_THE ROMANCE OF SCIENCE._
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SOUNDING
THE OCEAN OF AIR
_BEING SIX LECTURES_
DELIVERED BEFORE THE LOWELL INSTITUTE OF BOSTON
IN DECEMBER 1898
BY
A. LAWRENCE ROTCH, S.B., A.M.,
DIRECTOR OF THE BLUE HILL METEOROLOGICAL OBSERVATORY,
MASSACHUSETTS, U.S.A., AND MEMBER
OF THE INTERNATIONAL CLOUD AND AERONAUTICAL COMMITTEES.
PUBLISHED UNDER THE DIRECTION OF THE GENERAL
LITERATURE COMMITTEE.
LONDON:
SOCIETY FOR PROMOTING CHRISTIAN KNOWLEDGE,
NORTHUMBERLAND AVENUE, W.C.; 43, QUEEN VICTORIA STREET, E.C.
BRIGHTON: 129, NORTH STREET.
NEW YORK: E. & J. B. YOUNG & CO.
1900
This little Book is gratefully Dedicated
TO
The late AUGUSTUS LOWELL, Esq.
OF
BOSTON, U.S.A.
WHO, AS TRUSTEE OF THE LOWELL INSTITUTE,
ENABLED SCIENTIFIC MEN OF TWO CONTINENTS TO
PRESENT THE RESULTS OF THEIR INVESTIGATIONS
TO THE PUBLIC
CORRIGENDA
Page 50, line 11, _for_ "isolation" _read_ "insolation."
Page 59, line 21, _before_ "direction" _insert_ "opposite."
Page 112, line 2; page 115, lines 2 and 15, and Index, pages
175 and 183, _for_ "ViollÈ" _read_ "Violle."
Page 112, line 23; page 113, line 6, and Index, page 181, _for_
"Muntz" _read_ "M¸ntz."
Page 123, last line, _for_ "1889" _read_ "1891."
Index, page 177, _for_ "Cotte (T.)" _read_ "Cotte (L.)."
Index, page 179, _after_ "Hann (J.), 36," _add_ "173."
Index, page 179, _for_ "Hellman (G.)" _read_ "Hellmann (G.)."
Index, page 181, _after_ "Langley (S. P.)" _insert_ "28."
[Transcriber's Note: These corrections have been applied to the
current version]
CONTENTS
CHAP. PAGE
I. THE ATMOSPHERE--ANCIENT AND MODERN
KNOWLEDGE--METHODS OF INVESTIGATION 9
II. CLOUDS--FORMATION AND CLASSIFICATION--MEASUREMENTS
AT BLUE HILL--THE INTERNATIONAL OBSERVATIONS 38
III. BALLOONS--NOTABLE ASCENTS AND RESULTS
OBTAINED--CAPTIVE BALLOONS 68
IV. _BALLONS-SONDES_ FOR GREAT ALTITUDES--THE INTERNATIONAL
ASCENTS 98
V. KITES--HISTORY AND APPLICATION TO METEOROLOGICAL
PURPOSES AT BLUE HILL AND ELSEWHERE 117
VI. RESULTS OF THE KITE-FLIGHTS AT BLUE HILL--FUTURE WORK 145
INDEX 175
LIST OF ILLUSTRATIONS
PAGE
PLATE I. Comparative Altitudes 22
PLATE II. Optical Phenomena showing the Height of the
Atmosphere 26
PLATE III. Temperature at different Latitudes and Altitudes 31
FIG. 1. Nephoscope at Blue Hill Observatory. 52
FIG. 2. Cloud Theodolite at Blue Hill Observatory 54
PLATE IV. Heights and Velocities of Clouds 57
PLATE V. Atmospheric Circulation at different Heights in
Cyclones and Anti-cyclones 61
FIG. 3. German Balloon equipped for Meteorological
Observations 87
PLATE VI. Temperatures observed in Four High Balloon Ascents 91
FIG. 4. German Kite-balloon. 95
FIG. 5. Baro-thermograph of Richard 102
FIG. 6. The _AÈrophile_ rising 103
PLATE VII. Heights and Temperatures recorded in eight Ascents
of the _Cirrus_ 109
FIG. 7. Oriental tailless Kites. 119
FIG. 8. Eddy tailless Kite 129
FIG. 9. Hargrave Kite 130
FIG. 10. Modified Hargrave Kites at Blue Hill 133
FIG. 11. Lamson's Aero-curve Kite 135
FIG. 12. Meteorograph lifted by Kites at Blue Hill 138
PLATE VIII. Meteorogram from the Kite-flight of Oct. 8, 1896,
at Blue Hill 148
PLATE IX. Mean Changes with Height, and Changes during the
Kite-flight of Oct. 8, 1896 150
PLATE X. Changes with Height recorded by Kites at Blue Hill 155
PLATE XI. Kite Observations at Blue Hill, Sept. 5-11, 1897 163
PLATE XII. Automatic Records during a High Kite-flight
at Blue Hill 166
PLATE XIII. Results of Kite-flights at Blue Hill during an
Anti-cyclone and a Cyclone 168
SOUNDING
THE OCEAN OF AIR
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CHAPTER I
THE ATMOSPHERE--ANCIENT AND MODERN KNOWLEDGE--METHODS OF
INVESTIGATION
Concerning this most important element in which we live and move and
have our being, Pliny, in the first century of our era, wrote as
follows: "It is time to consider the other marvels of the heavens;
thus our fathers called that immense space where flows the vital fluid
to which we give the name of air, and which is not apparent to the
senses because of its great rarity. There clouds form, thunder and
lightning also; it is the region of tempests and of whirlwinds; from
there fall rains, hail, and hoar frost; from there come all those
phenomena, astonishing and often disastrous, which follow the combat
of Nature with herself.... The sun's rays strike the earth on all
sides, warming and strengthening it; they are reflected and detach
all the particles they can carry away; vapours descend and rise again;
the winds come empty and return laden with spoil; animals breathe in
from above this vital fluid which animates them, and the earth sends
it back to its source as if she would fill the void by this means. So,
by Nature acting everywhere and in all directions there results an
apparent discord from which is born the harmony of the Universe; it is
this general movement which puts all things in their places; some are
preserved by the destruction of others; all move, all act, the
struggle is continual, if it ceased an instant everything would fall
into chaos...."
From the earliest times, as far back as history extends, we find
mankind interested in meteorological phenomena. This appears natural
if we consider the importance of the weather to the ancient pastoral
nations, which, from the open-air life and keen perceptive faculties
of their people, were well fitted to study natural phenomena. The
beauty and grandeur of many of the phenomena occurring in the
atmosphere, and the curiosity excited concerning their causes,
probably contributed to interest people in them. Meteorology appears
to have been first treated systematically, as distinct from astronomy
and astrology, by the Greek philosopher, Aristotle, more than 2000
years ago. The word "meteor," derived from the Greek "elevated," was
applied to certain phenomena having their origin in the atmosphere.
These were classified into aÎrial, aqueous, and luminous meteors, and
were all included in the term Meteorology. In his treatise by this
name Aristotle gave a more detailed account of them than any preceding
or contemporary writer, and Theophrastus, his pupil, wrote two books
on the winds and on the signs of rain, which have been translated into
Latin and English. About the same period Aratus incorporated the
current weather proverbs in his poem, _Diosemeia_. The Greek
historians and poets frequently alluded to atmospheric phenomena, and
their example was followed by the Romans, of whom Pliny has been
quoted.
No doubt the desire to ascend into the air always possessed man, but
owing to the awe with which mountains seem to have inspired the
ancients, there is rarely mention in their writings of climbing
mountains, or of the physiological effects which could hardly have
failed to be apparent upon high summits. Citing one of the few
existing narratives, Aristotle relates: "Those which ascend to the top
of the mountain Olympus could not keep themselves alive without
carrying with them wet sponges, by whose assistance they could respire
in that air otherwise too thin for respiration." This mountain of
less than 10,000 feet was said to be so high that it never rained on
its summit, where, it was supposed, the air was always still. A still
higher mountain, easily accessible to the ancient world, and which we
know was ascended, is Etna.
Concerning the progress of meteorology, from the time of the ancient
Romans to the revival of knowledge in Europe, there is little to say
except that during the middle ages meteorology, like other learning,
was confined to the monasteries. Speculations were current as to the
extent of the atmosphere until, in the middle of the eleventh century,
Alhazen, a learned Arab, computed from the duration of twilight that
the atmosphere extended nineteen leagues above the earth. The same
method was applied with more precision by Tycho Brahe, Kepler, and
other astronomers of the sixteenth and seventeenth centuries. The
earliest weather chronicles were probably noted by monks from time to
time in almanacks or missals, although when this was done first we do
not know. The oldest daily chronicles of the weather extant are those
kept by William Merle in Oxford from 1337 to 1344. We owe it to the
late Mr. Symons, the English meteorologist and bibliophile, that this
MS. and many other old records have been brought to light and
published. Dr. Hellmann has done even more in Germany, and this
historical research is evidence of the growing importance of the
science of meteorology.
With the advent of the age of geographical discovery it was seen that
the climatic features of our globe depend chiefly upon distance from
the equator, proximity to the ocean, and height above it. In the
tropics especially, the luxuriant vegetation, which diminishes on
mountain slopes and higher up gives place to snow, must have been
visible proof of the decrease of temperature with altitude, for, as
Professor Daniell remarked, mountains are a gigantic registering
thermometer having for the freezing-point the line of perpetual snow.
The invention of instruments for measuring temperature and barometric
pressure made possible the quantitative observations that have
supplied the data for deducing the laws governing the atmosphere. The
oldest meteorological instrument is, no doubt, the weather or
wind-vane, which had its origin before the Christian era. The next
oldest is the hygrometer, or instrument for measuring moisture in the
air, the form which acts by absorption dating from the middle of the
fifteenth century, and the condensation hygrometer being a century
younger. Next in chronological order comes the rain-gauge, which
appears to have been used by Castelli, a friend of Galileo, in the
year 1639. The history of that important instrument, the thermometer,
is obscure, but it is certain that Galileo in Padua used an
air-thermometer in the latter part of the sixteenth century, which
Rey, a French physician, filled with liquid in 1631. This thermometer,
as well as other physical instruments, was perfected by members of the
Accademia del Cimento at Florence. These instruments are described in
_Saggi di Naturali Esperienza_, written in 1666, and translated into
Latin and English. The Florentine thermometers had one fixed point,
that of freezing water, and contained either spirits or mercury. In
1724 Fahrenheit, in Danzig, fixed three points on the scale of the
mercurial thermometer, viz. the cold produced by ice and sal-ammoniac
which he called 0∞, freezing water or 32∞, and the heat of the human
blood which he assumed to be 96∞. This thermometric scale, having 180∞
between freezing and boiling water, and that of Celsius, with 100∞,
are the only ones in scientific use to-day. It is a remarkable fact in
the history of thermometers that neither of these thermometers
remained in the country where it was invented; thus the thermometer of
Fahrenheit, a German, came into use exclusively in England and her
colonies, while that of Celsius, a Swede, is now used on the continent
of Europe except in Germany, where the thermometer of RÈaumur, a
Frenchman, is still in popular use. Of the four fundamental
meteorological instruments, the barometer was the last invented.
Aristotle had suspected that air had weight, but it was not
demonstrated until the middle of the seventeenth century, when the old
axiom "that Nature abhors a vacuum" was replaced by the rational
explanation, given by Galileo and Torricelli, his pupil, why water
will not rise in a suction pump more than thirty-two feet. In 1643
Torricelli executed this famous experiment: he took a glass tube,
sealed at one end, and filled it with mercury, then, closing the open
end with his finger, he inverted it in a basin of mercury. The mercury
fell to about thirty inches, which was recognized to be the weight of
a column having the area of the tube and of the height of the
atmosphere. The application of the barometer was due to Blaise Pascal,
who repeated at Rouen Torricelli's experiment with a much longer tube
filled with water, which being thirteen times lighter than mercury,
stood thirteen times higher, or thirty-two feet, in the tube. Pascal,
being himself at Paris in 1648, got his brother-in-law Perier to carry
a barometric tube filled with mercury to the top of the Puy de DÙme, a
mountain in Auvergne rising about 3500 feet above the city of
Clermont. The mercury fell in the tube with the ascent, and at the top
of the mountain it stood some three inches lower than at the base,
showing that the lower layers of the atmosphere are denser than the
upper. Pascal repeated the experiment on the Tower of St. Jacques in
Paris, and it is interesting to note that more than two hundred years
afterwards, meteorological stations were established both there and on
the Puy de DÙme. It was soon perceived that not only did the level of
the mercury in the tube change with height, but that it oscillated
continually at the same place, and from its observed relation to the
state of the weather its name "weather-glass" is derived. In 1650 the
weight of the air was demonstrated in another manner by Otto von
Guericke, burgomaster of Magdeburg, who by means of an air-pump of his
invention performed the experiment, which Aristotle had tried
unsuccessfully, of weighing a vessel full of air and the same vessel
exhausted of air. He also showed the pressure of the air in all
directions by the famous experiment of the Magdeburg hemispheres,
which, being hollow, were placed together, and after the air was
exhausted from the sphere so formed sixteen horses were unable to pull
them apart. Soon afterwards Robert Boyle experimented further upon the
weight and "spring of the air," as he called it, and gave the name to
the barometer. Both Boyle in England and Mariotte in France discovered
the law, bearing indifferently their names, that the pressure of gases
is proportional to their density. Halley, a few years later, showed
that the rate of decrease in pressure differed from the rate of
increase in height, and developed formulÊ for measuring heights by the
barometer, which were afterwards perfected by Laplace. Knowing the
heights of the barometer at a high and at a low-level station, and the
mean temperature of the air lying between them, it is possible to
compute accurately the difference of height of the two stations, or,
conversely, given this height, the difference in barometric pressure
can be calculated. By the middle of the seventeenth century the most
important meteorological instruments had been invented, and not only
can Italy claim to be their birthplace, but the Grand Duke Ferdinand
II., whose brother Leopold founded the Accademia del Cimento,
distributed the new instruments in Italy and even beyond the Alps, so
that in 1654 observations several times a day were begun at a dozen
stations. The observations in Florence from 1650 to 1670 were
preserved and constitute the commencement of instrumental meteorology.
It was the conquest of Peru which, by leading men over the high passes
of the Andes, first brought them to great heights, but although we
find mention in the history of the expeditions of the so-called
mountain sickness, caused by fatigue as well as by cold and rarefied
air, it does not appear that scientific observations were made.
Therefore, while it must be assumed that the atmospheric conditions at
considerable altitudes were familiar to travellers, yet not until the
middle of the last century did Bouguer, one of three French
Academicians sent to Peru on a geodetic mission, fix the height of the
freezing point in various latitudes, after observing that the
temperature fell below freezing at night upon the mountains near the
equator. During the latter part of the century, Kirwan, an English
chemist, calculated the temperature for various parallels of latitude,
and in 1817 Alexander von Humboldt, after a voyage around the world,
published his isothermal lines, or lines of equal temperature on the
surface of the globe, by which he showed that the deviation from the
normal, or calculated, temperature arose from the distribution of land
and water, and from the geographical relief of the former. This work
of von Humboldt formed the basis of all subsequent studies in
comparative climatology. Meanwhile chemistry had kept pace with
physics, and in 1774 the old theory, that air was one of the four
elements from which all things originated, was rendered untenable by
Priestley, who proved that oxygen gas, which he discovered, was a
constituent part of air. The other constituent, nitrogen, formerly
called azote from its destructiveness to life, was discovered soon
afterwards, and its proportion in the air determined by the French
chemist, Lavoisier.
In 1783 man became possessed of the long-sought-for means of rising
freely in the air, and he speedily availed himself of it. The first
balloons, filled with heated air, were called _MontgolfiËres_ from the
inventors, the brothers Montgolfier, living in Annonay, France. After
animals had been sent up attached to one, Pil‚tre de Rozier ventured
to ascend in the aerostatic machine, first tethered captive but then
set free, and before the close of the year a balloon, filled with
hydrogen gas, or "inflammable air" as it was called, carried M.
Charles 9000 feet above Paris. During more than a century the balloon
has been the most important agent for the exploration of the
atmosphere, and yet, notwithstanding the courage and devotion to
science of the early aeronauts, their ascents with unsuitable
instruments furnished much discordant and erroneous data. Some of the
most remarkable balloon voyages and the modern methods of sounding and
dredging the atmosphere, to borrow terms from the exploration of the
ocean, will be described in two future chapters.
Perhaps the chief reason for the slow progress of meteorology to the
status of a science is the variable character of its phenomena with
the place of observation. In this respect it differs from astronomy,
which was more easily cultivated in the restricted ancient world. Only
after many years of observation at different places had contributed a
foundation for climatology was it realized that man, in his relation
to the atmosphere, resembled marine organisms confined to the bottom
of the ocean, and that in order to discover the true conditions of the
atmosphere it was necessary to observe them at considerable heights.
In the last century the highest point at which physical observations
had been made was the summit of Mont Blanc, less than 16,000 feet
above the sea. The ascent of this mountain was first accomplished in
1787 by H. B. De Saussure and his guides with much difficulty and
suffering, and the observations, abridged and rendered less accurate
by the fatigue and sickness of De Saussure, were also influenced by
the proximity of the mountain itself. In 1802 von Humboldt and
Bonpland reached a height of about 18,000 feet in the Andes, where
they made important observations. The ascent of man was rapid during
the first years of the nineteenth century, for in 1804 Gay-Lussac rose
in a balloon, without exertion or discomfort, to the height of 23,000
feet, and there made observations which were assumed to give the true
atmospheric conditions. After an active campaign the conquest of the
air by balloons was temporarily abandoned, and the field was left free
to the mountaineer. But to-day supremacy rests with the aeronaut, for
no one has succeeded in getting higher than 24,000 feet on a mountain,
while the aeronaut has exceeded this altitude by a mile without great
hardship, and lately has sent his unmanned balloons twice as high as
the loftiest mountains.
Plate I., headed The Exploration of the Atmosphere, represents a
vertical section of the lower portion of our atmosphere. On the right
is a scale of miles above the sea, and on the left is a scale of
barometric pressures corresponding to the height. The right-hand half
of the diagram shows the eastern hemisphere with the Himalaya
mountains, the left-hand half the western hemisphere with the Andes.
There are seen the heights of the different kinds of clouds, measured
at Blue Hill, as described in the next chapter; the highest
meteorological stations, those on Mont Blanc and El Misti in Peru; the
highest permanently inhabited place, which is a monastery in Thibet;
and the greatest height to which man has climbed, namely, in the
Andes. The heights at which observations have been made in balloons,
carrying observers, or only recording instruments, may be compared
with the height attained by the Blue Hill kites, to be described
hereafter. Other altitudes can be noted, such as the height of the
snow-line on various mountains, and as a thousand-foot measure, the
Eiffel Tower in Paris, the tallest structure erected by man, may be
used.
[Illustration: PLATE I.--COMPARATIVE ALTITUDES.]
The development of meteorological knowledge to the commencement of the
present century has now been traced, but before beginning the
consideration of the methods of exploring the atmosphere that form the
subject of the book, let us, in order to understand this work better,
review the general knowledge which we possess of our atmosphere as
regards its origin, composition, extent, and conditions of heat and
moisture. First, then, regarding the =Origin of the Atmosphere=, or
vapour envelope which the name means. According to the nebular
hypothesis of Laplace, our earth, like all existing suns and planets,
was condensed from clouds of nebulous matter and became a
highly-heated globular mass rotating, like every celestial body, from
west to east. As the earth cooled, a crust was formed, and many of the
substances that now exist in the earth were suspended as clouds in the
cooler atmosphere surrounding it. Eventually, these substances were
condensed upon the crust; the oxygen, especially, must have been
diminished by combining with the rocks, while the lighter gases, such
as hydrogen, may have escaped from the earth's atmosphere. No doubt,
when vegetable and animal life began, the earth's atmosphere was
denser than now and much richer in carbonic acid, which, during the
carboniferous period, was absorbed by plants, and is now imprisoned in
coal and limestone. Within historic times, however, there is no
evidence of any change in the composition of our atmosphere, or the
climatic conditions as a whole.
M. Jourdanet, a distinguished French physiologist, maintained that man
appeared on the earth at the close of the tertiary period, when the
barometric pressure at sea-level was, he supposed, about forty-three
inches, or nearly a half more than it is to-day, and owing to the
greater density of the air its temperature was also considerably
higher. Under these circumstances he believed that man first occupied
the high regions of Central Asia, and only emigrated to lower levels
when the climatic conditions became ameliorated. In other words, M.
Jourdanet believed in a literal "descent of man," but if this be true,
many of the race have returned to their birthplace, for to-day
millions of people dwell on the great Asiatic plateau, and on the
South American Cordillera, at an average altitude of 10,000 feet,
while a few live throughout the year at extreme heights of 15,000
feet.
=Composition of the Atmosphere.=--Dry air is a mixture of about
one-fifth of a volume of oxygen to four-fifths of a volume of
nitrogen, besides a very small quantity (3/10,000) of carbonic acid,
traces of ammonia, ozone, argon, and other recently discovered gases.
The oxygen consumed, and the carbonic acid given off by animal life
and by combustion, are maintained in this fixed proportion in the free
air by the absorption of the carbonic acid, and the setting free of
oxygen by vegetation. By diffusion and the mobility of the air, a
thorough mixture is effected, with the result that the fundamental
composition of our atmosphere is everywhere nearly the same. In the
lower atmosphere the vapour of water is present in a varying quantity,
in the average about one per cent. in weight, with a volume depending
on the temperature. Dust is always suspended in the atmosphere; the
coarser particles settle, but the finer ones, that come from
volcanoes, may float for a long time in the high atmosphere. Dust is
an important factor in the production of clouds and rain, and
occasions many optical phenomena.
[Illustration: PLATE II.--OPTICAL PHENOMENA SHOWING THE HEIGHT
OF THE ATMOSPHERE. ]
=Extent of the Atmosphere.=--If the atmosphere were incompressible and
had throughout the density that it has at the earth, its height would
be about five miles only, but actually it is composed of gases that
follow Boyle's law and vary in volume inversely as the pressure upon
them. Since the pressure decreases with height in a geometrical
progression, it would be halved for each three and a half miles of
ascent were the temperature constant, but as the temperature also
decreases with height, the successive intervals, beginning with three
and a half miles, become shorter because the volume of a gas depends
on its temperature as well as on the pressure upon it. The decrease of
pressure with increasing height above the earth is shown by the
left-hand scale of Plate I., already described, and the subsequent
diminution of density to the limits of our measurable atmosphere is
indicated on the right of Plate II., Optical Phenomena showing the
Height of the Atmosphere. The gases composing the atmosphere probably
extend to heights proportional to their density; viz. oxygen to about
thirty miles and nitrogen to thirty-five miles, although water-vapour
nearly disappears at twelve miles. From these considerations it is
supposed that the atmosphere, as measurable by the barometer, vanishes
at about thirty-eight miles, and this is about the height indicated by
twilight, which is the reflected light of the sun when 18∞ below the
horizon. After the great eruption of the volcano Krakatoa in the South
Seas in 1883, the brilliant sunset glows and the longer twilight
showed that the dust emitted by the eruption remained for more than a
year suspended at a height of at least sixty miles. The so-called
"luminous clouds" seen at night during the same period, and which were
probably these same dust particles still illumined by the sun, were
found by trigonometrical measurements to have about the same altitude.
Although it is computed that at a height of seventy miles the air has
less than one-millionth of its density at sea-level--which is about
the density of the air remaining in the exhausted bulb of an
incandescent electric lamp--it is there sufficiently dense to render
meteors luminous by friction after they with great velocity enter our
atmosphere. The height of these meteors has been found, from
simultaneous trigonometrical measures at two stations, sometimes to
exceed one hundred miles, and if we suppose the aurora borealis to be
an electrical discharge in highly rarefied air, measures made in the
same way indicate as great a height for our atmosphere. The height of
the aurora varies enormously, but the average altitude of it and of
the other phenomena described, with the corresponding computed density
of the air, are shown in the preceding diagram, in which the depth of
the ocean of air may be compared with the deepest seas and the highest
mountains. While, as Professor Young says, it cannot be asserted that
the atmosphere has any defined upper limit, yet the kinetic theory of
gases seems to afford evidence that the molecules of oxygen and
nitrogen do not escape from the earth's attraction, and therefore the
hypothesis of Professor Fˆrster is unwarranted, that interplanetary
space is filled with _Himmelsluft_, or very thin air.
=Temperature of the Atmosphere.=--The warmth of the atmosphere is
derived chiefly from the sun's rays which, arrested by the earth's
surface, are partly reflected and partly radiated back through the
atmosphere. Not more than seventy-five per cent.--Professor Langley
says only sixty per cent.--of the heat of the sun, which is received
vertically on the upper surface of the atmosphere, penetrates to the
earth, and very much less than this when the angle of the sun is low.
The reason why temperature diminishes as we ascend, is partly owing to
the greater loss of heat by radiation through the thinner envelope of
the upper strata, and partly owing to the greater absorption of the
heat given off from the earth by the lower and denser strata. In
general, it may be said that there is a diminution of 1∞ Fahrenheit
for each three hundred and thirty feet that we rise vertically, but,
this rate varies greatly at different heights, places, and times. For
instance, the decrease is not the same on mountains as it is in the
free air, and in the northern hemisphere it is greater on the south
than on the north sides of mountains; it is usually greatest near the
ground, and is faster in summer than in winter. But in the average,
the temperature falls as much for three hundred and thirty feet of
elevation as it does for a change of seventy miles on the earth's
surface north or south of the equator. When dry air rises, because it
is heated and thereby is made lighter, the laws of thermo-dynamics
show that, by reason of its expansion, its temperature is decreased 1∞
Fahrenheit for each one hundred and eighty-three feet that it ascends,
and, by compression, its temperature is increased as much if it is
made to descend the same distance. This is called the "adiabatic rate
of change of temperature," because it is produced by an alteration in
the density of the air, due to variation in pressure, without the
addition or loss of heat. In the course of this book there will be
occasion frequently to refer to this law of heating and cooling. The
adiabatic rate of change is seldom observed on mountains because of
their influence upon the currents of air in contact with their flanks,
or even in balloons, on account of imperfect measurements, but, as
will be explained in the closing chapter, the adiabatic change of
temperature is confirmed by the observations with kites, which furnish
the best method of obtaining the temperature of the free air up to
moderate heights. The adiabatic cooling of rising currents of air is
another reason for the rapid decrease of temperature with height up to
a mile or more. The upper air alters its temperature from diurnal and
seasonal causes much more slowly than the lower air, and a mile above
the earth the daily change of temperature, apart from the passage of
"warm and cold waves," is less than one degree. At a height of six
miles above the earth a temperature much below zero constantly
prevails, while, at ten miles, 80∞ below zero has been recorded in a
balloon--this is approximately the temperature prevailing winter and
summer above pole and equator. These facts are expressed graphically
in Plate III., Temperature at Different Latitudes and Altitudes,
which represents half of a section of the earth, from the north pole
to the equator, with the superincumbent atmosphere.
[Illustration: PLATE III.--TEMPERATURE AT DIFFERENT LATITUDES
AND ALTITUDES.]
Perhaps it should be explained, that whereas the curvature of the
earth with respect to the height of the atmosphere in the previous
diagram was not exaggerated, in the present diagram the height of the
atmosphere over the radius of the earth is enormously increased. At
the north pole the mean annual temperature is about 0∞ Fahrenheit, and
at the equator it is about 80∞. It is seen that the atmospheric layer
having a temperature of 50∞ (here represented in section by a line)
touches the earth at 45∞ latitude, but is about two miles above the
equator. In the same way the line of freezing (32∞) leaves the earth's
surface at 58∞ latitude and rises to about three and a half miles over
the equator; the line of 0∞ rises from the pole to about seven miles
at the equator. This is familiarly illustrated by the fact that only
the highest mountains in the tropics are snow-capped, while within the
Arctic circle the snow-line descends nearly to sea-level. The lines in
the diagram show the mean annual temperatures, but the isothermal
surfaces rise in summer and sink in winter, the change of altitude
being greatest in northern regions and near the ground. Frequently
there is an inversion of temperature, that is to say, it is warmer
above than below. Notably, in Siberia, where the winter temperature is
60∞ below zero, there can be no immediate decrease of temperature with
height, and it is probable that there is a warmer layer of air
interposed between the very cold earth and the still colder upper air,
so that the temperature first rises rapidly with elevation and then
falls slowly to the limits of the atmosphere. In temperate latitudes
it often happens, with a high barometric pressure, in winter that the
mountain stations enjoy a long period of still and relatively warm
weather, as compared to that experienced in the valleys. But the
subject of inversions of temperature will be discussed at length in
considering the results of the balloon and kite observations.
The observations from balloons at great heights are neither
sufficiently numerous nor accurate to enable us to form an opinion as
to what is the temperature of interplanetary space, which the kinetic
theory of gases places at 460∞ Fahrenheit below zero. This temperature
is called "the absolute zero," and is calculated from the fact that
air under a constant pressure contracts 1/490 of its volume for each
degree Fahrenheit it is cooled below the temperature of freezing
water, and consequently under no pressure it should have an infinite
volume and a temperature of about 490∞ below freezing, or 458∞ below
zero. There are other hypotheses regarding the temperature of space,
but since it can never be measured directly, it will probably remain a
matter of speculation. It is certain, however, that if the earth were
deprived of its atmosphere, the temperature would fall very low, and
even with our atmosphere as a blanket our earth would be
uninhabitable were it not for the aqueous vapour which controls the
selective absorption of the solar rays, transforming them into obscure
rays so that they cannot escape from the atmosphere. Water-vapour,
then, is a very important factor in the physics of the atmosphere, but
it can only be considered briefly here.
=Moisture of the Atmosphere.=--The air is constantly absorbing
moisture from the water on the earth, but the tension of this aqueous
vapour decreases with elevation much faster than does the atmospheric
pressure. At the height of about a mile and a quarter half the
quantity of water-vapour is below, while we must rise about three and
a half miles to reduce the quantity of air one-half, as may be seen in
Plate I. The relative humidity, or the percentage of moisture in the
air, as compared to the amount which it could contain at that
temperature, is nearly the reverse at low and at high levels. It is
found from the kite-observations at Blue Hill, that up to the height
of a mile or two the air is drier during winter and at night, and
damper during summer and in the day-time than it is near the ground.
At great heights probably the air is always very dry. The condensation
of the invisible vapour into a visible form is considered in the next
chapter on clouds.
It is apparent that our observational knowledge of the atmosphere is
gained by two general methods of exploring it, viz. observations made
from the earth upon clouds and optical phenomena at a distance, and
observations made directly in the air itself. Although it was realized
at the beginning of this century that meteorological observations were
almost all conducted at the very bottom of our atmosphere--"in the
shoals and shallows of the ocean of air," von Humboldt said--yet only
within the past thirty years was it thought necessary to replace the
occasional observations on mountains by systematic and long-continued
ones, comparable to those so generally carried on at low levels. It is
an evidence of the zeal in America to advance the young science of
meteorology, that the first mountain-top station in the world was
established in 1871 upon Mount Washington, and that both this exposed
post of observation, 6300 feet above the sea, and the one more than
twice as high on Pike's Peak, which was for a long time the highest in
the world, were maintained for many years by the United States Signal
Service. The present highest station in the world is maintained by the
Harvard Observatory upon El Misti in Peru, where, at a height
exceeding 19,000 feet, a combination of self-recording instruments was
constructed by my assistant, Mr. Fergusson, to operate during three
months without attention. It must be admitted, however, that the
addition to our knowledge of the physics of the atmosphere afforded by
the American stations has been slight and incommensurate with the
expense incurred. More has been gained from the mountain stations in
Europe, notably from those in the Austrian Alps, which have furnished
data for Dr. Hann's splendid discussions of the thermo-dynamics of the
atmosphere. While mountain stations present the only means of
obtaining continuous observations at a considerable and constant
height, still they have serious drawbacks. Not only is the
distribution of mountains over our globe irregular, but since they
form part of the earth's crust, terrestrial influences affect all
observations made upon them. In the case of plateaux this was at once
admitted, but by placing the stations on the summits of high and
isolated peaks, it was hoped to approximate to the conditions of the
free air. It is now recognized that the equilibrium of the atmosphere
is so delicate that for its dynamical study exact and minute
measurements of temperature, moisture, and currents are required, and
the methods which will be described are intended to give the values of
these elements free from terrestrial disturbances.
Clouds, balloons, and kites naturally supplement one another. While
clouds indicate the direction and velocity of the air at different
heights, yet the lower clouds often conceal the upper strata, or there
may be no clouds at all, in which case balloons or kites will aid us
to determine the drift of the currents. When there is little wind at
the ground, or to reach heights of several miles, we must employ
balloons, but otherwise kites are preferable in most cases. The
thermal and hygrometric conditions of the free air can be ascertained
only by personal observations in balloons, or by raising
self-recording instruments with balloons and kites, and this latter
method it is predicted will be the path of greatest progress.