Silence would reign here, as probably it does reign on the moon.
Sound, as it commonly reaches our ears, depends for its very
existence upon air. Let the concussion of two bodies be ever so
mighty, if there were no air to bear away the vibrations of that
concussion, there could be no crash of sound. True, sound-waves can
be conveyed through a liquid or through a solid as well as through
air; and we might be conscious of the ground’s vibrations, but our
ears would hear no noise.
So an airless world would be a silent world. Without air, supposing
we could ourselves exist, we should hear no trickling brooks, no rush
of waterfalls, no breaking ocean waves, no sighing of the wind, no
whisper of leaves, no singing of birds, no voices of men, no music,
no thunder, no one of the thousand concomitant sound-waves which
together make up the babble and murmur of country and town. Those
only who are perfectly deaf can know what such silence means.
Without air our world would not be in darkness; for light does
not, like sound, depend mainly upon air for its transmission.
Light travels through regions where air is not; and if light is
communicated by waves, they are not waves of air. But though the
absence of air would not deprive the earth of light, it would make a
very great difference in the kind and degree of light received.
Without air the blue sky would be black as ink; stars would glitter
coldly in the daytime beside a glaring sun; deep shadows would
alternate with blinding dazzle, and all the soft tints of sunrise
and sunset would be wanting. Earth would be like the almost airless
moon--all fierce whiteness and utter blackness--with no gray shades,
no rosy gleams, no golden evening clouds; nay, without air there
could be no clouds. On the moon is no twilight; for no air-particles
float about, reflecting the sunlight from one to another, and forming
a soft veil of brightness, to reach further than the direct sunlight
alone can reach.
Sunbeams travel straight to earth, unbending as arrows in their
flight, and unaided they can not creep any distance round a solid
body, though they may be reflected or turned back from it. But the
air breaks up the sunbeams, bends them, diffuses them, spreads them
about, surrounds us with a delicate lacework of woven light. A
sunbeam traveling through space is invisible till it strikes upon
some object. If that object is solid, the light of the sunbeam is
partly absorbed, partly reflected; if the object is transparent,
the sunbeam passes through and onward. Few substances, if any, are
perfectly transparent. We call air transparent, yet it is so only in
a measure. Each sunbeam passing through the atmosphere loses part
of its brightness by the way, and so the great glare of the sun is
softened before it reaches the lower depths of the air-ocean.
The sun’s rays are rays of heat as well as of light. While the
atmosphere softens the glare, giving us shade and twilight, it also
modifies the extremes of temperature, from which, without air, we
should suffer.
When the sun goes down, although we are often conscious of a chill,
it is not the instant and overwhelming chill which we should feel but
for the atmosphere. All day long the sun has been warming the earth
and air. When his direct rays are withdrawn, the warm air for a while
keeps its warmth, and gives over of that warmth to us.
WEATHER
--SIR RALPH ABERCROMBY
The earliest records of weather among every nation are to be found in
those myths, or popular tales, which, while describing rain, cloud,
wind, and other natural phenomena in highly figurative language,
refer them to some supernatural or personal agency by way of
explanation.
The most interesting thing about these mythical stories is the
remarkable fidelity with which they reflect the climate of the
country that gave them birth. For example, from the mythologies of
Greece and Scandinavia we can almost construct an account of the
climate of those two countries by simply translating the figurative
phraseology of their legends into the language of modern meteorology.
Many survivals of mystic speech are still found among popular
prognostics, and especially in cloud names.
In England and Sweden “Noah’s Ark” is still seen in the sky, while
in Germany the “Sea-Ship” still turns its head to the wind before
rain. In Scotland the “Wind-Dog” and the “Boar’s Head” are still the
dread of the fisherman, while such names as “Goat’s Hair” and “Mare’s
Tails” recall some of the shaggy monsters of antiquity.
At a rather later period of intellectual development, the premonitory
signs of good or bad weather become formulated into short sayings,
or popular prognostics. A large number of these are still current in
every part of the world, but their quality and value are very varied.
Some represent the astrological attitude of mind, by referring
weather changes to the influence of the stars or phases of the moon;
others, on the contrary, are very valuable, and, in conjunction with
other aids to weather forecasting, prognostics will never be entirely
superseded, especially for use on board ship. Till within a very
recent period, their science and explanation had hardly advanced
since they were first recorded. In many cases the prognostics came
true; when they failed, no explanation could be suggested why they
did so; neither could any reason be given why the same weather was
not always preceded by the same signs. A halo sometimes precedes a
storm; why does it not always do so? Why is rain sometimes preceded
by a soft sky and sometimes by hard clouds?
About one hundred and fifty years ago the barometer was invented.
Very soon after that discovery, observation showed that, in a general
way, the mercury fell before rain and wind, and rose for finer
weather. Also that bad weather was more common when the whole level
of the barometer was low, independent of its motion one way or the
other, than when the level was high. But as with prognostics, so
with these indications, many failures occurred. Sometimes rain would
fall with a high or rising barometer, and sometimes there would be a
fine day with a very low or falling glass. No reason could be given
for these apparent exceptions, and the whole science of barometric
readings seemed to be shrouded in mystery.
The science of probabilities came into existence about the
commencement of the Nineteenth Century, and developed the science of
statistics. By this method the average readings of meteorological
instruments, such as the height of the barometer or thermometer, or
the mean direction and force of the wind, at any number of places
were calculated, and the results were sometimes plotted on charts so
as to show the distribution of mean pressure, temperature, etc., over
the world.
By this means a great advance was made. Besides giving a numerical
value to many abstract quantities, the plotting of such lines as the
isothermals of Dove conclusively showed that many meteorological
elements hitherto considered capricious were really controlled by
general causes, such as the distribution of land and sea.
Still more fruitful were these charts as the parents of the more
modern methods of plotting the readings of the barometer over large
areas at a given moment, instead of the mean value for a month
or year. Then by tabulating statistics the relative frequency of
different winds at sea, many ocean voyages--notably those across
the “doldrums,” or belt of calms near the equator--were materially
shortened.
Statistics also of the annual amount of rainfall became of commercial
value as bearing on questions of the economic supply of water for
large towns, and much valuable information was acquired as to
the dependence of mortality on different kinds of weather. Of
more purely scientific interest were the variations of pressure,
temperature, wind, etc., depending on the time of day, or what are
technically known as diurnal variations, which were brought to light
by these comparisons.
This branch of the subject is known as “Statistical Meteorology,” and
has advanced very little since it was first developed by Dove and
Kaemtz.
When the attempt was made to apply statistics to weather changes
from day to day, it was found that average results were useless.
The mean temperature for any particular day of the year might be
50°, if deduced from the returns of a great many years, but in any
particular year it might be as low as 40°, or as high as 60°. The
first application of the method was made by the great Napoleon, who
requested Laplace to calculate when the cold set in severely over
Russia. The latter found that on an average it did not set in hard
till January. The emperor made his plans accordingly; a sharp spell
of cold came in December, and the army was lost.
It has now been thoroughly recognized that statistics give a
numerical representation of climate, but little or none of weather,
and that large masses of figures have been accumulated, to which
it is difficult to attach any physical significance. The misuse of
statistics has done much to bring the science of meteorology into
disrepute.
But within the last thirty years a new treatment of weather problems
has been introduced, known as the synoptic method, by which the
whole aspect of meteorology has been changed. By this method, a
chart of a large area of the earth’s surface is taken, and after
marking on the map the height of the barometer at each place,
lines are drawn through all stations at which the barometer marks
a particular height. Thus a line would be drawn through all places
where the pressure was 30.0 inches, another through all where it
was 29.8 inches, and so on at any intervals which were considered
necessary. These lines are called “isobars,” because they mark out
lines of equal pressure. When these charts were first introduced,
the estimation of the value of the mean pressure was so great that,
instead of drawing lines where pressure was equal at the moment,
they were drawn through those places where the pressure was equally
distant from the mean of the day for each place. These lines were
called “is-abnormals”; that is, equal from the mean. After the
isobars have been put in, lines are usually drawn through all places
where the temperature is equal at the moment. These are called
“isotherms,” or lines of equal temperature. Then arrows to mark
the velocity and direction of the wind are inserted; and finally
letters, or other symbols, to denote the appearance of the sky, the
amount of cloud, or the occurrence of rain or snow. Such a chart is
called a “synoptic chart,” because it enables the meteorologist to
take a general view, as it were, over a large area. Sometimes they
are called “synchronous charts,” because they are compiled from
observations taken at the same moment of time.
[Illustration: Typical Forms of Clouds
1, Squall Cumulus; 2, Pillar Cumulus; 3, Cirrus; 4, High Stratus and
Cumulus]
When these came to be examined, the following important
generalizations were discovered:
1. That in general the configuration of the isobars assumed one of
seven well-defined forms.
2. That, independent of the shape of the isobars, the wind always
took a definite direction relative to the trend of those lines, and
the position of the nearest area of low pressure.
3. That the velocity of the wind was always nearly proportional to
the closeness of the isobars.
4. That the weather--that is to say, the kind of cloud, rain, fog,
etc.--at any moment was related to the shape, and not the closeness,
of the isobars, some shapes inclosing areas of fine, others of bad,
weather.
5. That the regions thus mapped out by isobars were constantly
shifting their position, so that changes of weather were caused by
the drifting past of these areas of good or bad weather, just as on
a small scale rain falls as a squall drives by. The motion of these
areas was found to follow certain laws, so that forecasting weather
changes in advance became possible.
6. That sometimes in the temperate zone, and habitually in the
tropics, rain fell without any appreciable change in the isobars,
though the wind conformed to the general law of these lines.
Observation also showed that, though the same shapes of isobars
appear all over the world, the details of weather within them, and
the nature of their motion, are modified by numerous local, diurnal,
and annual variations. Hence modern weather science consists in
working out for each country the details of the character and motion
of the isobars which are usually found over it; just as the geologist
finds crumplings and denudation all over the world, and works out the
history of the physical appearance of his own scenery by studying the
local development of these agencies.
So far the science rests on pure observation--that such and such
wind or weather comes with such and such a shape of isobars. But it
has been found, still further, that the seven fundamental shapes
of isobars are, as it were, the product of so many various ways in
which an atmosphere circulating from the equator to the poles may
move. Just as the motion of a river sometimes forms descending eddies
or whirlpools, sometimes back-waters in which the water is rising
upward, or yet at other times ripples in which the circulation is
very complex, so it now appears that the general movement of the
atmosphere from the equator to the pole sometimes breaks up into a
rotating and descending movement round that configuration of isobars
known as an anticyclone, sometimes into a rotating and ascending
movement round that known as a cyclone, or at other times quite in a
different way during certain kinds of squalls and thunderstorms.
_Isobars, therefore, represent the effect on our barometers of the
movements of the air above us, so that by means of isobars we trace
the circulation and eddies of the atmosphere._
By carrying the general laws of physics into the conception of a
circulating gas, we find that a cold mixed atmosphere of air and
vapor descending into a warmer soil would remain clear and bright;
while a similar atmosphere rising into cooler strata would condense
some of its vapor into rain or cloud. It is by reasoning of this
nature that the origin of some of the most beautiful and complex
forms of clouds has been discovered.
Following out these lines of research, a new science of meteorology
has grown up, which entirely alters the attitude of mind with which
we regard weather changes, and gives rise to an entirely new method
of weather forecasting that far surpasses all previous efforts, and
which explains and develops all that was known before.
On the one hand, the new method not only explains why certain
prognostics are usually signs of good or bad weather, and the reason
why the indications sometimes fail; but also the reason why rain, for
instance, is sometimes foretold by one prognostic and sometimes by a
totally different one.
On the other hand, it not only gives a more extended meaning to all
the statistics which partially represent the climate of a place, and
to the relation of the diurnal to the general changes of weather; but
it also enables new inferences to be drawn, which had hitherto been
impossible from some observations, and explains why other sets of
figures must always remain without any physical significance.
We may notice here an attempt which has been made by one school of
meteorologists to deduce all weather _à priori_ from changes in the
radiative energy of the sun; that is to say, that from a knowledge of
greater or less heat being emitted by the sun, they would treat the
consequent alteration of weather as a direct hydrodynamical problem.
Given an earth surrounded by fifty miles of damp air, and a sun at
varying altitude, and of varying radiative energy, deduce from that
all the diverse changes of weather. This is doubtless a very tempting
ideal, for there is no doubt that the sun’s heat is the prime mover
of all atmospheric circulation; but when we have explained what the
nature of weather changes is, we see that there is little hope that
this method will ever lead to satisfactory results.
Other meteorologists, who lay less stress on the varying power of
the sun, have taken up the indications of synoptic charts, and
endeavored to construct a mathematical theory of cyclones and the
general circulation of the atmosphere. Ferrel, Mohn, Gulberg, Sprung,
and others have all started with the analysis of the motion of a
free mass of air on the earth’s surface, first given by Professor
Ferrel, and worked out, from that and other general principles,
schemes of the nature and propagation of cyclones, and of the general
distribution of pressure over the world.
THE ROMANCE OF A RAINDROP
--ARTHUR H. BELL
Depth of rainfall is, of course, ascertained by means of a
rain-gauge, which measures the amount of water precipitated from
the atmosphere during certain definite periods--usually twenty-four
hours. Sir Christopher Wren has the credit of constructing the
first rain-gauge; but they have been made in various shapes and
sizes since his time; and perhaps none of the instruments in the
meteorologist’s armory is so familiar to the general public as the
rain-gauge. The methods of using the instrument and the meaning of
rainfall statistics are also thoroughly understood nowadays. However,
behind these statistics and the methods of obtaining them, there
are questions of great interest that obtrude themselves when we are
watching the falling rain, and we desire to learn about the history
of the raindrop--for example: Why is a raindrop round? How are
raindrops formed? At what particular time does vapor become visible
as mist? And what are the causes which change this mist into cloud
and subsequently into rain?
The two prime causes of rain are, of course, the sun and the ocean;
and since these two factors do not appreciably vary from year to
year, it follows that the annual rainfall on the earth as a whole,
if it could be measured, would also be found to be invariable. It
is obvious, however, that the rainfall at all places is not equal.
In London, for instance, the average yearly rainfall is twenty-two
inches; but on the Khasi Hills in India it is no less than six
hundred inches. Similar contrasts are observable in other parts of
the world, the differences being due to local geographical conditions.
The starting points in the history of rain are, therefore, heat
and moisture. From the surface of land and water tiny globules or
vesicles of moisture are continually rising into the atmosphere by
the force of the sun’s heat; and the warmer the air the greater the
number of these globules of water the atmosphere is able to absorb.
In this respect the atmosphere may be likened to a sponge, for it
is from the moisture thus retained that the subsequent raindrops
are formed. Most persons are well acquainted with the very familiar
phenomenon which is to be noticed when a glass of very cold water is
brought into a warm room: the drops of moisture which form on the
outside of the glass being among the commonest phenomena in what may
be termed domestic meteorology. There is a similar transformation in
the outside atmosphere; so that when the warm, moist currents of air
flow against the sides of a cold mountain, or it may be against a
body of cold air, there is a reduction in temperature, the atmosphere
is squeezed like a sponge, and the particles of moisture are forced
out of it. The particles then assume the form of cloud, fog, mist,
rain, snow, and hail, as the case may be.
Now, as regards the globules of moisture, the most recent experiments
and observations point to the conclusion that before the drops of
vapor can form, there must be a tiny nucleus of dust upon which the
condensed water may settle. At the centre of every drop of vapor in
a cloud there is probably a little core of dust; and without these
little atoms there could be no rain. These atoms of dust are visible
only under the strongest microscopes; and so minute are they that in
a cubic foot of saturated air it has been calculated that they number
one thousand millions, their total weight being only three grains.
It is commonly considered that the particles of moisture within
a cloud are quite motionless; and when looking at a huge cloud
floating serenely in a summer sky it is difficult not to think of its
constituent parts as being quite at rest. The apparently stationary
cloud is all commotion and movement, the particles within it being
always on the move, some going up and others down. The particles of
moisture, moreover, being probably only about the four-thousandth
part of an inch in diameter, the resistance offered by the air to
their movement is very slight; indeed, as soon as they are condensed
they immediately begin to fall downward, and were it not for the
atoms of dust waiting to catch them the particles would at once
fall to the ground. It is often asked why the vapor, if so readily
condensed in the atmosphere, does not continually fall to the earth.
The answer to this question, it will be seen, is that the moisture,
instead of always pouring down on the earth, settles on the surface
of the atoms of dust. Thus the first downward movement of the
incipient raindrop is arrested by the dust-nuclei which swarm in all
parts of the atmosphere; so that instead of being destroyed as soon
as it is formed, the particle of moisture is preserved and stored for
future use. In realizing the fact that a cloud is always in motion,
the first step has been taken in discovering how a raindrop is formed.
It might be supposed that the raindrops would evaporate as quickly as
they were condensed; but observation of the drops of moisture running
down a window-pane and forming larger drops gives a good idea of
what occurs in the clouds; as also does the fact that in a bottle of
soda-water the bubbles of air overtake one another and, colliding,
make larger bubbles.
One of the principal causes of the manufacture of a raindrop is
to be found in the circumstance that there is a similar process
of amalgamation at work in every part of the atmosphere. It often
happens that a drop of moisture falls downward through a cloud for a
distance of a mile or more; and although it may pass through strata
of very warm air, thus running a great risk of being evaporated
and destroyed, it has also many collisions by which its bulk is
considerably increased, and eventually becomes so heavy that its rate
of progress is very much accelerated. Then, no longer able to float
in the air, it plumps down to the earth as a full-grown raindrop.
Supposing it were possible for an observer to occupy a position
immediately below a cloud, and close enough to see all that was
taking place, he would notice raindrops of all sizes leaping from the
under side of the cloud and plunging toward the earth. The simplest
experiment to get some idea concerning the variation in raindrops
is to expose an ordinary slate for a few minutes during a shower of
rain, and it will be seen by the different-shaped blotches on the
slate that, although the raindrops have all made a similar journey,
they have, nevertheless, contrived to acquire an individuality during
their downward passage. That the raindrops are round admits of a
very simple explanation. They are this shape owing to the action of
capillarity, which in the case of the raindrop acts equally in all
directions.