THE CLIMATIC IMPORTANCE OF WATER VAPOUR
When Aristotle, for two thousand years our leading savant in
cosmography, about twenty-three centuries ago stated the foundation of
his natural science, he laid down as the important principles: moisture
and heat and their opposites; because the four elements out of which
everything was made, were: earth, characterized by dryness and cold;
water, which was moist and cold; air, which combined moisture with
heat; and lastly fire, which stood for dryness and heat. Undoubtedly
he was considering the requisites of life, which may be designated as
humidity and heat. We have seemingly agreed that all life originates
in the sea, so that moisture can be considered the first requirement
for its appearance on the earth. As to heat, life is destroyed by
frost and favoured by increased warmth, at least to a certain point,
about 35° to 40° C. (95° to 104° Fahrenheit), which temperature is
most propitious to the development of life, while a further increase
is detrimental, so that already below the boiling point of water life
suffers more harm than at temperatures below freezing. In fact, the
geologists have found that the different epochs in earth’s evolution
are best characterized by their humidity or dryness. To arrive at a
clear conception in these matters we shall briefly survey our present
knowledge of the importance for the evolution of life on earth that we
should attach to the humid and to the dry periods or localities.
We are all familiar with the heavy, moisture-laden warmth which meets
us when we enter a hothouse. It is particularly favourable to the
growth of plants and to the prosperity of the lower animals. To the
higher animals and to man, the humid heat is not so beneficial. In the
open such hothouse air exists only in the tropics. Particularly the
Congo region and the parts of Brazil adjoining the Amazon River are
remarkable for their humid heat and for their fabulously luxuriant
vegetation. From our greatest living climatologist, Julius Hann, I have
borrowed the following description of such a clime:
“The changes of temperature between the coldest and the hottest month
are very small in the Congo, from .5° to 5° C. (.9° to 9° Fahrenheit),
with an average of about 3.5° C. (6.3° F.). The difference between day
and night reaches nearly thrice this value, or 9.5° C. (17.1° F.).
The dry season becomes shorter the more we approach the equator
and in Equatorville and Bangala it shrinks to nothing. During the
rainless months, a dense humid fog settles morning and evening over
the savannahs. Low hanging clouds of uniform thickness frequently hide
the Sun for weeks at a time. It is during the rainy season alone that
we see a clear sky between the showers. This season opens and closes
with magnificent thunderstorms coming from the east. In Luluaburg,
lightning occurs during not less than 106 days in the year. In the
dry season the wind carries with it clouds of dust which falls to the
ground. The cloudiness is enormous in the Congo basin, so that there
are veritably no months with a clear sky in this part of the world. In
Vivi, the number of overcast days averages 74 per cent., fluctuating
between 63 per cent. in August and 83 per cent. in November. The
humidity is very high, varying in Vivi from 70 to 79 per cent., with a
mean value of 75 per cent., and in Bolebo the mean itself reaches 79
per cent. During the rainy season, the heat is sometimes unbearably
oppressive; suffocating fumes rise from vegetable matter which rapidly
decays in the excessive humidity. The annual precipitation does not
reach very startling figures; it varies between 120 and 180 cm. (47 to
71 inches). In Gabun, close by, the sky is almost continuously covered
with clouds during the dry season.
“Corresponding regions in South America are in parts characterized by
an even higher humidity. In Iquitos by the Amazon River, it reaches not
less than 83 per cent. of saturation. The annual change of temperature
is only about 5° C. (9° F.); in Para (1.08° south latitude on the
coast) it shrinks to 1° or 1.5° C. (1.8° to 2.7° F.). In the course
of twenty-four hours the variation is considerably larger. The sky
is remarkably clear between showers during the rainy season. In the
interior of Guiana, the rains continue from the end of April well into
July or even into August. Abundant dew is common during the rainless
part of the year, thus maintaining the humidity. Sun and Moon are
rarely visible, and gigantic lightning storms announce the arrival of
the rainy period.”
Similar conditions apparently prevailed during the carboniferous
period, which was characterized by a luxuriant vegetation. The mighty
tree-trunks of that time fell into the water-covered marshes out of
which they had grown and their decay was thereby prevented. Instead
they turned into coal like the peat in the mosses of today. This
was for some time thought to indicate that the temperature was not
particularly high--Frech estimated about 12° C. (53.6° F.) (1910). But
since the discovery and subsequent description by Keilhack (1914) of
peat-beds on Ceylon, where the average yearly temperature is 26° C.
(78.8° F.), a return is to be expected to the older conception,
which held that the vegetation during the carboniferous period is
evidence of a very warm climate. Judging by the appearance of fossil
plants, the temperature should have been nearly the same all over
the globe. Carthaus remarks that the air was stirred by only feeble
winds because the trees of that age with their great dimensions but
frail root-systems could not have withstood a fresh breeze. The sky
was hidden behind a continuous thick cover of clouds which only let a
faint light sift through to the ground. The motionless air was almost
saturated with moisture. The luxuriance of the vegetation, transcending
anything existing today, indicates a favourable high percentage of
carbonic acid in the air. This combined with the humidity and the dense
clouds caused the heat radiation from the Sun to be almost entirely
absorbed by the upper strata of the atmosphere in which thereby a
strong circulation was maintained. As a result, the heat was nearly
equalized between the poles and the equator and under the cloud cover
an almost constant temperature reigned day and night, summer and
winter. The damp air stood wellnigh still and was filled with dense fog
at the smallest changes in temperature. Lack of light prevented the
development of flowers, and the thriving plants belonged mainly to the
ferns and to the horsetails. Pine and fir trees were yet comparatively
few. The conditions in the swampy regions where plant life flourished
were nearly identical with those in a hothouse if we were to draw a
dense veil in front of the windows in walls and ceiling so that a
continuous twilight would prevail.
In this uniform climate, plant life developed enormously faster than
animal life. The dense clouds could store considerable quantities of
heat in the equatorial belt through evaporation in their upper layers
and the violent wind storms above the clouds would carry the aqueous
vapours to colder regions where the heat would be liberated through
new cloud formations. Currents in the oceans now largely attend to
this heat transportation and give for instance to the coast of Norway,
and indeed to the whole of Western Europe, its remarkably mild, and to
life and civilization, propitious climate, but in the carboniferous age
humid air currents fulfilled the same task. They moved considerably
faster and more evenly than the ocean currents, were not checked
or deflected by coasts or islands, and could therefore produce the
extraordinarily uniform temperature and the marine climate all over
the globe. Such a heat distribution takes place also in our days at a
height of about 10,000 m. (6.2 miles) in the so-called “stratosphere,”
but the temperature here is very low, about -60° C. (-76° F.), so that
the vapour suspended is hardly worth mentioning, and cannot give rise
to cloud formations. The quantities of heat carried in these higher
strata of the atmosphere are too insignificant to influence the masses
of air below, whose temperature, therefore, is almost entirely governed
by that of the sun-heated surface of the earth, except where the
ocean currents equalize matters, as for instance in the almost wholly
water-covered latitudes south of the 30th parallel on the Southern
Hemisphere. Even during the carboniferous period at its height, there
existed, of course, a temperature difference between pole and equator,
but it was very small, some 10° C. (18° F.) perhaps. Undoubtedly, the
formation of coal beds was mainly confined to those regions where the
climate was most uniform all the year around.
The opposite extremity, the dry desert climate, is far more pronounced
in the present time. This condition is well known in all continents
except Europe, where we hardly can claim a desert but instead have
steppes, with a vegetation abundant after the spring rains but fast
disappearing with the arrival of the burning summer heat. A particular
type of plant life has adapted itself to this periodic change from rain
to drought, from bitter cold during the winter to parching sun during
the summer. Perennial plants, and particularly trees, can rarely endure
the rigors of such climatic upheavals. Animal life on the other hand
has proved fairly adaptable and displays considerable wealth.
This steppe climate is only an intermediate stage towards the desert
climate proper, which is hostile to all life. Its temperature is
subject to enormous changes in the course of the day and the year.
The annual variation is less pronounced near the equator and the
daily variation less on the approach to the poles, on account of the
small changes in the sun’s radiation during corresponding periods.
The difference between day and night in Sahara is frequently 30°
to 40° C. (50° to 70° F.). The lowest temperature observed by
Foureau-Lamy, 1898–1899, was -20° C. (-4° F.) or nearly the same as on
the Scandinavian coasts. The highest amounted to 48° C. (118.4° F.) or
a total variation of nearly 70° C. (126° F.). In Upper Egypt (21.9°
N. Lat.) the mean temperature changed from 16.3° C. (61.3° F.) in
January to 34.1° C. (93.2° F.) in July, and nearer the equator in
Central Africa (8.1° N. Lat., 23.6° E. Long.) the difference amounted
to only 6.9° C. (44.6° F.), 22.7° C. (72.5° F.) in December, 29.6° C.
(85.1° F.) in April, while in Kiachta (50.4° N. Lat., 106.5° E. Long.)
in Siberia, the yearly change reaches 45° C. (81° F.), -26.6° C.
(-15.7° F.) in January, 19.1° C. (66.2° F.) in July. The average daily
variation at continental stations is about 12° C. (21.6° F.). All this
refers to the temperature of the air, while the surface temperature in
the course of twenty-four hours may change 50° C. (90° F.) and in the
desert even more. Frost occurs in the Sahara as late as May when the
maximum temperature may reach 50° C. (122° F.). While in Scandinavia
the diurnal difference between highest and lowest temperature averages
only 6° to 7° C. (11° to 13° F.), a maximum in July of 10.4° C.
(18.7° F.) and a minimum in November of 4° C. (7.2° F.), Hedin on his
journey in Tibet, 1899–1902, observed a daily variation of 19° C.
(34.2° F.) and no appreciable difference with change in altitude.
The result of such a violent temperature change in the course of a day
is a breaking up of the rocks which subsequently and gradually are
ground to fine dust by unobstructed winds wherever vegetation does not
bind the soil. In this manner the sand deserts are formed. The arid
wastes of Asia have lately been vividly described by Sven Hedin. The
mountains eroded by the sandstorms resemble dilapidated ruins, standing
as monuments of an ancient highland. The sand in East Turkestan is
reduced to such a fine powder that it can float in the air for several
days after a storm, revealing itself in gorgeous sunsets. Winds sweep
the sand into long dunes, which shift in the direction of the blast.
It is ferruginous and therefore red or if powderized reddish-yellow.
When moistened it assumes a brown to black shade. After rain, the water
descends toward the valley, carrying with it the sand in the form of
silt. This, through evaporation, is transformed into a plastic black
dough, slides like a glacier slowly down the hillsides, and finally
comes to rest in some broad hollow which it gradually fills. Such a
silt aggregation is called in Persia a “Kevir.” Its surface dries,
but the interior remains moist. As evaporation continues it becomes
richer in salt so that white crusts of this substance are formed during
dry periods. In other districts, as in the basin of the Tarim River,
the water occasionally appears in the lowest parts, the so-called
“Bayirs” (see Fig. 9), formations similar to the Kevirs, or in salt
lakes between the sand dunes. Sand carried by the winds quickly fills
these lakes so that they too move in the direction of prevailing
winds. They lie with their longest dimensions parallel to each other
and at right angle to the course of the Tarim River. The sketch map
taken from Hedin’s work shows the Bayirs strung out in line with the
lakes somewhat like panels in a tapestry pattern. This reticulation
of the landscape is the result of the dune formations. The main dunes
with steep western slopes run in the direction N.N.E.-S.S.W. They
stand at right angle to the prevailing winds. Nearly perpendicular to
their crest lines dunes of smaller height are thrown up by winds in
another common direction but less frequent than those which raised the
fundamental dunes. This system brings to mind the cloud formations
called mackerel sky, clouds rippled in two directions frequently almost
at right angle to each other. They owe their peculiarity to two series
of wave motions propelled by winds from two different directions in
the upper strata of the atmosphere. The cloud patches correspond to
the wavecrests on a surging sea. The map of the Bayirs suggests a
chessboard with squares somewhat elongated and irregular.
[Illustration: Fig. 9. Tarim river with adjacent lakes and Bayirs,
drawn by Sven Hedin.
(solid arrow) Predominating direction of winds.
(dashed arrow) Direction in which the river is shifting.
(hollow box) Vegetation.
(dotted box) Sand.
(horizontal lines in box) Lakes.
(vertical lines in box) Bayirs.
]
We may now return to a closer study of the largest of these
formations, the great Kevir in Persia. This mud-lake, with a dry
surface, measures 500 km. (310 miles) in length and 200 km. (124
miles) in width over its largest dimensions. Hedin estimates its area
at 55,000 sq. km. (21,142 sq. miles) or the same as that of the great
Lake Michigan on the boundary between the United States and Canada.
Due to the continuous growth of the salt proportion through the inflow
of Kevirs and through superficial evaporation a salt crust of varying
thickness is formed near the surface of the lake. Hedin caused a hole
to be cut with an iron bar. He first encountered a 10 cm. (3.9 in.)
deep covering of clayey paste, and then the salt crust about 7 cm. (2.8
in.) thick resting on a semi-dry layer of clay with a depth of 15 cm.
(5.9 in.). Farther below, softer strata of clay followed, becoming more
watery the deeper he went. The iron bar carelessly wielded would have
disappeared in the mire. Another investigator, Buhse, examined a piece
of the crust, which when dry is fairly solid and of a yellowish-grey
colour. One half consisted of sand (probably quartz-sand), one sixth of
limestone, 6.1 per cent. oxide of iron (causing the yellow colour), 5.3
per cent. common salt, 2.5 per cent. sulphate of sodium, and 2.1 per
cent. clay. Rain converts this surface layer into a plastic mass, which
persistently sticks to the clothes of the traveller or to the bodies of
the camels if they should slip and fall into the mud. Not the slightest
trace of vegetation or of any life exists. On the shore of the mud-lake
small flat elevations and depressions may be found; otherwise, the
surface is as level as that of an ordinary lake.
The Kevir battles with the drifting sand as does the water in East
Turkestan. The sand appears to gain in the contest. After storms, vast
portions of the Kevir are covered with yellow desert sand. “If the
climatic change in Persia continues in the present direction,” says
Hedin, who, however, is dealing with large spans of time inasmuch
as in his opinion no appreciable alterations have taken place since
the invasions of Alexander the Great, “then it may be taken for
granted that the slough of the Kevir will lose in moisture and afflux
of water and in time will become more solid and that the drift sand
with greater ease will gain foothold and territory. The final outcome
of the physico-geographical transformation now in progress will no
doubt be to convert the entire Kevir into a sand desert of the kind
predominating in East Turkestan. And conversely we may infer that
East Turkestan, once a part of the central Asiatic Mediterranean sea,
in the course of time was filled with the finely divided products of
disintegration, such as we now find in the Kevir, and further that its
expanse of watery mud and clay finally dried and hardened to such an
extent that it could support the load of the encroaching sand. That the
extension of the sand formerly was smaller, is also borne out by the
archæological discoveries in East Turkestan which several travellers
besides myself have made. The hardpan laid bare in the ‘Bayirs’ of the
Cherchen desert strongly reminds of the Kevir-ground. In both cases
the same dark, fine powder forming a nearly plane surface. In both
cases this material when mixed with water is transformed into a slough
in which one hopelessly sinks, but in East Turkestan the water has
receded to a greater depth and as rains are extremely rare travel all
over the smooth ‘Bayir’-ground may be undertaken with impunity.”
These formations are of the greatest interest because they picture
the changes taking place on a slowly desiccating planet. In 1858 the
Geographical Society in Petrograd despatched an expedition under
the command of Khanikoff to visit these regions. From Hedin’s work,
_Overland to India_, from which the preceding quotation is taken, we
borrow the following vivid description by Khanikoff: “At last, in
the morning of April fourth, and during the most oppressive heat, we
reached Bala-haus. At this place remnants of a ruined reservoir, long
since bereft of its water, could be seen. The desert had here assumed
the perfect character of the ‘the accursed land,’ which name it bears
among the natives. Not the smallest tuft of grass, not a sign of animal
life gladdened the eye, not a sound interrupted the deathlike, awful
silence but that of the marching caravan.
“Because of the slow procession of the camels and the delay suffered
when we lost our way, we only covered 25 km. (15.5 miles) in the night
stage. After four hours’ rest we resumed our march and directed our
steps toward the hills, called Kellehper and situated 20 km. (12.5
miles) from Bala-haus; they were plainly visible but positively seemed
to take flight on our approach. I was somewhat ahead of the caravan and
sat down at the foot of this sand-elevation; and never can I describe
the feeling of weariness and of depression that I was unable to ward
off as I looked into the ghastly solitude that engulfed me. Scattered
clouds intercepted the rays of the sun, but the air was hot and heavy;
the diffused light lent a monotonous and disconsolating hue to the
greyish, burning surface of the desert; hardly a single variation
of colour gave relief to the immense expanses that the vision
embraced. The absolute immobility of every point in this mournful
landscape, combined with the complete absence of any sound, produced
an overwhelming depression of the spirit; one felt as set upon a place
that had been struck inanimate for ever, a place to which organic life
would never return safe through some terrible catastrophe of nature.
One witnessed the beginning, so to speak, of the death-agony of our
planet.”
If a drying out has taken place in these regions,--which seems probable
from Hedin’s observation that the water in a Tibet lake, Lakker-tso,
formerly reached a 133 m. (435 ft.) higher level than at present,--such
a process is nevertheless not so obvious here as in the salt inland
lakes, for instance the Great Salt Lake in Utah, the Dead Sea, and
the Caspian Sea, where the saltness has greatly increased due to
evaporation. Concerning the Great Salt Lake we know that even at a
comparatively late time it had a much wider extension than now. Its
water contains 22 per cent. common salt besides other compounds. The
Dead Sea holds 25 per cent. salt. A very variable percentage is to be
found in the Caspian Sea. Near the mouth of the Volga it is of course
low, only 0.15, and increases southward to 1.32 at the peninsula
Apsheron and to 5.63 in the bay of Kaidak. In the gulf of Karaboghaz on
the Asiatic side it reaches 28.5 per cent. It has been computed that
this gulf receives annually from the inrushing waters of the Caspian
Sea 350,000 tons of salt which is partly deposited on its shores and
bottom.
This desiccation, however, is a mere trifle when contrasted with the
process by which the mighty salt deposits in Germany were formed. It
took place, we believe, in a shallow bay extending southward from the
Arctic Sea. As the salts, first gypsum, then common salt, and lastly
more soluble potassium and magnesium compounds, crystallized by degrees
new water masses entered from the sea. At the same time, the bottom of
the gulf slowly receded, giving room for fresh evaporating floods. The
salt layers deposited in this manner sometimes reach a depth of more
than 1000 m. (about one half a mile). We can thus gain a conception
of the immense quantities of water evaporated and the enormous time
required therefor. The deposits would long ago have been carried away
from their original place were it not for the fact that they finally
were covered with a layer of slime nearly impervious to water. The most
soluble salts, such as chloride of magnesium, have nevertheless been
extracted to a great extent.
The extremes of aridity or humidity have of course not occurred
during the brief time known to history. Of a special interest is the
question in what direction the climate at present is tending. In this
connection, Huntington has aroused great attention by propounding the
theory that the earth is now in a period of rapid desiccation.
Judging by the testimony of geology, it seems beyond doubt that an age
of humidity prevailed simultaneously with the ice-period in northern
Europe, over several parts of the globe, in fact everywhere as far as
we know except in Australia. This is clearly borne out by the higher
levels of the lakes and their consequent greater extension in earlier
days. So far as Tibet and Central Asia are concerned we have already
mentioned this fact. But in America and Africa the humid period was
even more patent. The Great Salt Lake has covered an area many times
greater than at present as testify the picturesque terraces in its
surroundings (compare Fig. 10). According to the researches by Passage,
this period was also strongly pronounced in Africa. A large fresh-water
body occupied the Congo basin, the Tsad lake had a far greater
expansion than now, and mighty rivers traversed Sahara.
[Illustration: Fig. 10. Extension of the great Lake Bonneville in Utah,
of which the Great Salt Lake is a remnant.]
It is often assumed that the climate of Africa has been more humid
even in historic times. The geographer Leo Berg in Petrograd, however,
is emphatically opposed to this theory. He points out that the
writers of antiquity, Diodoros, Polybios, and Pausanias, have given
descriptions of the rivers on the North African coast which nearly
agree with conditions today. The location of two ancient cities on
the shores of Lake Chott-el-Djerid in Tunis (Lacus Tritonis of old),
which lake, it is claimed, reached a level very much higher than now
500 years B.C., plainly demonstrates that the shore-line then ran very
close to its present position. Students of ancient Egypt are unable to
find evidence of any distinct difference in the climate of that country
from the earliest times to our days. It is true that marshes in the
Nile delta have changed into splendid meadows--but this is the work of
man. The humid period must have ended long before history commenced.
A few of the old writers, such as Herodotos, Aristophanes, and Philo,
assert that rain never falls in Egypt, but this must be classed as an
exaggeration when contrasted with references to rain, snow, and hail
in this section made by other ancient authors, for instance Plutarch,
Pliny, and Ælianus. At any rate it seems as if precipitation was as
rare an occurrence in the land of the Pharaohs as in the land of the
Nile of today.
Against the statement by Huntington that the climate of Palestine
has become very much more torrid during historic times, stands the
assertion by Hilderscheid, who has made a thorough study of these
questions, that no reason whatever exists for such a conclusion.
Of greatest interest to us in this connection are perhaps Italy and
Greece. Huntington believes that the river Alpheios, which flooded
Olympia and covered it with a 4 m. to 5 m. (4 or 5 yards) thick
sediment, carried a far greater volume of water formerly than today.
This flood, however, was caused by an earthquake accompanied by a
fall of rocks whereby the river was dammed. There is no necessity for
assuming a greater abundance of the waterflow. According to Strabo,
the streams Kefissos and Ilissos, between which Athens is situated,
dried out during the summer then as they do now. If we are to believe
Pausanias, the brooks which traversed the Argive plain behaved
similarly, and so they behave today. From all we can judge, the climate
of Greece has not changed perceptibly since the days of Homer.
In regard to Sicily, it is stated that several of its rivers were
navigable in the Middle Age, while such is not the case now. But this
is explained by the devastation of the forests which formerly equalized
the flow of these rivers, and perhaps also by the size of the vessels
of that time. Cultivation in these regions has sharply declined since
antiquity. As a consequence, the loose soil, which formerly was
planted, has been washed away and the dams and retaining walls, built
to prevent a too rapid drain of the water, have disappeared. Thus
the country has grown increasingly arid. Large cities, as Palmyra,
existed in desert regions where lack of water now prevents habitation.
But water was brought to the metropolitan cities of old through long
magnificent aqueducts, the ruins of which partly remain today. We
have all reasons to believe that the marked decrease in cultivation
and population, laid to changes in conditions of humidity, depended
altogether on man’s interference with nature. On certain rocks in
Morocco, carvings have been found portraying in a simple way large
mammals such as elephants, rhinoceroses, and giraffes, which do not
exist in these regions now because of lack of nourishment. But these
rough works of art, resembling those of the bushmen of today, date
from prehistoric time, the so-called paleolithic era, when the climate
admittedly was more humid in these regions than it is now.
Similar conditions obtain, according to Hedin, in Central Asia and in
Persia. The climate there has without doubt been more humid but not in
historic times. The march of Alexander toward India took place under
as adverse conditions as now are found in these regions (Baluchistan).
Their cities, now in ruins, received their water supply through
conduits from rivers some of which were then adjacent to the cities,
although they later have changed their course as pointed out by Leo
Berg.
In Western and Central Europe numerous marshes and morasses have
indeed been drained and rendered available for cultivation, but this
does not prove that the climate is become more dry. On the contrary
all observations, for instance those made by Tycho Brahe on the
island Hven, indicate that the difference between summer and winter
temperature has decreased during historic time; that is the climate is
become less continental, or more humid, than formerly. Furthermore,
many circumstances, such as the occurrence of hazel and of water
chestnut in far more northerly latitudes and the higher altitude of
the timber line in earlier time, prove that the summer in prehistoric
ages was warmer than now. Simultaneously it was dryer. A study of the
lacustrine pile dwellings in Switzerland shows that the levels of the
lakes then were not higher than now but very nearly the same, which
demonstrates that the precipitation has not changed perceptibly in
Switzerland since these buildings were made; the period in question
occurred, we believe, about 7000 years ago.
While great climatic changes have taken place since man’s first
appearance on earth, presumably before the end of the ice period,
historic time is too short to record any distinct modifications.
Local ones may be in evidence such as West Europe’s transition to a
less continental climate. A variation of this nature has been found
not longer back than since thermometrical observations commenced.
Thus the winters in Berlin during the period 1746–1847 were colder
and the summers warmer than during 1848–1907. The difference for
January amounted to -1.5° C. (-2.7° F.) and for May to +0.6° C.
(+1.08° F.). The tabulation below, quoted from Ekholm, shows the mean
temperature in Stockholm, Lund, London, and Paris, during winter
(December-February), spring (March-May), summer (June-August), and
autumn (September-November) and for the following periods:
-------+-------------------+------------------
| _Stockholm_ | _Lund_
|-------------------+------------------
| _1799– | _1849– | _1753– | _1799–
| 1848_ | 1898_ | 1798_ | 1898_
-------+---------+---------+---------+--------
Winter |+25.5° F.|+26.8° F.|+30.2° F.|30.9° F.
| -3.6° C.| -2.9° C.| -1.0° C.|-0.6° C.
-------+---------+---------+---------+--------
Spring | 37.9° F.| 37.9° F.| 41.2° F.|41.5° F.
| 3.3° C.| 3.3° C.| 5.1° C.| 5.3° C.
-------+---------+---------+---------+--------
Summer | 60° F.| 60° F.| 61° F.|60.2° F.
| 15.6° C.| 15.6° C.| 16.1° C.|15.7° C.
-------+---------+---------+---------+--------
Autumn | 43.9° F.| 43.5° F.| 45.9° F.|45.9° F.
| 6.6° C.| 6.4° C.| 7.7° C.| 7.7° C.
-------+---------+---------+---------+--------
Year | 41.9° F.| 42.1° F.| 44.6° F.|44.6° F.
| 5.5° C.| 5.6° C.| 7.0° C.| 7.0° C.
-------+---------+---------+---------+--------
-------+-----------------+-----------------
| _London_ | _Paris_
|-----------------+-----------------
| _1799– | _1849– | _1806– | _1849-
| 1848_ | 1898_ | 1848_ | 1898_
-------+--------+--------+--------+--------
Winter |38.5° F.|39.2° F.|37.9° F.|37.9° F.
| 3.6° C.| 4.0° C.| 3.3° C.| 3.3° C.
-------+--------+--------+--------+--------
Spring |48.2° F.| 48° F.|50.5° F.|50.3° F.
| 9.0° C.| 8.9° C.|10.3° C.|10.2° C.
-------+--------+--------+--------+--------
Summer |61.9° F.|62.2° F.|64.6° F.|64.8° F.
|16.6° C.|16.8° C.|18.1° C.|18.2° C.
-------+--------+--------+--------+--------
Autumn |50.7° F.|50.5° F.|54.2° F.|51.8° F.
|10.4° C.|10.3° C.|11.3° C.|11.0° C.
-------+--------+--------+--------+--------
Year | 9.8° F.| 50° F.|51.3° F.|51.3° F.
| 9.9° C.|10.0° C.|10.7° C.|10.7° C.
-------+--------+--------+--------+--------
The difference is not great. For Stockholm the winter has grown warmer,
the autumn colder, for London the winter warmer and so slightly also
the summer, but spring and autumn a trifle colder, and for Paris the
summer a little warmer while the autumn is considerably colder. Lund
shows the least variation. The winter has grown 0.4° C. (0.72° F.)
warmer and the summer colder by the same amount. The annual mean
remains nearly constant, only slightly increased, but the climate is
become more marine. (This is hardly apparent from the figures cited as
far as Paris is concerned.)
From Tycho Brahe’s observations of the number of days when snow or rain
fell in the place where his observatory was situated on the island
Hven in Öresund not far from Copenhagen, Ekholm has calculated that
the temperature there during the time 1582–1597 was 1.4° C. (2.5° F.)
lower in February and 1° C. (1.8° F.) lower in March than in later
years (1881–1896). On the other hand the first autumn frost occurred at
the same time as now and the same was the case with the last frost in
spring, so that the temperatures on these dates in autumn and spring
were nearly identical at the end of the sixteenth century and now.
Ekholm drew the conclusion that the climate is become more marine.
Hildebrandson makes the objection that Tycho Brahe’s observations
were confined to an abnormally cold period judging by the tables
Speerschneider has prepared showing the ice formation in Danish
navigable waters. Nine out of the sixteen years in which Tycho Brahe
gathered his data were notable for abnormally cold winters, while only
nineteen out of the hundred years composing the sixteenth century were
characterized by equally severe winters.
Thus, the conclusion is not warranted that the winters of the sixteenth
century as a whole averaged colder than those of the nineteenth. Later
investigations (in 1917) in regard to the dates when the ice would
break up in Lake Mälar at Vesterås, in Neva River at Petrograd, and
in Dwina River at Riga, have led Ekholm to believe that he has found
a periodicity in winter temperatures of not less than 212 years, a
conclusion which would agree with Speerschneider’s statistics. If
so, we are at present living in a period remarkable for its mild
winters while a series of extremely severe winter seasons occurred at
the time of Tycho Brahe. This law would also have a bearing on the
preceding table of temperatures in Stockholm, Lund, London, and Paris
as a succession of severe winters came around in the beginning of the
nineteenth century while the reverse is true towards its end. On the
whole climatic variations during historic time have been insignificant,
if present at all, provided we extend our comparisons over two or more
centuries. Such is also the opinion of Hildebrandson.
The idea of a slow deterioration of the climate due to increasing
desiccation is of old lineage and is most likely related to the
venerable conception of a bygone golden age. Aristotle even at that
early date believed that a gradual arefaction of the Earth took
place. In recent times this faith has been particularly fostered by
Huntington in a number of treatises where he endeavours to prove that
Asia, represented for instance by Palestine, Syria, and Persia, and
further Africa and North America are subject to a rapid exsiccation
clearly traceable through historic time. The contrary, however, is true
about Western Europe. It was often said that Southern Russia in recent
times suffered a slow arefaction manifesting itself in the formation of
steppes. This led to careful investigations showing the assertion to
be erroneous and culminating in the work of Leo Berg. Rather, a slight
shifting in the opposite direction is detectable as the forest region
has expanded at the expense of the steppes in conformity with the
development toward the end of prehistoric time. The renowned American
astronomer, the late Dr. Lowell, embraced the idea of increasing
aridity which he observed himself in Arizona where his observatory
is located. The drying out of Arizona undoubtedly took place in
long bygone prehistoric time. The disappearance of a high culture in
Syria and Mesopotamia was the result of hostile devastation of their
waterworks; a compensation is now offered in the reclamation of the
deserts along the Nile, in California and Arizona, and in numerous
other places.