PHYSICAL GEOGRAPHY. METEOROLOGY.
THE ATMOSPHERE—WINDS AND AIR CURRENTS—WIND
PRESSURE—STORMS—RAIN-CLOUDS—WATER-SPOUTS—ATMOSPHERICAL PHENOMENA.
Under this heading we shall find the atmosphere playing a very
important part. The air is composed of oxygen and nitrogen with some
carbonic acid gas and aqueous vapour. We have, under the Chemistry
section, discussed these constituents which unite to make up the air or
atmosphere in the following proportions:—
Oxygen 210·0
Nitrogen 775·0
Aqueous Vapour 14·2
Carbonic Acid 0·8
-------
1000·0
=======
It is a fact that all over the world the same chemical result is found.
Whether we bottle up the air in the valley, or, as Gay-Lussac did, go
up to an elevation of 21,000 feet in a balloon, we shall find the air
of the same chemical composition. In Europe, Asia, Africa, and America,
it is all the same. The pressure is less as we ascend, and we cannot
manage to breathe in very high altitudes so well as upon the ground
_for which we were fitted_, but the air is the same.
The atmosphere, then, is not always equal in density, nor is it quite
transparent. The light from sun and stars is, to a certain extent,
lost, and it has been calculated that the sun’s rays lose one-fifth
part of their brightness passing through the atmosphere. We all
know what the air is. We breathe it, we feel it blowing, we witness
its effects. Were it not composed as it is we should die or go mad;
plants would not live, and the earth would become a desert. Air is
everywhere—invisible; a so-called empty vessel is full of air because
an animal will live in it till the atmosphere has become vitiated
by the carbonic acid from the lungs. Yet air, or rather its watery
portion, is visible when condensed.
Vapour is not perceptible. But how does it become so? We cannot see
the air, how can we see a portion of it? We can answer this question
by illustration. The steam from an engine is not visible on a very hot
day. But when the day is damp and dull the vapour is _condensed_, and
becomes visible; then air appears and is resolved into vapour again.
This change was effected by heating water and then cooling it, when it
came back to water again. This watery vapour is always present in the
atmosphere. Heat, also, is present in the atmosphere, and the sun is
the origin of that heat.
[Illustration: Fig. 714.—High tide and storm on the coast of Schleswig.]
Heat, we know, is the effects of the rapid motion of small particles
of matter, and is radiated from our bodies—so we feel cold; it reaches
our bodies, and we feel warm. So air is heated or cooled by the sun,
not in its absence, except when the earth and air have been so warmed
during the day that the heat is given out by them long after sunset.
We have read of the pressure of the atmosphere in the PHYSICS section,
and that warm air is lighter than cold air, as shown in the ascension
of the Montgolfier balloon. It is this variation of temperature of the
atmosphere that gives birth to one great meteorological agent—viz., the
WIND, which we will now consider.
WINDS AND AIR CURRENTS.
We can easily illustrate the cause of winds. Suppose we have a hot
room and a cold one, and we suddenly open the door of communication
between them, the heated air which has risen to the ceiling of one room
will rush out through the upper part of the opening of the door, and
the cooler current will flow in just above the floor. If we place a
lighted candle in the upper and lower part of the opening we shall see
the flame tending outwards from the heated room, and in an opposite
direction from the cold room. In the centre of the open door there will
be but slight disturbance. So it is in nature. The warm air ascends,
cooler air rushes in to fill the space, and a storm or a breeze is
created. The balance must be restored. The upper current probably
moves one way, and the lower the other way. Thus clouds are said to be
“coming up against the wind” when they are moving in an upper current,
or in a different direction to that the wind is blowing just above the
earth’s surface.
The wind moves, with varying velocity. We have a gentle breeze when the
motion of the air is about five or six miles an hour, a good breeze at
twenty-five miles an hour, a high wind at thirty-five, and a gale at
fifty. Hurricanes travel at sixty and seventy miles an hour, and do
enormous damage. Near the Equator we do not find much wind, and this
fact has caused the name of the Region of Calms, or “The Doldrums” of
sailors, to be bestowed upon that portion of the globe, but this belt
of calm has no fixed position. It follows the sun’s course, and is the
region of greatest heat, and, as it were, the centre of a concentric
circle of currents. The hot air rises and goes away; air rushes
in north and south, and causes what are called the North-East and
South-East Trades, or Trade Winds, owing to their being so useful in
commerce for ships, or to the old meaning of the word trade, a “regular
course.” The calms of the Tropic of Cancer are called the “Horse
Latitudes.”
Readers of the life of Columbus will remember how his crew were
affrighted at the persistency of the wind which bore him across, for
no sail requires shifting, nor is a sheet altered while the vessel
is making way with the “Trades.” Were the earth covered with water,
we should find the trade-winds blowing equally over the surface, but
the varying temperature of the land diverts them. The rarefaction of
the air in the Sahara causes a westerly wind to prevail, which blows
towards the land, instead of the trade wind we might expect to find.
The MONSOONS, again, are caused in like manner, for the ordinary
“trade” from the south-east is changed by the elevation of the heated
air in Central Asia into a south-west wind, and so in the south, in
consequence of the heated air from Australia, the north-west trade
appears as a north-east monsoon, but is altered to a north-west wind.
Nearly all the year round, therefore, we find the two winds, which are
modifications of the “trades,” blowing in different directions and from
different quarters. From November to March there is a north-east wind
north of the Equator, and a north-west wind blows south of the Equator.
From April to September a south-west wind blows at the north, and a
south-east wind at the south of the line. The term monsoon signifies a
“season,” and the changes of these winds give rise to tremendous storms
causing great havoc.
SEA and LAND breezes are really little monsoons; they are caused
by the heat of the sun in just the same way, but with miniature
results. We all know the sea-breeze which comes in as the land gets
hot during the day, for the land warms more quickly than the sea
under equally existing circumstances. So again, in the evening, the
land loses its heat more quickly, and then the cool air flows out
again to take the place of the warmer sea air which is continuing to
ascend. The intensity and regularity varies when the degrees of heat
are most different between land and sea and in tropical regions; and
the varied coast formation will of course affect the wind, but as a
rule the fact may be accepted as plainly explained, sea-breeze in the
morning, land-breeze at night, and amateur sailors in boats at our
watering-places will do well to bear this in mind.
There are a great number of local winds deriving their names from their
direction or influence. We may mention them briefly. The special terms
for winds are—
The North Wind, or Tramontana.
The North-East Wind, or Greco.
The East Wind, or Levanter.
The South-East Wind, or Sirocco.
The South Wind, or Ostro.
The South-West Wind, or Libeccio.
The West Wind, or Ponente.
The North-West Wind, or Maestro-Mistral.
[Illustration: Fig. 715.—On a lee shore.]
The Mistral, or Maestrale, is well known at Nice as the north wind,
while at Toulon it is a north-east wind. The other winds, such as the
Sirocco, which in some places is a warm, damp wind, in Madeira is a hot
wind, and likewise in Sicily, where it is equally warm and damp like
steam. It has different names in various countries, such as Samiel in
Turkey, and sometimes as Föhn in Switzerland, where it may, however, be
a north wind—which, as all travellers know, is a dry and a hazy-weather
breeze, yet sometimes moist. The _Simoon_ is a very hot wind raising
sand-storms in the deserts, and experience has shown it to be very
prejudicial to life in consequence of the fine sand and the tremendous
heat it carries with it. Egypt is subject to another hot wind,
called the _Khamsin_, and the west coast of Africa is subject to the
_Harmattan_, a dry, easterly wind. The cold, dry wind of the Himalaya
is known as the Tereno. In South America there is the same wind, the
_Pampero_ blowing east and south-east. The Euroclydon, mentioned by
St. Paul, is the modern _bora_ over the Adriatic. Malta rejoices (or
laments) in the Gregale, a north-east wind. There are several other
terms, such as the Puna of Peru, a very drying wind; the Purgas in
Labrador, the Tourmente in France, and Guxen in Switzerland. Then we
have the _Hurricane_, from “Ouracan” of the Caribs; the _Typhone_, or
Tae-fun of China, so called from the dreaded god Typhon of Egypt; and
the _Tornado_—all very violent winds, and circling round, causing, so
to speak, whirlwinds, by which trees are uprooted, and houses destroyed.
The measure of the velocity of wind is performed by anemometers, which
record the velocity in feet per second, and the amount of pressure. The
anemometer is a well-known apparatus, with its four arms terminating
in “cups” and a “tablet” anemometer, which is more or less disturbed
or deflected from the vertical line by each gust of wind, and thus
the score of degrees is marked by an indicator, which is moved as the
tablet is deflected. We annex a table of wind pressure and velocity—
PRESSURE OF THE WIND.
+------------------+------------+----------------------+
| Velocity. |Pressure per| |
+---------+--------+ sq. foot. | |
| Miles |Feet per| Rouse and | Description of wind. |
|per hour.| second.| Smeaton. | |
+---------+--------+------------+----------------------+
| 1 | 1·47 | 0·005 | Very gentle. |
| 2 | 2·93 | 0·020 | |
| 3 | 4·40 | 0·044 | Light airs. |
| 4 | 5·87 | 0·079 | |
| 5 | 7·33 | 0·123 | Light breeze. |
| 10 | 14·67 | 0·492 | |
| 15 | 22·00 | 1·107 | Brisk breeze. |
| 20 | 29·34 | 1·968 | |
| 25 | 39·67 | 3·075 | Strong breeze. |
| 30 | 44·01 | 4·429 | |
| 35 | 51·34 | 6·027 | High wind. |
| 40 | 58·68 | 7·873 | |
| 45 | 66·01 | 9·963 | Gale. |
| 50 | 73·35 | 12·300 | |
| 60 | 88·02 | 17·715 | Heavy Gale. |
| 80 | 117·36 | 31·490 | Hurricane. |
| 100 | 146·70 | 49·200 | Tornado. |
+---------+--------+------------+----------------------+
The south-west wind is more constant than any other, and the west
wind in our islands is more frequent than the east; tables have been
compiled showing the average number of days upon which the winds blow
from different quarters, but need not be quoted. Storms can generally
be anticipated by the barometer, which falls very quickly for “wind.”
The quarter whence the breeze may be expected is often indicated by the
streamers of clouds, or “mare’s tails,” across the sky; though we must
admit the _opposite_ direction to that anticipated by casual observers
may often prove the right one.
Hurricanes and tornadoes are really whirlwinds in motion. The rotatory
movement of the air is from right to left in the northern hemisphere,
and from left to right in the southern—that is, in the opposite
and same directions respectively as the hands of a watch move. The
whirlwinds are caused by two currents of air meeting at a certain
angle, just as a whirlpool is the result of opposing currents of water.
[Illustration: Fig. 716.—Effects of storm at Halligen in 1834.]
The use of the wind in nature cannot be over-estimated. It is
frequently destructive and terrible in its effects, but these
comparatively trifling damages are as nothing when weighed against the
advantages conferred upon mankind by the wind and the currents of the
atmosphere. The north cold is tempered by the warm south wind. The
pollen and the seeds of plants are borne on the wings of the wind, and
the clouds are carried over the land to “drop fatness” upon our fields.
The want of free circulation of air is very injurious. Witness the
terrible affliction of _goitre_, so prevalent in the closely shut-in
valleys such as the Rhone Valley, where cretinism or congenital idiotcy
is distressingly prevalent.
VAPOUR AND CLOUDS.
Vapour, as we have heard, is invisible, and is produced by heat. As the
visible steam (which is invisible as it issues from the safety valve
at the actual aperture, and nearly invisible altogether on a hot day)
is produced by combustion, so vapour is produced by the heat of the
sun’s rays. But there are some observations to be made respecting these
rays, which are the cause of vapour, and therefore of cloud, rain, dew,
frost, ice, snow, and water all over the earth; and we must look at the
circumstances closely.
Those who have followed us through this volume will remember that at
the end of Chapter VIII. we remarked upon the spectrum, and made a few
observations respecting the _heat_ spectrum, and the velocity of light
rays, which became too rapid to be observed, and then they developed
heat—_invisible heat_—produced by non-luminous waves, which proceed
from the sun as surely as visible rays or light. Professor Tyndall has
written very pleasantly upon this subject, and, with his clear leading,
any reader can study for himself.
We have now arrived at the conclusion that there are visible and
invisible rays giving us respectively light and heat. These latter are
the means whereby the ice is melted, and by which water is evaporated
to vapour, and formed into CLOUDS when it is chilled or condensed. Here
is another link in the beautiful chain constructed by Nature. We cannot
penetrate far into any portion of the system of the universe without
being struck with the wondrous harmony that exists between every
portion of it. Thus heat and light, vapour, cloud, rain, dew, and ice
are all intimately connected.
A cloud, then, is a visible body of vapour in the atmosphere, which
is supported by an invisible body of vapour. It will remain thus
invisible so long as the atmosphere is not saturated with moisture.
The air can contain a great quantity of moisture without its being
rendered visible, and so when the day is hot we see no steam from the
locomotive. It is absorbed into the dry atmosphere. But when the day
is “damp” we find that the air has nearly as much moisture as it can
carry, and the steam is condensed, a portion falling in tiny drops like
rain. This is proved every day in cold weather when ice is found in the
windows—the cold air has condensed and frozen the water breathed out
from our lungs, and _snow_ has been known to fall in a ball-room when a
cold current of air was admitted.
People are sometimes apt to think that if the sun were very hot,
glaciers, and such icy masses, would diminish; but we think after what
has been said respecting the power of the sun’s rays to evaporate
water, all will see that the contrary is the fact. Without sun-heat we
should have no cloud, and as clouds give us rain and snow and ice and
glacier, we must come quickly to the conclusion that glaciers and snow
are the direct _results_ of the heat of the sun. The “light” rays of
the sun do not penetrate snow, and that is why our eyes are so affected
in snowy regions. The poor _Jeannette_ sufferers a short time since
were blinded by reflected light, and dark spectacles are worn on all
Alpine expeditions. The _invisible_ rays, as we have said, dissolve the
ice into rivers.
The atmosphere produces clouds by expansion of vapour, which chills or
cools it, and it descends as rain. To prove that expansion cools air
is easy by experiment, but if we have no apparatus we must make use of
our mouths. In the body the breath is warm, as we can assure ourselves
by opening our mouths wide and breathing upon our hands. But close the
mouth and blow the same breath outwards through a very small aperture.
It is in a slight degree compressed as it issues from the lips, and
expanding again in the atmosphere feels colder. Air compressed into a
machine and permitted to escape will form ice.
[Illustration: Fig. 717.—Cumulus cloud.]
Water is present in clouds which assume very fantastic and beautiful
forms. We know nothing more enjoyable than to sit watching the masses
of cumuli on a fine afternoon. The grand masses built up like the Alps
appear to be actual mountains, and yet we know they are but vapour
floating in the air, and presently to meet with clouds of an opposite
disposition, and produce a thunderstorm with torrents of rain. Those
who will devote a few minutes every day to the steady examination of
clouds, will not be disappointed. They give us all the grandeur of
terrestrial scenery. Mountains, plains, white “fleecy seas,” upon
which tiny cloudlets float, and low upon the imaginary yet apparent
horizon, rise other clouds and mimic mountains far and farther away in
never-ending distance.
A pretty, light, feathery cloud, with curling tips and fibres, is known
as _cirrus_, and exists at a very great elevation. Gay-Lussac went up
in a balloon 23,000 feet, and even at that height the cirri was far
above him in space. We can readily understand that at such an extreme
elevation they must be very cold, and they are supposed to consist
of tiny particles of ice. Such clouds as these are very frequently
observed at night, as cirro-cumulus around the moon, and a yellowish
halo, apparent to all observers, is thought to be coloured by the icy
particles of the lofty _cirrus_. The beautiful and varied phenomena of
perihelia, etc., are due also to the snowy or icy flakes of the cirri
and cirri-cumuli, caused by the refraction of light from the frozen
particles. These cirri clouds are indicative of changeable weather as
“Mares’ tail” skies, and long wisps of cloud, foretelling storm.
The _cirro-cumulus_ is the true “mackerel” sky, and is formed by the
cirri falling a little and breaking off into small pieces of _cumulus_,
which is a summer (day) cloud generally, and appears in the beautifully
massive and rounded forms so familiar. The _stratus_ is, as its name
implies, a cloudy layer formed like strata of rock. It is generally
observable at night and in the winter. It often appears suddenly in the
sky consequent upon diminished pressure or a rapid fall of temperature.
It is low-lying cloud sometimes, and at night forms fogs.
[Illustration: Fig. 718.—Cirrus cloud.]
The _cirro-stratus_ is perceived in long parallel lines, and indicates
rain; when made-up rows of little curved clouds it is a certain prophet
of storm, and when viewed as haze is also indicative of rain or snow.
“Mock-suns” and halos are often observed in the cirro-stratus.
The _nimbus_ is the rain-cloud, or condition of a cloud in which rain
falls from it. It is upon this rain-cloud we can perceive the rainbow,
and on no other cloud, but otherwise only in the sky.
We have now seen the varieties of cloud and their common origin with
fogs and mists, which differ from them only in the elevation at which
they come into existence, according to the condition of the atmosphere.
The uses of clouds are many and varied. Their first and most apparent
use seems to be the collection and distribution of rain upon the earth.
But besides this, they shelter us from the too great heat of the sun,
and check the evaporation at night. Supposing we had no clouds we
should have no rain. If we had no rain the earth would dry up, and the
globe would appear as the side of the moon appears—a waterless desert.
The invisible vapour in the atmosphere will produce cloud, but the moon
can have no atmosphere in that sense. Vapour will also absorb heat, and
intercept the sun’s heat rays, acting much as clouds do in preventing
radiation and great changes of temperature.[33]
All animals and plants depend upon moisture in the atmosphere as much
as upon the varying degrees of warmth. A dry east wind effects us
all prejudicially; warm, soft airs influence us again in other ways.
Air will be found drier as a rule in continents than in islands or
maritime districts, and this will account for the clearness of the sky
in continental regions. Fogs and mists arise when the air is what is
termed saturated with moisture, and colder than the earth or waters
upon it. So the celebrated and dangerous fogbanks of Newfoundland arise
from the warm water of the Gulf Stream, which is higher in temperature
than the air already saturated. And the same effect is produced when
a warm wind blows against a cold mountain; the air is cooled, and
condenses in cloud.
The cooling of the breath by the exterior air is exemplified in winter
when we can perceive the vapour issuing from our mouths as we speak.
[Illustration: Fig. 719.—Storm clouds.]
RAIN, SNOW, AND DEW.
Rain is produced by the condensation of vapour. “Vesicular vapours, or
minute globules of water filled with air,” compose the clouds, and at
last these vesicles form drops, and get heavy enough to come to the
ground. Perhaps they are not sufficiently heavy to do so, and then
they are absorbed or resolved into vapour again before they can get so
far, because the lower strata of air are not yet saturated, and can
therefore contain more moisture.
On the other hand, we may experience rain from a cloudless sky. This is
no very uncommon case, and occurs in consequence of the disturbance of
the upper strata when warm and cold currents come into collision and
condense the vapours.
Rain is very unequally distributed. We shall find that the region of
calms, which we mentioned in a former page, is also the zone of the
greatest amount of rain. The heated air rises and falls back again,
there being little or no wind to carry it away. The rainy season,
therefore, sets in when a place enters the zone of calms. Equatorial
districts have two rainy seasons, as they enter twice a year into the
region of calms, but most places have only a wet and dry season, while
north and south of the calm region we find rainless districts, or zones
tempered by the trade winds, which are dry winds.
But if we suppose—as indeed is the case in South America—that these
dry winds happen to come in contact with a cool mountain, the moisture
of the air is precipitated in rain. In Australia, on the contrary, we
have portions of land actually burnt up for want of rain, because the
mountain chain breaks the clouds, so to speak, on a limited corner of
the island, while the interior is parched. The winds also coming over
India from the Bay of Bengal discharge clouds and rain in the Himalayan
slopes. So we perceive that the situation of mountain chains have
much to do with the rain-fall, and of necessity, therefore, with the
vegetation and fertility of the land. This is another noticeable link
in the great chain of Nature.
[Illustration: Fig. 720.—Meteorological Observatory, Pic du Midi.]
Perhaps it may now be understood why westerly and south-westerly winds
bring rain upon our islands, and why the counties such as Westmoreland
and Cumberland and those in Wales receive more rain than any other part
of the United Kingdom. Seathwaite, so well known to tourists in the
lake district, has the proud position of the wettest place in these
islands. We find that when the westerly wind sets in it has come across
the warm Atlantic water and become laden with moisture, which, when
chilled by the mountains, is precipitated as rain.
The amount of rain that falls in the United Kingdom is carefully
measured by rain-gauges, some of which are extremely simple. The water
is caught in a funnel-mouthed tube, and measured in a measuring glass
every four-and-twenty hours. Thereby we can tell the annual rainfall
in any given district, whether it be twenty inches or a hundred. One
inch of rain actually means one hundred tons of water falling upon one
acre of land. Therefore, if the annual report of rainfall (including
all moisture) be twenty inches, we have an aggregate of 2,000 tons of
water upon every acre of surface within the district. Twenty inches
is a very low estimate. Some places have an annual rainfall of forty
or fifty inches. In Cumberland we find 165 inches has been recorded!
If we then multiply these last figures we get the enormous quantity of
16,500 tons of water upon every acre of land in the district in one
year. It is reported from India that in the Khasia Hills the average is
610 inches, which must be the maximum rainfall in the world. At other
places, in the north-west provinces, the fall is only seven inches.
Sometimes in tropical rains we find fifteen inches of rain in a day,
and that has been exceeded.
We can now judge of the enormous amount of moisture carried up by the
sun and dispersed over the earth in rain, which swells our brooks and
rivers, cleanses the air of its impurities, supplies our springs,
carries with it into the sea lime from the rocks for the shells of
marine animals, and then leaving its salts, is again evaporated to form
clouds, which discharge the fresh water continually upon the earth in a
never-ceasing rotation.
SNOW.
“We all know what SNOW is,” you will say, perhaps. Well, then, will
any ordinary young reader tell me what he knows about snow? “It falls
from the sky in white flakes,” says one. “It’s frozen rain,” remarks
another. “Why, snow _is_ snow,” says a third. “There’s nothing like it;
it’s white rain-water frozen.”
[Illustration: Fig. 721.—Crystals of snow.]
The last answer we received is the nearest of all. Snow is not
snow, paradoxical as that sounds. _Snow is Ice!_ Flakes of snow are
ice-crystals—white, because reflecting light. In the section of
MINERALOGY we mentioned crystals, which are certain definite shapes
assumed by all substances, and we gave many examples of them. Just as
alum crystallizes and rock crystal assumes varied and beautiful forms,
so ice crystallizes into six-rayed stars.
It is to Professor Tyndall that the world is chiefly indebted for the
descriptions of snow crystals and ice flowers. In his work upon “Heat
as a Mode of Motion,” this charming writer shows us the structure
of ice flowers. He describes a snow shower as a “shower of frozen
flowers.” “When snow is produced in calm air,” he says, “the icy
particles build themselves into stellar shapes, each star possessing
six rays.” We annex some drawings of snow crystals, which are, indeed,
wonderfully made. Hear Professor Tyndall once again:—
“Let us imagine the eye gifted with a microscopic power sufficient to
enable us to see the molecules which compose those starry crystals: to
observe the solid nucleus formed and floating in the air; to see it
drawing towards it its allied atoms, and these arranging themselves as
if they moved to music, and ended by rendering that music concrete.”
This “six-rayed star” is typical of lake ice also.
[Illustration: Fig. 722.—Ice crystal.]
Snow sometimes reaches us in a partly melted condition; under these
circumstances it is called _sleet_, and snow being much lighter than
rain (ice is lighter than water), it descends less directly, and
represents about one-tenth the depth of the rain-fall. The use of snow
in warming the earth is universally acknowledged, and as it is such a
bad conductor, a man in a snow hut will soon become unpleasantly warm.
[Illustration: Fig. 723.—Ice crystal.]
Ice is only water in another form, and snow is ice; and it is the air
in the snow that gives it warming properties. These are all simple
facts, which any one by observation and careful reading and study may
soon ascertain for himself. We have another frozen fall of water from
the clouds—viz., hail, which may possibly be the development of _sleet_.
HAIL is formed by the falling rain being frozen in its descent, or when
different currents meet in the atmosphere. A hail-storm is accompanied
with a rushing sound, as if the hail-stones were striking against each
other. They are very destructive, and actual hail showers occur in
summer more frequently than in winter, and a peculiarity noticeable
with regard to hail is its infrequent occurrence during the night.
Records of destructive hail storms are plentiful. The hail assumes a
great size, weighing sometimes as much as two ounces, and measuring
several inches round. Thunder and lightning are very frequent
accompaniments of hail showers.
DEW is moisture of the atmosphere deposited on a cool surface—another
form of condensation, in fact. Cold water in a tumbler will produce
a “dew” upon the _outside_ of the glass when carried into a warm
atmosphere. Such is the dew upon the grass. It is produced by the air
depositing moisture as it becomes colder after a warm day when much
vapour was absorbed. Warm air can hold more water than cold air, and,
the saturation point being reached, the excess falls as _dew_ at the
dew (or saturation) point. We have previously remarked that one use of
clouds was to prevent rapid radiation of heat which they keep below.
Under these circumstances—viz., when a night is cloudy—we shall find
much less dew upon the grass than when a night has been quite clear,
because the heat has left the atmosphere for the higher regions, and
has then been kept down by the clouds; but on a clear night the air has
become cooled rapidly by radiation, and having arrived at saturation
point, condensation takes place.
Dew does not _fall_, it is deposited; and may be more or less according
to circumstances, for shelter impedes the radiation, and some objects
radiate less heat than others. Hence some objects will be covered with
dew and others scarcely wetted.
When the temperature of the air is very low,—down to freezing
point,—the particles of moisture become frozen, and appear as
_hoar-frost_ upon the ground. Thus dew and hoar-frost are the same
thing under different atmospheric conditions, as are water and ice and
vapour.
We have now come round again almost to whence we started. We have
seen the land and water, and the parts that water, in its various
forms, plays upon the land, and its effects in the air as rain, etc.
We have noticed the winds and air currents as well as the ocean and
its currents. We know what becomes of rain and how it is produced, and
how the sea works upon the shore, and how clouds benefit us. There are
besides some less common phenomena which we will now proceed to examine.
FOOTNOTES:
[33] _See_ “Molecular Physics” (Tyndall).