[Illustration: _A tornado near Elmwood, Nebraska. A painting made from
what is probably one of the most remarkable photographs ever taken
of a tornado. The original photograph was made in two parts, as the
photographer was too close to get the whole funnel cloud into the field
of his camera._
(Photograph by G. B. Pickwell)]
POPULAR SCIENCE LIBRARY
EDITOR-IN-CHIEF
GARRETT P. SERVISS
AUTHORS
WILLIAM J. MILLER HIPPOLYTE GRUENER A. RUSSELL BOND
D. W. HERING LOOMIS HAVEMEYER ERNEST G. MARTIN
ARTHUR SELWYN-BROWN ROBERT CHENAULT GIVLER
ERNEST INGERSOLL WILFRED MASON BARTON
WILLIAM B. SCOTT ERNEST J. STREUBEL
NORMAN TAYLOR DAVID TODD
CHARLES FITZHUGH TALMAN
ROBIN BEACH
ARRANGED IN SIXTEEN VOLUMES
WITH A HISTORY OF SCIENCE, GLOSSARIES
AND A GENERAL INDEX
_ILLUSTRATED_
[Illustration]
VOLUME ONE
P. F. COLLIER & SON COMPANY
NEW YORK
Copyright 1922
BY P. F. COLLIER & SON COMPANY
MANUFACTURED IN U. S. A.
METEOROLOGY
THE SCIENCE OF THE ATMOSPHERE
BY
CHARLES FITZHUGH TALMAN
CHAIRMAN, COMMITTEE OF PUBLIC INFORMATION,
AMERICAN METEOROLOGICAL SOCIETY
[Illustration]
P. F. COLLIER & SON COMPANY
NEW YORK
PREFACE
Meteorology is the science of the atmosphere and its phenomena,
including weather.
Nowadays, when we speak of a “meteor,” we generally mean a shooting
star; but formerly this term was applied (and it still often is in
technical literature) to a great variety of phenomena and appearances
in the atmosphere, including clouds, rain, snow, rainbows, and so
forth. That is how the science of the atmosphere came to have its
present name.
Meteorology is not a branch of astronomy. These two sciences are as
different from each other as zoölogy is from botany. They are both
founded on physics, and they “overlap” each other to some extent, just
as every science does certain others; but if you want information about
the atmosphere, weather and climate, an astronomical observatory is not
the place to seek it; while if you wish to make inquiries about comets,
sun spots, eclipses, standard time, or the date on which Easter fell in
the year 1666, do not apply to the Weather Bureau.
In the city of Washington the Government maintains an astronomical
and timekeeping institution known as the Naval Observatory, and it
maintains in the same city the central office of the United States
Weather Bureau. The two establishments are a mile apart in space and
nearly a whole library apart in the subjects with which they are
concerned. The fact that their functions are persistently confounded by
the public indicates the necessity of writing this preface to a popular
book on meteorology.
CONTENTS
CHAPTER PAGE
I. THE ANATOMY OF THE ATMOSPHERE 9
II. THE RESOURCES OF THE ATMOSPHERE 24
III. THE ATMOSPHERE AS A HIGHWAY 39
IV. DUST AND SMOKE IN THE ATMOSPHERE 52
V. WEATHER AND WEATHER INSTRUMENTS 66
VI. CLOUDLAND 90
VII. PRECIPITATION 106
VIII. WINDS AND STORMS 123
IX. ATMOSPHERIC ELECTRICITY 141
X. ATMOSPHERIC OPTICS 164
XI. ATMOSPHERIC ACOUSTICS 186
XII. CLIMATE AND CLIMATES 197
XIII. ORGANIZED METEOROLOGY 212
XIV. WEATHER MAPS AND FORECASTS 224
XV. AGRICULTURAL METEOROLOGY 245
XVI. COMMERCIAL METEOROLOGY 261
XVII. MARINE METEOROLOGY 271
XVIII. AERONAUTICAL METEOROLOGY 284
XIX. MILITARY METEOROLOGY 306
XX. MEDICAL AND PHYSIOLOGICAL METEOROLOGY 316
XXI. WEATHER-MAKING 332
XXII. ATMOSPHERIC BYWAYS 346
GLOSSARY 365
LIST OF ILLUSTRATIONS
FUNNEL-SHAPED CLOUD OF A TORNADO _Frontispiece_
_Painted from an unusual Photograph_
FACING PAGE
EXPLORING THE UPPER AIR--BEGINNING OF A PILOT BALLOON FLIGHT 32
EXPLORING THE UPPER AIR--SENDING UP A SOUNDING BALLOON 32
METEOROGRAPH FOR USE WITH A SOUNDING BALLOON 33
KITE METEOROGRAPH TO BE SENT ALOFT ATTACHED TO A KITE 33
ALTO-CUMULUS--FLEECY MASSES SHOWING BLUE SKY BETWEEN 96
CUMULUS, OR WOOL-PACK CLOUD 96
MAMMATO-CUMULUS, OR “RAIN BALLS” 97
CUMULO-NIMBUS--THE THUNDERCLOUD 97
CLOUD BANNER OVER MOUNT ASSINIBOINE, CANADIAN ROCKIES 100
CIRRUS, WITH LOWER CLOUDS IN THE FOREGROUND 100
LENTICULAR CLOUD OVER MOUNT RAINIER 101
ICE STORM AT PHILADELPHIA--TREES COVERED WITH GLAZE 116
NIEVE PENITENTE IN THE ARGENTINE ANDES 117
CLOUDS OR FOG CASCADING THROUGH LAST FORK CAÑON INTO SANTA
ANITA CAÑON 136
CLOUDBURST IN SOUTHERN UTAH 137
PHOTOGRAPHIC ANALYSIS OF LIGHTNING--SINGLE AND DOUBLE
DISCHARGES 160
PHOTOGRAPH OF LIGHTNING, SHOWING “BLACK” FLASHES 161
ATMOSPHERIC ELECTRICITY INSTRUMENTS 161
GLASS WEATHER MAP IN THE UNITED STATES WEATHER BUREAU 224
THE SUN DRAWING WATER 225
ORCHARD HEATERS IN OPERATION AS PROTECTION AGAINST EXPECTED
FROST 256
SNOW SURVEYOR AT WORK, USING A CYLINDRICAL SNOW SAMPLER 257
SNOW ROLLERS, OR WIND-BLOWN SNOWBALLS 257
BED OF THE POTOMAC RIVER PHOTOGRAPHED FROM THE AIR 288
DRILLING WITH COMPRESSED AIR IN A COPPER MINE 289
LAUNCHING A WEATHER BUREAU KITE TO EXPLORE THE AIR OVER THE
OCEAN 289
WEATHER BUREAU KIOSK, UNION SQUARE, SAN FRANCISCO 320
UNITED STATES WEATHER BUREAU STATION AT PEORIA, ILLINOIS 320
CENTRAL OFFICE OF THE UNITED STATES WEATHER BUREAU AT
WASHINGTON 321
CHAPTER I
THE ANATOMY OF THE ATMOSPHERE
Two quite different conceptions of the substance called “air” are
current in the world. One has prevailed from time immemorial. The other
is wholly modern. One is the popular view, the other the scientific.
Ancient philosophers regarded air as one of the four “elements” of
which all things were supposed to be made. Average humanity, though it
did not concern itself with philosophy, must have begun, almost as soon
as it realized the existence of air at all, to think of it as something
that, however it changed its state from hot to cold, dry to moist, pure
to impure, was fundamentally uniform--a single entity. Certainly this
idea is in full vigor today. The air that we breathe, supply to our
fires, stir with fans, pump into bicycle tires, fly in--the air that
asserts its independence of our will in the wind and the weather--gives
us the impression of individuality. We instinctively rank it with water
among the simple, definite things in the repertory of nature.
Even the man of science often finds it convenient to discuss and deal
with air as if it were a single substance, but he is well aware that
it is nothing of the kind. He knows that it is, in fact, a jumble of
gases having very different properties. Some are heavy, others light.
Some are chemically very active, others extremely inactive. Some
are abundant, others very rare. These gases constitute the earth’s
atmosphere. Other planets have atmospheres that are quite different in
composition from ours. The sun itself has a very complex atmosphere.
The earth’s atmosphere is, then, a collection of gases, which are mixed
but not chemically combined. Some of them are themselves chemical
compounds. Each of these gases behaves very much the same as if the
others were not present, and each of them has its separate business to
perform in the economy of nature. For example, a tree draws upon the
store of carbon dioxide gas in the atmosphere to build up its tissues.
Presently the tree is cut down and its wood is burned for fuel. In this
process a different atmospheric gas is brought into play. We often say
that the “air” supports combustion--that we supply “air” with a bellows
to make a fire burn more brightly--but it is not the air as a whole
that enables things to burn. Four-fifths of the atmospheric substance
takes no part in the process. We burn with oxygen alone. So it is with
breathing. Oxygen and not air constitutes the breath of life.
Near the surface of the earth the proportions of the more abundant
gases mixed together in the air are remarkably constant. Ignoring a
variable admixture of water vapor, oxygen is always about 21 per cent,
by volume, and nitrogen about 78 per cent. The remaining 1 per cent
is mainly argon. At great altitudes, however, these percentages no
longer obtain. The atmospheric gases differ greatly among themselves
in weight, and in the high atmosphere, where they are not mixed by
the winds, as they are below, the heavier tend to settle to the
bottom and the lighter to float on top, as oil floats on water. It
is calculated that at a height of thirty miles above sea level the
percentage of nitrogen is about 86½ and of oxygen only 10, while at
the same altitude the gas hydrogen, which at low levels constitutes
less than one-hundredth of 1 per cent of the atmosphere amounts to more
than 2½ per cent. Going higher, the percentage of hydrogen is supposed
to increase rapidly, until, at an altitude of forty-eight miles, the
atmosphere is more than half hydrogen, and at eighty miles above the
earth this gas forms 99 per cent of the whole. These figures are not
necessarily final; for some authorities believe that the atmosphere
contains an unknown gas lighter than hydrogen, while others think that
the hydrogen found in the lower air enters into chemical combinations
before it can reach the higher levels; but it is beyond doubt that the
composition of the upper atmosphere is quite different from that of the
lower.
Of course almost any gas may be found locally and occasionally in the
atmosphere, but there are several that are always found wherever a
refined analysis of the air is made, and others that are generally
present. The following is a fairly complete list: Nitrogen, oxygen,
water vapor, argon, carbon dioxide, hydrogen, helium, neon, krypton,
xenon, niton (radium emanation), ozone, hydrogen dioxide, ammonia and
other compounds of nitrogen.
A number of these substances have only become known to science within
the last quarter of a century. Argon, though it constitutes nearly 1
per cent of the atmosphere, escaped detection until the year 1894. The
investigation of argon led to the discovery of some of the others. In
1895 it was found that the air, as well as certain minerals, contains
helium. This substance was not new to science, but it had never before
been found on earth. It was discovered in the atmosphere of the sun, by
means of the spectroscope, as early as 1868. Terrestrial helium, neon,
krypton, and xenon were all discovered by Sir William Ramsay, who also
shared with Lord Rayleigh the distinction of discovering argon.
Ramsay has published the following figures for the proportions in which
some of the rare gases exist in the atmosphere:
Helium 1 part in 245,320 by volume
Neon 1 “ 80,800 “ “
Krypton 1 “ 2,000,000 “ “
Xenon 1 “ 17,000,000 “ “
Niton, or radium emanation, is one of the products of the
disintegration of radium. Niton itself disintegrates very rapidly,
one-half of any given quantity disappearing in about four days, and one
of its products is helium. The amount of niton in the atmosphere is
never more than an infinitesimal trace. Thus we are told that the total
quantity of this substance present in the atmosphere of the whole earth
up to an altitude of one kilometer (0.6 mile) weighs less than nine
ounces, and that each cubic centimeter of air contains among its thirty
million million million molecules only between one and two molecules of
niton, on an average.
Turning, now, to the more abundant constituents of the atmosphere, we
find that oxygen and nitrogen differ strikingly from each other in
the fact that, while the former has a strong chemical affinity for
nearly all other elements, the latter is chemically inert, having
little tendency to unite directly with other elements, though by
indirect processes, and chiefly through the agency of plants and
animals, a large number of nitrogen compounds are produced. Oxides of
nitrogen are formed directly from the atmospheric gases by lightning
discharges, and these unite with the moisture of the air to form
nitric and nitrous acids. A certain amount of ammonia (a compound of
nitrogen and hydrogen) may also be formed by lightning from nitrogen
and atmospheric water, but most of the ammonia in the air is derived
from the decomposition of plant and animal matters. The compounds of
nitrogen that occur in the air are washed down by rain in considerable
quantities. Analyses of rain water made in different parts of the world
show from one to nine pounds of such substances per acre per annum.
Carbon dioxide (more familiarly known as carbonic acid gas) occurs in
the atmosphere in the almost constant proportion of three parts in
10,000 by volume. It is a little more abundant in the air of towns
than in the open country or over the ocean, and it undergoes slight
periodic variations, but the fact that it is not much more variable
is rather surprising, considering that it is continually being added
to and abstracted from the air by numerous agencies that have no
dependence upon one another. It is supplied to the air by volcanoes,
mineral springs, the combustion of fuel, the respiration of animals and
plants, and the decay of organic matter. The amount supplied annually
by the burning of coal alone is estimated to be equivalent to more
than one-thousandth of the total volume of the gas present in the
atmosphere at any one time. On the other hand, all green plants, in the
presence of sunlight, withdraw carbon dioxide from the air, abstract
the carbon from it for the use of the plant, and return the oxygen to
the atmosphere. Thus it is estimated that an acre of beech forest takes
a ton of carbon out of the air annually. A vast amount of atmospheric
carbon dioxide enters into chemical combination with certain rocks
at the earth’s surface. Lastly, a large quota of this atmospheric
gas is absorbed by sea water, and certain authorities have seen in
this process a regulator of the total amount in the atmosphere, the
hypothesis being that the ocean gives back some of the carbon dioxide
whenever this substance becomes deficient in the air.
Water vapor--i. e., water in an invisible gaseous form--is always
present in the atmosphere, but its amount is subject to wide
fluctuations. An important fact in this connection is that, at any
given temperature, the air can hold only a definite amount of this
vapor. This maximum amount increases rapidly with temperature. When the
air is fully charged with water vapor it is said to be “saturated.”
Properly speaking, the temperature limits the amount of the vapor that
can occur in a given space, regardless of the presence of the other
constituents of air, and in scientific language it is the vapor itself
that is said to be saturated, and not the air; but in a popular book
about the atmosphere, where much has to be said about atmospheric water
vapor, adherence to scientific usage in this matter invariably leads
to awkward complications. Speaking, then, in familiar terms--when the
air is saturated with water vapor, a fall in temperature causes some
of the vapor to condense in visible form, as cloud, fog, rain, dew,
snow, hail, etc. As the sole source of these various forms of moisture,
and on account of the important part it plays in many atmospheric
processes, water vapor is, from a meteorological point of view, the
most interesting constituent of the atmosphere.
One more atmospheric gas requires notice here, both on account of
the great popular interest attaching to it, and because of recent
scientific discoveries concerning it--viz., ozone. This substance may
be described, in nontechnical language, as a concentrated form of
oxygen. It is one of the most powerful oxidizing agencies known, and
has found useful applications in medicine and various industries. Its
popular renown, however, is due to the fact that for many years it was
regarded as a great natural purifier of the atmosphere. “Life-giving
ozone” was reputed to be abundant in the air of forests, mountains, and
the seashore. Systematic observations were made of the prevalence of
ozone at different places throughout the world, generally by noting the
change of color of test-papers exposed to the air. These “ozonometric”
observations are now a closed chapter in the history of meteorology,
for it has been found that the reactions of so-called ozone papers
are due chiefly or entirely to atmospheric substances other than
ozone. Moreover, direct examination of the air by more accurate
methods--including samples collected with the aid of kites and balloons
up to a height of several thousand feet above the earth--shows that the
amount of ozone in the whole of the lower atmosphere is exceedingly
small--much too small to be of hygienic significance. Whatever ozone is
produced from oxygen at such levels by lightning discharges or other
possible agencies probably enters promptly into chemical union with
oxidizable substances and therefore has only a brief existence.
On the other hand, the spectroscope has brought us evidence that far
aloft in the atmosphere, many miles above the earth, ozone is quite
abundant. Here it is supposed to be generated by two agencies--the
electrical discharges of the aurora and ultra-violet radiations from
the sun. The ultra-violet rays that help to produce it are prevented
from reaching the earth, and astronomers are thus deprived of much
interesting information they might otherwise obtain concerning the
spectra of the sun and stars. However, as the present Lord Rayleigh
has pointed out, we can console ourselves for this fact by reflecting
that if the ozone did not shut off much of the ultra-violet light from
the sun, this light would probably ruin our eyesight; or, rather, we
should be put to the inconvenience of constantly wearing some sort of
protective spectacles in the daytime.
The high-level ozone is further interesting because of exercising a
certain control over the temperature of the lower air. It is more
transparent for incoming solar radiation than for outgoing earth
radiation. Hence, when it is unusually abundant, it should raise the
general temperature of the earth. This presumably happens when the
condition of the sun is such that an unusual amount of ultra-violet
radiation reaches the upper atmosphere, a fact that must be taken into
consideration in any attempt to establish a relation between climatic
fluctuations and the sun-spot period.
The lowest part of our atmosphere is the densest because it is
compressed by the weight of the air above it. Thus it happens that,
although the atmosphere is at least several hundred miles in height,
one-half of its mass--i. e., one-half of the quantity of matter in
it, as expressed in terms of weight--lies below an altitude of about
3½ miles above sea level, while about seven-eighths lies below the
ten-mile level. Above about five miles the atmosphere is too rare to
support life. The highest clouds seldom occur higher than ten miles.
Storms hardly ever reach that height. In short, the phenomena of life
and the phenomena of weather are confined to a layer of air so shallow,
in proportion to the dimensions of our globe, that on the surface of an
orange it would be represented by a sheet of thin paper.
The actual height of the atmosphere is not even approximately known.
There are theoretical reasons for believing that even at a height of
thousands of miles above the earth there are molecules of atmospheric
gases still under the control of the earth’s gravity, while at such
levels yet other atmospheric molecules are constantly escaping into
outer space. At an altitude of fifty miles the atmosphere is less
than 1/75,000 as dense as at sea level--i. e., more than seventy-five
times as attenuated as the best “vacuum” obtainable with an ordinary
mechanical air pump. At 300 miles it is computed to be about one
two-millionth as dense as at sea level.
The loftiest atmospheric phenomenon that we can observe directly is
the aurora, which has been photographed up to heights of more than
300 miles. The altitude of the aurora is determined by simultaneous
observations made at two or more points, and the same is true
of shooting stars and their trails, which seem to be especially
numerous between the levels of sixty and ninety miles. The so-called
“noctilucent clouds,” which shone by reflected sunlight throughout the
night for some years after the great eruption of Krakatoa and were
supposed to consist of fine dust from that volcano, were probably about
fifty miles above the earth. From the duration of twilight we infer
that above about forty-five miles the air is so tenuous that it cannot
reflect sunlight to the earth. Clouds furnish information concerning
the movements of the air at various levels up to ten miles or more.
Observations on mountains contribute further to our knowledge of the
atmosphere above the ordinary levels of habitation.
Of all methods of exploring the atmosphere in a vertical direction,
the most fruitful is the use of kites and balloons. In recent years
investigations of this character have become so extensive and so highly
specialized that they are regarded as forming a separate department
of meteorology, known as Aerology. It is by virtue of developments in
this field that meteorology has become “a science of three dimensions.”
Formerly meteorologists could do but little more than study the
_bottom_ of the weather, so to speak; but now they observe it and
chart it at all levels. The weather forecaster has daily reports of
conditions aloft to aid his predictions both for dwellers on _terra
firma_ and for the aeronaut; while the accumulated data of upper-air
observations are throwing new light on many difficult atmospheric
problems.
Scientific balloon ascents are no novelty. Some were made in the
eighteenth century, and many famous ones in the nineteenth, including
those of Biot, Gay-Lussac, Glaisher, Tissandier, and other daring
_savants_. The “record” height for such personal ascents was attained
in 1901, when Berson and Süring rose to 35,400 feet above Berlin.
Kites were sent up for meteorological purposes even before Benjamin
Franklin’s immortal experiment in 1752. Modern aerological methods
have, however, little in common with these pioneer undertakings.
Existing types of box kites, pilot balloons, sounding balloons, and
self-registering meteorological apparatus for upper-air research were
developed in the latter part of the nineteenth century, but their use
did not begin to bulk large in meteorology until about the beginning of
the present century. The epoch-making event in these undertakings was
the discovery of the _isothermal layer_.
It is a matter of common knowledge that the air is found to be colder
the higher one ascends in the atmosphere. Thus, even in equatorial
regions, the tops of high mountains are mantled in perpetual snow. The
rate of this temperature decrease averages about 1 degree Fahrenheit
per 300 feet. Previous to the year 1902 meteorologists supposed
that the atmosphere continued to grow steadily colder in an upward
direction indefinitely; but in that year a Frenchman, M. Teisserenc de
Bort, who had sent aloft hundreds of small unmanned balloons carrying
self-recording thermometers, announced that above a height of about
six and one-half miles the temperature ceased to fall. In fact, he
found that at about that level there was often a slight _increase_ of
temperature with increasing altitude for a certain distance upward, and
then a nearly uniform temperature as high as the balloons ascended.
This announcement was at first received with considerable skepticism,
but very soon similar observations were reported from other parts of
the world. A new “shell” of the atmosphere had been revealed--which,
as subsequent investigations proved, differs from the lower air in
other respects besides temperature--and it was at first named by
its discoverer the isothermal layer. He afterward substituted the
name _stratosphere_, now generally employed. In distinction from the
stratosphere, the part of the atmosphere lying below it is called the
_troposphere_.
The stratosphere has been explored in widely scattered parts of the
earth, and information concerning it is daily accumulating. Although
it extends over the whole world, the altitude at which it begins is by
no means uniform. The altitude is greater in summer than in winter; it
varies with the barometric pressure at the earth’s surface; and it is
decidedly greater over the equator than over the poles. The last fact
leads to an interesting paradox. Since over the equatorial regions
the temperature keeps on falling with ascent to a greater height than
in other latitudes, it is here that the lowest temperatures in the
atmosphere are found. A sounding balloon sent up from Batavia, Java,
in November, 1913, recorded 113° below zero Fahr., the lowest air
temperature ever observed. In middle latitudes the temperature of the
stratosphere averages something like 68° below zero Fahr.
The temperature of this interesting upper atmosphere varies a good
deal, both vertically and horizontally, but never shows the steady
vertical variation that characterizes the lower air. The stratosphere
contains no clouds (except occasional dust clouds), and has a
circulation quite distinct from that of the troposphere, the exact
nature of which, however, has not yet been determined.
The sounding balloon, already mentioned, is one of the four principal
types of aerial vehicle used in the study of the atmosphere, the
others being the pilot balloon, the captive balloon, and the kite.
The sounding balloon, or _ballon-sonde_, is a small free balloon
that carries no human aeronaut, but instead a set of superhuman
meteorological instruments, which register the temperature, the
barometric pressure, and sometimes the humidity continuously and
automatically through the whole course of their journey. The record
is traced on a revolving drum or disk, usually coated with lampblack.
In its commonest form the balloon is made of india-rubber, and when
launched is inflated to less than its full capacity with hydrogen. As
it rises to regions of diminished air pressure it gradually expands,
and it finally bursts at an elevation determined approximately in
advance. A sort of parachute, or sometimes an auxiliary balloon,
insures a gentle fall to the ground. Attached to the apparatus
there is generally a ticket offering the finder a reward for its
return, and giving instructions as to packing and shipping. Sooner
or later it generally comes back. In fact, the large percentage of
records recovered, even in sparsely settled countries, is not the
least remarkable feature of this novel method of research. Thus, of
seventy-two balloons sent up by a Franco-Swedish expedition in Lapland,
forty-one were eventually recovered with their instruments. One of
these fell into a lake and was found after three years.
No instruments are carried by the pilot balloon, which merely serves
to show, by its observed drift, the speed and direction of the air
currents at different levels. The pilot balloon is sighted, while in
flight, through a special form of theodolite, or, preferably, two
theodolites some distance apart. Several ingenious methods have been
devised for computing and plotting its actual course through the air.
Such balloons, apart from their use in scientific research, have become
one of the principal adjuncts of aeronautical undertakings all over
the world, and are also used by artillerists to enable them to make
proper allowance for the deflective effect of the wind on the flight of
projectiles. Hundreds of thousands of pilot balloons were sent aloft
for military purposes during the world war.
Meteorological instruments are sent up attached to kites or captive
balloons whenever--as in connection with weather forecasting--the
observations must be obtained more promptly than would be possible with
the aid of sounding balloons, but such devices can attain only moderate
altitudes. Kites have been raised to about four and one-half miles
above sea level, as compared with nearly twenty-two miles reached by a
sounding balloon and twenty-four miles by a pilot balloon. The average
height of sounding-balloon ascents is about ten miles. As already
stated, balloonists have risen to 6.7 miles. This is a little higher
than the best aeroplane record.
The use of the aeroplane for making meteorological observations is
still quite limited, but will inevitably increase. One other device
gives promise of yielding valuable aerological information, on account
of its ability to rise to extraordinary altitudes. This is a special
form of rocket, recently invented by Prof. R. H. Goddard, which is
propelled by several successive discharges of an explosive in the
course of its upward flight, and with which the inventor thinks it will
be possible to explore the whole vertical extent of the atmosphere.
Meteorological apparatus for use with the Goddard rocket has been
planned by Mr. S. P. Fergusson of the Weather Bureau.
The atmosphere presses down upon the earth with a weight that, at sea
level, amounts to about 14.7 pounds to the square inch, on an average.
This pressure is, at any point, exerted equally in all directions; it
acts, for example, on the whole surface of the human body, and this
means that a man of average size lives under a burden of some seventeen
tons of air. He is not incommoded because the pressure from without is
balanced by that of the air that permeates his body.
The pressure of the atmosphere decreases upward at nearly the same
rate as its density. Thus on mountains and plateaus it is considerably
less than in lowlands. At no place is the pressure invariable, nor
is there a constant relation between pressure and altitude, but,
knowing approximately the average atmospheric pressure over the
earth’s surface, and knowing also the area of the latter, we can
compute in round numbers the total weight of the atmosphere--about
5,000,000,000,000,000 tons. This is about 1/1,200,000 of the entire
weight of the earth.