence of a former large population. A gradual
desiccation of the region is therefore believed to have taken place, but
to-day the equatorial rain belt seems to be again advancing farther
north, giving an increased rainfall. Farther south, several lakes have
been reported as decreasing in size, e.g. Chad and Victoria; and wells
and springs as running dry. In the Lake Chad district A. J. B. Chevalier
reports the discovery of vegetable and animal remains which indicate an
invasion of the Sudan by a Saharan climate. It is often held that a
steady decrease in rainfall has taken place over Greece, Syria and other
eastern Mediterranean lands, resulting in a gradual and inevitable
deterioration and decay of their people.
_What Meteorological Records show._--As concerns the popular impression
regarding change of climate, it is clear at the start that no definite
answer can be given on the basis of tradition or of general impression.
The only answer of real value must be based on the records of accurate
instruments, properly exposed and carefully read. When such instrumental
records are carefully examined, from the time when they were first
kept, which in a few cases goes back about 150 years, there is found no
good evidence of any progressive change in temperature, or in the amount
of rain and snow. Even when the most accurate instrumental records are
available, care must be taken to interpret them correctly. Thus, if a
rainfall or snowfall record of several years at some station indicates
an apparent increase or decrease in the amount of precipitation, it does
not necessarily follow that this means a permanent, progressive change
in climate, which is to continue indefinitely. It may simply mean that
there have been a few years of somewhat more precipitation, and that a
period of somewhat less precipitation is to follow.
_Value of Evidence concerning Changes of Climate._--The body of facts
which has been adduced as evidence of progressive changes of climate
within historical times is not yet sufficiently large and complete to
warrant any general correlation and study of these facts as a whole. But
there are certain considerations which should be borne in mind in
dealing with this evidence before any conclusions are reached. In the
first place, changes in the distribution of certain fruits and cereals,
and in the dates of the harvest, have often been accepted as undoubted
evidence of changes in climate. Such a conclusion is by no means
inevitable, for many changes in the districts of cultivation of various
crops have naturally resulted from the fact that these same crops are in
time found to be more profitably grown, or more easily prepared for
market, in another locality. In France, C. A. Angot has made a careful
compilation of the dates of the vintage from the 14th century down to
the present time, and finds no support for the view so commonly held
there that the climate has changed for the worse. At the present time,
the average date of the grape harvest in Aubonne is exactly the same as
at the close of the 16th century. After a careful study of the
conditions of the date tree, from the 4th century, B.C., D. Eginitis
concludes that the climate of the eastern portion of the Mediterranean
basin has not changed appreciably during twenty-three centuries.
Secondly, a good many of the reports by explorers from little-known
regions are contradictory. This shows the need of caution in jumping at
conclusions of climatic change. An increased use of water for irrigation
may cause the level of water in a lake to fall. Periodic oscillations,
giving higher and then lower water, do not indicate progressive change
in one direction. Many writers have seen a law in what was really a
chance coincidence.
Thirdly, where a progressive desiccation seems to have taken place, it
is often a question whether less rain is actually falling, or whether
the inhabitants have less capacity and less energy than formerly. Is the
change from a once cultivated area to a barren expanse the result of
decreasing rainfall, or of the emigration of the former inhabitants to
other lands? The difference between a country formerly well irrigated
and fertile, and a present-day sandy, inhospitable waste may be the
result of a former compulsion of the people, by a strong governing
power, to till the soil and to irrigate, while now, without that
compulsion, no attempt is made to keep up the work. A region of
deficient rainfall, once thickly settled and prosperous, may readily
become an apparently hopeless desert, even without the intervention of
war and pestilence, if man allows the climate to master him. In many
cases the reports of increasing dryness really concern only the decrease
in the water supply from rivers and springs, and it is well known that a
change in the cultivation of the soil, or in the extent of the forests,
may bring about marked changes in the flow of springs and rivers without
any essential change in the actual amount of rainfall.
Lastly, a region whose normal rainfall is at best barely sufficient for
man's needs may be abandoned by its inhabitants during a few years of
deficient precipitation, and not again occupied even when, a few years
later, normal or excessive rainfall occurs.
_Periodic Oscillations of Climate: Sun-spot Period._--The discovery of a
distinct eleven-year periodicity in the magnetic phenomena of the earth
naturally led to investigations of similar periods in meteorology. The
literature on this subject has assumed large proportions. The results,
however, have not been satisfactory. The problem is difficult and
obscure. Fluctuations in temperature and rainfall, occurring in an
eleven-year period, have been made out for certain stations but the
variations are slight, and it is not yet clear that they are
sufficiently marked, uniform and persistent over large areas to make
practical application of the periodicity in forecasting possible. In
some cases the relation to sun-spot periodicity is open to debate; in
others, the results are contradictory.
W. P. Koppen has brought forward evidence of a sun-spot period in the
mean annual temperature, especially in the tropics, the maximum
temperatures coming in the years of sun-spot minima. The whole amplitude
of the variation in the mean annual temperatures, from sun-spot minimum
to sun-spot maximum, is, however, only 1.3 deg. in the tropics and a
little less than 1 deg. in the extra-tropics. More recently Nordmann
(for the years 1870-1900) has continued Koppen's investigation.
In 1872 C. Meldrum, then Director of the Meteorological Observatory at
Mauritius, first called attention to a sun-spot periodicity in rainfall
and in the frequency of tropical cyclones in the South Indian Ocean. The
latter are most numerous in years of sun-spot maxima, and decrease in
frequency with the approach of sun-spot minima. Poey found later a
similar relation in the case of the West Indian hurricanes. Meldrum's
conclusions regarding rainfall were that, with few exceptions, there is
more rain in years of sun-spot maxima. S. A. Hill found it to be true of
the Indian summer monsoon rains that there seems to be an excess in the
first half of the cycle, after the sun-spot maximum. The winter rains of
northern India, however, show the opposite relation; the minimum
following, or coinciding with, the sun-spot maximum. Particular
attention has been paid to the sun-spot cycle of rainfall in India,
because of the close relation between famines and the summer monsoon
rainfall in that country. Sir Norman Lockyer and Dr W. J. S. Lockyer
have recently studied the variations of rainfall in the region
surrounding the Indian Ocean in the light of solar changes in
temperature. They find that India has two pulses of rainfall, one near
the maximum and the other near the minimum of the sun-spot period. The
famines of the last fifty years have occurred in the intervals between
these two pulses, and these writers believe that if as much had been
known in 1836 as is now known, the probability of famines at all the
subsequent dates might have been foreseen.
Relations between the sun-spot period and various other meteorological
phenomena than temperature, rainfall and tropical cyclones have been
made the subject of numerous investigations, but on the whole the
results are still too uncertain to be of any but a theoretical value.
Some promising conclusions seem, however, to have been reached in regard
to pressure variations, and their control over other climatic elements.
_Bruckner's 35-Year Cycle._--Of more importance than the results thus
far reached for the sun-spot period are those which clearly establish a
somewhat longer period of slight fluctuations or oscillations of
climate, known as the Bruckner cycle, after Professor Bruckner of Bern,
who has made a careful investigation of the whole subject of climatic
changes and finds evidence of a 35-year periodicity in temperature and
rainfall. In a cycle whose average length is 35 years, there comes a
series of years which are somewhat cooler and also more rainy, and then
a series of years which are somewhat warmer and drier. The interval in
some cases is twenty years; in others it is fifty. The _average_
interval between two cool and moist, or warm and dry, periods is about
35 years. The mean amplitude of the temperature fluctuation, based on
large numbers of data, is a little less than 2 deg. The fluctuations in
rainfall are more marked in interiors than on coasts. The general mean
amplitude is 12%, or, excluding exceptional districts, 24%. Regions
whose normal rainfall is small are most affected.
The following table shows the dates and characters of Bruckner's
periods:--
Warm 1746-1755 1791-1805 1821-1835 1851-1870 ..
Dry 1756-1770 1781-1805 1826-1840 1856-1870 ..
Cold 1731-1745 1756-1790 1806-1820 1836-1850 1871-1885
Wet 1736-1755 1771-1780 1806-1825 1841-1855 1871-1885
Interesting confirmation of Bruckner's 35-year period has been found by
E. Richter in the variations of the Swiss glaciers, but as these
glaciers differ in length, they do not all advance and retreat at the
same time. The advance is seen during the cold and damp periods.
Bruckner has found certain districts in which the phases and epochs of
the climatic cycle are exactly reversed. These exceptional districts are
almost altogether limited to marine climates. There is thus a sort of
compensation between oceans and continents. The rainier periods on the
continents are accompanied by relatively low pressures, while the
pressures are high and the period dry over the oceans and vice versa.
The cold and rainy periods are also marked by a decrease in all pressure
differences. It is obvious that changes in the general distribution of
atmospheric pressures, over extended areas, are closely associated with
fluctuations in temperature and rainfall. These changes in pressure
distribution must in some way be associated with changes in the general
circulation of the atmosphere, and these again must depend upon some
external controlling cause or causes. W. J. S. Lockyer has called
attention to the fact that there seems to be a periodicity of about 35
years in solar activity, and that this corresponds with the Bruckner
period.
It is clear that the existence of a 35-year period will account for many
of the views that have been advanced in favour of a _progressive_ change
of climate. A succession of a few years wetter or drier than the normal
is likely to lead to the conclusion that the change is permanent.
Accurate observations extending over as many years as possible, and
discussed without prejudice, are necessary before any conclusions are
drawn. Observations for one station during the wetter part of a cycle
should not be compared with observations for another station during the
drier part of the same, or of another cycle.
There are evidences of longer climatic cycles than eleven or 35 years.
Bruckner calls attention to the fact that sometimes two of his periods
seem to merge into one. E. Richter shows much the same thing for the
Alpine glaciers. Evidence of considerable climatic changes since the
last glacial period is not lacking. But as yet nothing sufficiently
definite to warrant general conclusions has been brought forward.
_Geological Changes in Climate._--Changes of climate in the geological
past are known with absolute certainty to have taken place: periods of
glacial invasion, as well as periods of more genial conditions. The
evidence, and the causes of these changes have been discussed and
re-discussed, by writers almost without number, and from all points of
view. Changes in the intensity of insolation; in the sun itself; in the
conditions of the earth's atmosphere; in the astronomical relations of
earth and sun; in the distribution of land and water; in the position of
the earth's axis; in the altitude of the land; in the presence of
volcanic dust;--now cosmic, now terrestrial conditions--have been
suggested, combated, put forward again. None of these hypotheses has
prevailed in preference to others. No actual proof of the correctness of
this or that theory has been brought forward. No general agreement has
been reached.
_Conclusion._--Without denying the possibility, or even the probability,
of the establishment of the fact of secular changes, there is as yet no
sufficient warrant for believing in considerable _permanent changes over
large areas_. Dufour, after a thorough study of all available evidence,
has concluded that a change of climate has not been proved. There are
periodic oscillations of slight amount. A 35-year period is fairly well
established, but is nevertheless of considerable irregularity, and
cannot as yet be practically applied in forecasting. Longer periods are
suggested, but not made out. As to causes, variations in solar activity
are naturally receiving attention, and the results thus far are
promising. But climate is a great complex, and complete and satisfactory
explanations of all the facts will be difficult, perhaps impossible, to
reach. At present, indeed, the facts which call for explanation are
still in most cases but poorly determined, and the processes at work are
insufficiently understood. Climate is not absolutely a constant. The
pendulum swings to the right and to the left. And its swing is as far to
the right as to the left. Each generation lives through a part of one,
or two, or even three oscillations. A snapshot view of these
oscillations makes them seem permanent. As Supan has well said, it was
formerly believed that climate changes locally, but progressively and
permanently. It is now believed that oscillations of climate are limited
in time, but occur over wide areas.
LITERATURE.--Scientific climatology is based upon numerical results,
obtained by systematic, long continued, accurate meteorological
observations. The essential part of its literature is therefore found
in the collections of data published by the various meteorological
services. The only comprehensive text-book of climatology is the
_Handbuch der Klimatologie_ of Professor Julius Hann, of the
university of Vienna (Stuttgart, 1897). This is the standard book on
the subject, and upon it is based much of the present article, and of
other recent discussions of climate. The first volume deals with
general climatology, and has been translated into English (London and
New York, 1903). Reference should be made to this book for further
details than are here given. The second and third volumes are devoted
to the climates of the different countries of the world. Woeikof's
_Die Klimate der Erde_ (Jena, 1887) is also a valuable reference book.
The standard meteorological journal of the world, the _Meteorologische
Zeitschrift_ (Braunschweig, monthly), is indispensable to any one who
wishes to keep in touch with the latest publications. The _Quarterly
Journal of the Royal Meteorological Society_ (London), _Symons's
Monthly Meteorological Magazine_ (London), and the _Monthly Weather
Review_ (Washington, D.C.) are also valuable. The newest and most
complete collection of charts is that in the _Atlas of Meteorology_
(London, 1899), in which also there is an excellent working
bibliography. For the titles of more recent publications reference may
be made to the _International Catalogue of Scientific Literature
(Meteorology)_. (R. DE C. W.)
CLIMATE IN THE TREATMENT OF DISEASE.--The most important qualities of
the atmosphere in relation to health are (i.) the chemical composition,
(ii.) the solids floating in it, (iii.) the mean and extreme
temperatures, (iv.) the degree of humidity, (v.) the diathermancy, (vi.)
the intensity of light, (vii.) the electrical conditions, (viii.) the
density and pressure, and (ix.) the prevailing winds. Generally
speaking, the relative purity of the air--i.e. absence of septic solid
particles--is an important consideration; while cold acts as a stimulant
and tonic, increasing the amount of carbon dioxide exhaled in the
twenty-four hours. Different individuals, however, react both to heat
and cold very differently. At health resorts, where the temperature may
vary between 55 deg. and 70 deg. F., strong individuals gradually lose
strength and begin to suffer from various degrees of lassitude; whereas
a delicate person under the same conditions gains vigour both of mind
and body, puts on weight, and is less liable to disease. And a
corresponding intensity of cold acts in the reverse manner in each case.
Thus a health resort with a moderate degree of heat is very valuable for
delicate or elderly people, and those who are temporarily weakened by
illness. Cold, however, when combined with wind and damp must be
specially avoided by the aged, the delicate, and those prone to gouty
and rheumatic affections. The moisture of the atmosphere controls the
distribution of warmth on the earth, and is closely bound up with the
prevailing winds, temperature, light and pressure. In dry air the
evaporation from both skin and lungs is increased, especially if the
sunshine be plentiful and the altitude high. In warm moist air strength
is lost and there is a distinct tendency to intestinal troubles. In
moist cold air perspiration is checked, and rheumatic and joint
affections are very common. The main differences between mountain air
and that of the plains depend on the former being more rarefied, colder,
of a lower absolute humidity, and offering less resistance to the sun's
rays. As the altitude is raised, circulation and respiration are
quickened, probably as an effort on the part of the organism to
compensate for the diminished supply of oxygen, and somewhat more
gradually the number of red blood corpuscles increases, this increase
persisting for a considerable time after a return to lower ground. In
addition to these changes there is a distinct tendency to diminished
proteid metabolism, resulting in an increase of weight owing to the
storage of proteid in the tissues. Thus children and young people whose
development is not yet complete are especially likely to benefit by the
impetus given to growth and the blood-forming organs, and the
therapeutic value in their case rarely fails. For older people, however,
the benefit depends on whether their organs of circulation and
respiration are sufficiently vigorous to respond to the increased
demands on them. For anaemia, pulmonary tuberculosis, pleural
thickening, deficient expansion of the lungs, neurasthenia, and the
debility following fevers and malaria, mountain air is invaluable. But
where there is valvular disease of the heart, or rapidly advancing
disease of the lungs, it is to be avoided. Light, especially direct
sunlight, is of primary importance, the lack of it tending to depression
and dyspeptic troubles. Probably its germicidal power accounts for the
aseptic character of the air of the Alps, the desert and other places.
Sir Hermann Weber has defined a "good" climate as that in which all the
organs and tissues of the body are kept evenly at work in alternation
with rest. Thus a climate with constant moderate variations in its
principal factors is the best for the maintenance of health. But the
best climate for an invalid depends on the particular weakness from
which he may suffer. Pulmonary tuberculosis stands first in the
importance of the effects of climate. The continuous supply of pure
fresh air is the main desideratum, a cool climate being greatly superior
to a tropical one. Exposure to strong winds is harmful, since it
increases the tendency to cough and thus leads to loss of body
temperature, which is in its turn made up at the expense of increased
metabolism. A high altitude, from the purity and stimulating properties
of the air, is of value to many mild or very early cases, but where the
disease is extensive, where the heart is irritable, or where there is
any tendency to insomnia, high altitudes are contra-indicated, and no
such patient should be sent higher than some 1500 ft. Where the disease
is of long standing, with much expectoration, or accompanied by
albuminuria, the patient appears to do best in a humid atmosphere but
little above the sea-level. The climate of Egypt is especially suitable
for cases complicated with bronchitis or bronchiectasis, but is
contra-indicated where there is attendant diarrhoea. Madeira and the
Canaries are useful when emphysema is present or where there is much
irritability of constitution. Bronchitis in young people is best treated
by high altitudes, but in older patients by a moist mild climate, except
where much expectoration is present.
The influence of atmospheric conditions on the functions of the nose is
very marked. Within the ordinary ranges of humidity and temperature the
nasal mucous membrane completely saturates the air with aqueous vapour
before it reaches the pharynx. In cold and dry mountain climates there
is a very free nasal secretion, far beyond what is needed for the
saturation of the air; and at low levels the reverse action takes place,
the nose becoming "stuffy." The mechanism on which this depends is found
in the erectile tissue, and anything favouring the engorgement of the
veins, such as weak heart action, chronic bronchitis or kidney troubles,
&c, leads to a corresponding turgidity of the nose and sinuses. In
addition to barometric and other influences, it has been found that
light produces collapse of this tissue, smoke having a similar effect.
On this latter effect probably depends the fact that many asthmatics are
better in a city like London than elsewhere, the smoke relieving the
turgescence of the inferior turbinals of the nose. In the treatment of
pathological nasal conditions, all cases of obstruction from whatsoever
cause are best in a dry atmosphere, and where there is atrophy and a
deficient flow of mucus in a moist atmosphere. If the mucous membrane is
irritable a dry sheltered spot on a sandy soil and in the neighbourhood
of pine trees is by far the best.
Scrofulous children, namely, those in whom the resistance to
micro-organisms and their products is low, pre-eminently require sea
air, and had better be educated at some seaside place. Where the child
is very delicate, with small power of reaction, the winter should be
passed on some mild coast resort. Gouty and rheumatic affections require
a dry soil and warm dry climate, cold and moist winds being especially
injurious.
For heart affections high altitudes are to be avoided, though some
physicians make an exception of mitral cases where the compensation is
good. Moderate elevations of 500 to 1500 ft. are preferable to the
sea-level.
In diseases of the kidneys, a warm dry climate, by stimulating the
action of the skin, lessens the work to be done by these organs, and
thus is the most beneficial. Extremes of heat and cold and elevated
regions are all to be avoided.