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:—

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.

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.

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.

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.

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.

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.

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.

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.

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.”

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.

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.

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.