 Chapter 14 of The Oceans of Air, Meteorology for Beginners This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by Elizabeth Miles. The Oceans of Air, Meteorology for Beginners by Agnes Gebern. About Evaporation Chapter 14 About Evaporation If there were no water vapor hidden away in the secret recesses of the ocean of air, we should not only have no dew, no clouds, no rain, we should not only see all green things wither for lack of moisture, but also the sun would shine down upon us with a fierce and glaring heat, such as now we can hardly even imagine. The veil of floating moisture in the atmosphere acts in two ways as a protection. It shelters us from excess of heat and also from excess of cold. It keeps off some of the sun's burning glare from us and it also keeps in a great deal of the earth's warmth for us. As a screen, it gently dims the glare and steals some of the heat from the traveling sunbeams. The tiny particles of invisible water help greatly to tone down the fierce straight radiance of the king of day, but perhaps the work which is done by the vapor at night is even more important. All day long the ground gathers heat from the sun's rays as they beat downward and when the sun disappears the stored up warmth begins at once to pour itself out into space. If there were then no floating vapor above to act as a sheltering screen, the ground would lose its heat with very great speed and the suddenness of the chill would be fearful. The shielding vapor prevents this like a soft unseen blanket holding in the earth's warmth. We all know how much warmer we feel on a cloudy night than on a clear night. The clouds act as a visible blanket or a counterpane radiating back to earth its escaping heat. If there are no clouds the ground loses warmth far more rapidly. Though even then there is always the soft fine veil of vapor, one of earth's outer garments. A man walking up a mountain or rising in a balloon will find certain differences in the ocean of air as he ascends if he has with him the right instruments for testing the state of the atmosphere. He will find that the air has grown more thin or rare. He will find that it has grown colder. He will find that it has grown drier. On the summits of great mountains the glare of sunshine is often almost overpowering even while the frost is so intense that the sun's rays are powerless to melt the snow. One reason for the dazzling glare is that the air contains there much less vapor than at lower levels. The shielding screen of vapor has grown thin and poor. The floating vapor is not only more in amount below than above. It also varies exceedingly in different places and it keeps on varying. Sometimes the air around us is damp. Sometimes dry. Sometimes it is ringing wet. Sometimes parching. These changes we know from our own sensations. Everything wet or moist on earth gives off gentle streams of vapor into the air. More or less abundant streams according to its degree of warmth. Day and night, summer and winter, the air is at work receiving or drinking up water from seas and rivers, ponds and streams, clouds and fogs, earth and grass, plants and animals. In short, from all damp surfaces of every imaginable kind and description. There are a few solid substances which steadily give off their material somewhat after the fashion of liquids. A lump of camphor, for instance, wastes slowly away, growing smaller as its particles pass into the air. All liquids dry up slowly or fast. If there were no air, they would still dry up at the same rate or even more rapidly. We may talk and talk truly in a sense of the air taking in or drinking up moisture, but the air does not cause evaporation. A damp surface evaporates of itself and the air receives the particles of water as they leave the damp surface. This is constantly going on, where water is to be found there as a rule a soft vapor creeps gently away into the atmosphere. The same thing happens though this is less generally known with water in the solid form. Both ice and snow evaporate, creeping slowly away, particle by particle. Sometimes a whole slight fall of snow will vanish thus without any apparent thaw. More commonly, the fall is too heavy or the thaw comes too soon for this gentle drying up to be noticed. So the air is ever at work taking in moisture from every possible quarter until full, only until full. Air will not hold unlimited supplies of hidden vapor. It will only hold a certain amount, just so much and no more. Directly air is well soaked, saturated is the right word. It refuses to take in another particle. Then evaporation grows languid and perhaps even ceases. Alondra knows well from sad experience the difference between drying her clothes on a fine day and on a wet day. She may be ignorant of the scientific cause, but she is aware of the fact. One week she hangs out her rows of wet garments and a soft dry wind sweeps past, stealing all the moisture out of them with delightful rapidity. Another week she leaves them to hang for hours and the damp motionless air has almost no effect, except indeed to hinder evaporation. After hours of exposure the clothes are scarcely less wet than when first put out, as she tersely expresses it, they won't dry. But the garments are not willful. The cause lies in the overladen condition of the air, already so full of vapor that it can receive no more. A wind is always good for drying purposes. The air around the wet clothes takes in some of the moisture, becomes saturated and declines any more. If no breeze stirs, the soaked air remains hanging about the clothes and drying is at a standstill. But if a wind blows, the wet air passes on and fresh dry air comes to carry off further supplies of vapor and to be in its turn speedily replaced. Thus the work proceeds merely. If you hold a sponge over a basin of water, letting it touch the surface, the water will soak upward into the sponge till the ladder is full and can't hold no more. Just so with the atmosphere, it hangs like an enormous sponge over all water surfaces, and the moisture soaks upward into the air till it is full. There is, of course, a difference. A sponge draws the water up into itself by means of what is called capillary attraction. The air does not draw up vapor, but merely receives the vapor which passes off from water. Still, there are points of resemblance. For instance, the drier a sponge, the more water it can hold, and the drier the air, the more rapid evaporation is found to be. This drying process goes on over the whole world, even in the far north and the far south, where ice and snow reign supreme, the same continues. The surfaces of ice and snow are passing slowly away into the atmosphere. In our temperate regions, we are perpetually aware of the fact of evaporation. More particularly, it is apparent in hot and dry summers. Then streams, vanish, and rivers fall low, ponds disappear, and springs fail. But it is in more southern lands and seas that evaporation is seen in full vigor. The Mediterranean Sea is almost enclosed by land, having only one narrow opening into the outside ocean through the Straits of Gibraltar. The whole of South Europe and the whole of North Africa are drained into the Mediterranean, countless streams and rivers pouring thither by day and night, their volumes of water gathered from surrounding tracts of country. Under these circumstances, when we picture to ourselves what is meant by such a watershed, we should certainly expect to find a current flowing from the Mediterranean into the outer ocean through the Straits of Gibraltar. But no, the drying up of the surface of the Mediterranean is so enormous that apparently it more than balances the mighty influx of rivers from north and south, east and west. A steady, sometimes very powerful current sets in from the Atlantic through the Straits to keep the Mediterranean surface level with the ocean outside. Water always seeks persistently to maintain everywhere the same level. It can never rest for an instant with a slanting surface. The smallest inequality means an immediate flow of water from the higher to the lower part, and the greater the inequality, the more rapid will be the balancing rush. So long as the Mediterranean is joined to the ocean, though by only one inlet, its surface must be level with that of the ocean. If its river and rain supply is greater than its drying up, a stream must pour out to the ocean. If its drying up is greater than its river and rain supply, a stream must flow in from the ocean. If the two are exactly equal, no stream need pass either way. Since we find a stream setting in from the ocean, we judge that more water dries up from the surface of the Great Sea than is supplied by all the vast expanse of land and rivers to north and south, not to speak of the rainfall on the sea itself. There may be, however, a further complication in the shape of a probable undercurrent flowing outward as the upper current flows in. So we must not speak too positively. Along the shores of India, careful experiments have been made to test the amount of evaporation. It is found that about three-quarters of an inch in depth daily passes upward from the whole sea surface. This, if carried on equally all the year round, would give an annual drying up of over 21 feet in depth. Even if, allowing for inequalities, we put the quantity at 15 or 18 feet, it is startling enough. Some mighty power must be at work to lift this tremendous weight of water out of the ocean. When one pictures what is meant by a mass of water 15 feet in depth and hundreds of miles in extent, one gains some dim idea of the force which is needed to bring about such a result. On Earth, no such force exists. If our Earth stood alone in the universe, this vast upheaving of ocean waters into the atmosphere would not be accomplished. When we see the steam which pours from a kettle or a boiler changing fast into white mist, we know well enough what force is at work. It is the force of heat. The glowing fire has caused the water to boil, driving it forth the steam into the air. In the drying up of waters upon Earth, in the evaporation of ocean surfaces, heat is again the working power. The sun is the fire which supplies the needed heat. The heat supplied by the sun enters into the water, drives its particles farther apart, and causes it to rise gently as invisible vapor into the air. The heat which works on Earth as fire all comes from the sun. If no sun had ever shown in the heavens, we could have on Earth no burning coals, no flaming gases. In earlier chapters, it has been explained how combustion may be either a quick or a slow process. Quick, giving out much heat in the short time, slow, giving out the same amount gradually in a longer time. When combustion springs from earthly fire derived from the sun, it is generally more sharp and quick. When the sun acts directly, the combustion is more usually calm and slow. The same difference is seen in the changing of water into vapor. It may be a quick or a slow process, giving out much heat or little heat in a short time. If the transformation is brought about by earthly heat derived from the sun, but acting by earthly methods, it is short and sharp. Much heat is given out and the water rushes away in scalding steam. But if the great sun himself does the work acting directly on the water, he does it softly, calmly, with no fuss or flutter. The transformation takes place in silence and the giving out of heat is gentle because gradual. End of Chapter 14 Chapter 15 of the Ocean of Air, Meteorology for Beginners This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by Elizabeth Miles, The Ocean of Air, Meteorology for Beginners by Agnes Shabern, About Condensation. Chapter 15 About Condensation Just as a sponge will hold a certain quantity of water and no more, so the air will hold a certain quantity of vapor and no more. But the sponge will hold the same amount of water at all times. The air will not hold the same amount of vapor at all times. Whatever supply may be hidden in the air at any particular moment, one never can be sure how long that supply will be able to stay there. For the question, how much water can the air contain, hinges on another question. How warm is the air? Warm air can hold more vapor than cold air. Hot air can hold more vapor than warm air. The carrying capacities of air for vapor depend upon its warmth or coldness. Happily, the very warmth which makes water to evaporate more quickly also makes the air able to hold a larger supply of vapor. So, in the long run, though not instantly, a change of temperature tells in both ways. Suppose a layer of cold air lies over a lake, receiving more and more vapor until it is quite full. The drying of the lake then grows very languid, from the fact of the saturated air weighing down upon it and refusing to take in any more vapor. Perhaps it may even stop altogether. If by any means you could suddenly make the cold mass of air warmer, it would be full no longer. It would receive from the lake fresh vapor supplies until again saturated. Another rise of temperature would bring about the same result. These variations of temperature do in fact happen constantly from different causes. Now let us view the question the other way. A layer of warm air floats over a pond and water is vapor, passes freely into the air till the latter is, in child's parlance, as full as it will hold. Then a current of cold air from elsewhere comes creeping into the warm air, gaining some heat for itself and also cooling the warm air rapidly down. Perhaps two or three degrees. What is to happen next? Cold air cannot possibly hold so much vapor as warm air. The warm air, having been saturated to its utmost extent, must, now it has grown colder, be more than saturated. If you have a large sponge full of water, as full as it will hold, and you give it a slight squeeze, what happens? You make the sponge smaller by your squeeze. Consequently, it is unable to hold so much water. Consequently, a few drops are pressed out. Something analogous to this happens with the atmosphere. The current of cold air acts upon the warm as your hand acts upon the sponge, giving it virtually a gentle squeeze. Thereupon, some drops of water are pressed out. Occasionally, no doubt, the new current of air, though cold, might be so dry as to have power to drink in all the moisture which the other air could no longer hold. In that case, no change would be apparent. Very often, however, the results of the squeeze are plainly manifest. Manifest? How? As already said, by drops of water being squeezed out. The analogy of the sponge, however, fails us here, for the drops from the sponge are large and they fall to the ground. The drops pressed out of the atmosphere are extremely small and light, so much so that they float in the air, like very fine water dust, as one has said. All of them, so floating in close company, make a mist or fog. This is how the mist comes from the steam which pours from the funnel of a steam engine. The heat of the engine fires turn the water in the boiler to steam. When the steam first rushes out into the air, it is invisible, but the air outside, already more or less laden with vapor, cannot in one moment receive such a quantity of vapor. So a good deal of it has to turn hastily into tiny floating drops of water streaming like a white cloud from the engine. As fresh relays of unsoked air come in contact with this cloud of fog, the minute drops are sucked up and soon the whole vanishes. Whether the mist vanishes slowly or fast depends on the kind of day. If the air is dry and thirsty, the newly made fog seems to melt away as if by magic. But if the air is nearly saturated with damp, the tail of fog from the engine hovers a long while in the air before it can be hidden away. On a dry day by the seaside, one may see the funnels of distant steamers carrying a short banner of steam which fast evaporates. On a damp day, long trails of white or gray fog from those same funnels will lie far around the horizon. We have already learned that the passing of water into vapor is called evaporation. The passing of vapor into water is called condensation. These two evaporation and condensation are exactly the opposite each of the other. If a tumbler of water is emptied through drying up, all the water passing into the air is vapor, that is evaporation. If the escaping vapor were all caught as it left the tumbler, were imprisoned, and through the power of cold were restored as water to the tumbler, that would be condensation. Evaporation is caused by heat. The greater the warmth of a liquid, the more quickly it evaporates. Condensation is caused by cold. The greater the cold of a vapor, the more quickly it condenses. When water is evaporated, not one particle of that water is destroyed. It has only changed into vapor, vanishing from our sight. When vapor is condensed, not one particle of that vapor is destroyed. It has only changed into water, returning to our sight. No fairy tales or conjuric tricks were ever so truly wonderful as this disappearing and reappearing of water. We are also used to the everyday marvels of nature that we forget to be surprised. Yet if this were less common, if we had never known or heard of the transformations of water until today, if we suddenly were to see, for the first time, the whole circle of water changes acted out, they would be, to us, a series of miracles. It was explained earlier how, by the burning of a candle, water is formed. Not liquid water, but invisible vapor, slipping away silently into the atmosphere. Also, it was explained that if a cold tumbler is held over the flame, the glass is dimmed by a soft mist made of tiny water drops. This is a case of condensation. The cold glass acts like the cold breeze, cooling down the warm air and pressing out a little of its moisture. On a frosty winter's day when we take a walk, a tiny fog or a cloud rushes from our lips with every breath. Warm air laden with moisture pours from the lungs. Meeting the cold air without, it is abruptly chilled and has to part with some of its vapor, which changes into a mist of water drops. The mist floats for a second till the dry surrounding air has had time to drink it up. Here again is condensation, followed by renewed evaporation. In a railway train compartment filled with people, if both windows are shut, the glass will become thick with haze. This haze is formed of water, and the colder the outside air, the more rapidly it forms. Within the compartment, the air grows warm and damp, though heat and moisture poured from several throats. Being warm, it has no difficulty in carrying the supply of vapor, but the cold air outside chills the windows and the cold glass chills the layer of air next to it. Then some of the vapor is pressed out by the shrinking air and is laid upon the panes as a collection of minute drops. If this goes on, one layer of air after another making its deposit for where several people are breathing and moving, the atmosphere cannot be at rest. The tiny drops become presently so abundant as to run one into another, joining to make big drops which flow downward. When the windows are thrown open, the moisture on the panes soon vanishes. The outside air may be cold, but unless extremely damp, it will be able to receive so small a quantity of additional vapor. There once more is seen condensation, followed by evaporation. The two are perpetually at work on earth, alternating and in opposition, neither knowing repose. The amount of vapor in the air at any one place is always changing, rarely for two minutes, precisely the same, unless we accept the dead level of dryness attained to by a tropical desert. All over the whole earth, water is vanishing and reappearing, going out of sight into the air and coming into sight out of the air, being evaporated and being condensed, passing from the liquid to the gaseous form and from the gaseous to the liquid form. Evaporation and condensation work in apparent opposition, yet each works into the other's hands, so to speak. Between the two is a perfect balance, which results in order and beauty, circulation and life upon our globe. When through condensation actual drops of water appear, whether is mist or fog, dew or rain, the process is also described as precipitation. The vapor-laden air gives out or drops or precipitates some of its surplus moisture. End of Chapter 15 Chapter 16 of The Ocean of Air Meteorology for Beginners This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. The Ocean of Air Meteorology for Beginners by Agnes G. Byrne Chapter 16. Dew, Mist and Fog On a fine autumn day, the sun beats down for hours, warming the earth and the atmosphere, and the warmed air drinks steadily at every damp surface within reach, till by evening it is well filled with moisture. Then the sun sets, and there are no sheltering clouds to hold in, or reflect back the heat of the ground. It pours out fast into space, despite all that the soft veil of floating vapor in the air can do. As the ground cools, it helps to cool the layer, footnote. When I speak of a layer of air, I do not mean to imply separate layers in the atmosphere, like rows of bricks or sheets of cardboard laid one over another. Air has no such divisions in its make, but any substance might be pictured as divided into possible sections, and, in speaking of the air, it is convenient to imagine successive layers, end of footnote. Layer of air next above it. The touch of cold acts as a squeeze, and immediately drops of water are pressed out, minute floating drops, born up by the air, because they are so small and light, thus an evening mist is formed. Light as the floating drops of a mist are, it is questionable whether even they would be supported by absolutely motionless air, since they are not actually lighter than air. But motionless air is a thing almost unknown. On the stillest day imaginable the air still moves. We often have such mists towards night, after sundown, more especially when the ground is low and damp, where therefore the air has been well soaked. Mists and fogs are much alike, being both neither more nor less than ground clouds. A mist is commonly distinguished from a fog, as being made of rather larger drops, therefore feeling more wet. Mists are often seen in the evening or early morning, lying over rivers, sometimes most sharply divided in their outlines, scarcely reaching beyond either bank, but piled up within like a pack of cotton wool. Water will not part quickly, with its heat like the solid ground, and long after the earth and air have cooled down, the river water keeps moderately warm. Being warm it continues to part with a good deal of vapor, more than the chilled air is able to take in. So then a little bank of mist or fog is formed, all along the course of the stream, more or less dense according to the warmth of the water and the coldness of the air. The river keeps offering water to the air, and the air keeps refusing to drink because it is not thirsty. Between the two, the rejected vapor turns into a cloud and hovers in a waiting attitude till air and water shall come to a definite conclusion. This patience is generally rewarded after sunrise by the warmed atmosphere drinking up the fog with eagerness. Sea fogs are brought about in much the same manner as medial mists. The air which lies over the sea, having a free water surface to feed on, gets well filled with vapor. Then a cold breeze from some new quarter blows into the warm air chilling it, and at once the hidden supply of vapor becomes too great. A wet mist or fog is quickly developed, sickening the atmosphere and blotting out the horizon. The perils of such a fog are sharply felt by those at sea, and way are sharply evidenced lately by a collision between the Austin and Dower mailboats in mid-channel. Suddenly and without warning, the passengers of the lesser boat saw a great steamer burst through the gloom close at hand, bearing down upon them at full speed. There was no time to get out of the way, no time to await the peril, and the worst that any man expected was not worse than the reality. For the heavier vessels struck the other full amid ships, and cut clean through her as was a knife, so that the two halves of the stricken vessel fell away from the boughs of her assailant as a divided bank of snow before the snowplough. Strange to say, one of the severed halves floated for hours after the collision. Sea fogs are often swept by a wind a short distance inland, but over the dry ground they soon die away. London fogs, too, spring into being after much the same method. Two currents of air meet and mingle, one warm and moist, the other cold. If the cold current is dry enough to receive all the superfluous moisture of the warm current, no marked effect is seen. At the most only a slight haze is formed, rapidly to be dissolved. But if, as is often the case, the cold air already has its full complement of moisture, then, when by contact it cools the warm damp air, a fog must be the result. The water of the Thames, there, is often warmer than the air above it, and the same effect is produced, as elsewhere, by the sudden cooling of the ground after sunset. In town, however, an additional and very unpleasant element is found, from which country and ocean are free. The air of London is always more or less full, of countless floating black specks, minute portions of carbon, dropping from the smoke, which thousands of chimneys are ever pouring into the atmosphere. Innumberable water drops, squeezed out of the cooling air, form around, or cling to these tiny specks, and they lend to the fog, its peculiarly thick and yellow or black look. Once let a fresh breeze sweep over the city, bringing fresh supplies of unsaturated air, able to hold more moisture, and the fog quickly vanishes. It is sucked up drop by drop into the interstices of the atmosphere. But if no such breeze comes, and the air remains at the same temperature, the fog may linger long, growing probably worse and worse, as more smoke is weighed down by the heavy damp air, to supply fresh carbon centers for new little drops. Sometimes an unhappy variety for the Londoner occurs, in the shape of a fog overhead, not resting on the ground, but most effectually cutting off the sun's rays. It is a little startling for a country cousin, running up to town for a few hours, out of a clear atmosphere, to find the mighty city plunged at midday into Egyptian darkness. This was my own experience one April morning of the current year, to reach the city proved not difficult, though the outskirts looked murky, but during a two-hours business interview the air grew darker and yet more dark. Gas had to be resorted to, and by one o'clock it was, to all intents and purposes, absolute midnight. No fog worth mentioning was in the streets, and the lamps gave out their light well, but a dense pole overhead shut away all daylight. It was as easy, no doubt, to go about as in any gas-lighted winter night. Only it was not exactly what one expected on an April day, and there was an unpleasant consciousness that the overhanging pole of fog might at any moment descend to the level of the pavement, putting an end to traffic. It seems not at all improbable that even with country and sea fogs, the floating drops of water always form around light specks of solid substance, born up in the air, too small and light to be perceived. Some maintain that no fog, no mist, no cloud, can ever be condensed out of the vapor in the air without the help of these minute particles for the water drops to cling to. If so, the floating dust of the air has a great and important part to play in the vast water circulation of Earth. It is only in large cities, however, that these specks are so considerable in size and number as visibly to thicken the fog and to endow it with color. As London waxes yearly bigger, as chimneys grow yearly more multitudinous, as fogs become yearly more dense and overpowering, the future of the city has a serious look. No doubt, however, when things have reached such a pitch of misery that human nature can endure it no longer, Englishmen will at length put their heads together and will devise some method for burning instead of breathing their smoke. One might well ask why wait so long, but the average Londoner is a much enduring individual. There is another kind of evening condensation of moisture often seen and nonetheless singular because common. This is the dropping of dew. Dew drops have something about them very pretty and poetical. They arrive so softly without stir, not battering like rain or clattering like hail or accompanied by rough winds. Dew comes on warm and still nights, creeping gently into existence, undercover of the dusk and fading quietly out of existence under the early sunshine. Dew does not fall from clouds overhead. It is commonly formed only when no clouds are there. Clouds act as a blanket, holding in the warmth of the earth, reflecting that warmth to the ground and for dew the quick cooling of earth is needful. It comes into being much after the same fashion as a meadow mist. Sometimes the two are found together, dew dropping out of the lower layers of air close to the ground and mist being formed out of the next layers of air above. Properly speaking, dew is hardly dropped since that would imply the falling of the drops for at least a little distance and it appears that they do not fall. The moisture is rather placed or deposited by the air on any surface ready to receive it. Much in the same fashion that moisture is deposited upon the window of a room or a railway carriage when the air was in is warmer than the air was out. The sun having set, the ground cools fast if there are no clouds. The air also lying on the earth is quickly lowered in temperature. On a still evening, when it does not move on, it reaches presently a stage called dew point. By this is meant that no dew has yet been formed but that the air can endure no further cooling if it is to hold still all the vapor it now contains. It is in fact completely saturated. If the cooling continues, the next step is the appearance of dew. Drops of water larger and smaller are squeezed out of the air and cling to grass blades, leaves, twigs, spider webs and all else in their way. The dew drops are by no means equally distributed over the ground. They favor most those substances which part with heat most readily which therefore grow cold most quickly. Grass and plants lose heat much faster than paving stones or the road. So dew is to be seen closing grass and leaves with shining drops while pavement and road are still dry. Sometimes the dew point is down below the freezing point. Then the air, instead of depositing drops of water, dresses fields and hedges, perhaps even the whole landscape in a coat of white whorefrost. Whorefrost is not, as many suppose, dew first dropped as water and then frozen. It is deposited at once in the frozen and solid form. Like dew, it clings most readily to those objects which part most quickly with their warmth. Whorefrost is often seen on lawn and bushes when the gravel walk is free from it. Generally the coating is slight, but sometimes it may be seen so thick as to look like a fall of snow. On the shutters of the observatory at the top of the Poydodome it was on one occasion actually three feet thick. In tropical countries the amount of dew formed is greater than anything we ever see in temperate climates. Travelers have found it possible by puring the dew from one large leaf into another to obtain water enough for washing their hands. Much of this abundant moisture sinks into the earth and feeds the thirsty roots of plants. When once the tropical sun rises, all the dew which has not soaked downward vanishes into the atmosphere like a dream. In certain American forests, dew is sometimes formed at the level of the three tops. The abundance of it is extraordinary, owing to the quantity of vapor in the air and the rapid changes of temperature. It is said that an actual shower of dew like a shower of rain is occasionally felt below. There's no doubt results from the running together of the drops when formed. As moisture deposited on a window will run together and flow downward. The slow heating and cooling of water has been alluded to in this chapter and it will be repeatedly spoken of again in connection with winds, climate and weather. That peculiar characteristic of water which is known as its great specific heat, in other words, the fact that water requires more heat to raise it to a certain temperature than perhaps any other known substance has widespread results. The touch of an overruling and all-wise providence may be seen here. If water could be warmed and cooled with the ease and rapidity of other substances, the climates of many parts of Earth would be in consequence so changed that lands, now more or less densely inhabited, would become almost uninhabitable. End of chapter 16 This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by Elizabeth Miles. The Ocean of Air. Meteorology for Beginners. By Agnes Gebern. The Mountains of Cloudland. Chapter 17. The Mountains of Cloudland. Wonderful scenery is found in Cloudland by those who love to study it. Fast mountain masses and jagged peaks alternating with lakes and hills dressed up in glowing sunlight. Yet the solidity of Cloudland exists only in our imagination. Clouds are, as a general rule, neither more nor less than masses of wet mist or fog, floating in the atmosphere, perpetually forming, diminishing, vanishing, reforming, growing, lessening again, never remaining for any length of time in the same shape or the same place. Evaporation and condensation are unceasingly at work in Cloudland. Every mist cloud that we look upon is either growing or melting away. It is never a fixture. True, some clouds seem to us to remain long in one position and to keep long one shape if a few hours or less can be called long. But in reality there is not even that small amount of fixity. When they seem to be at rest and changeless it is only because they are so far away that we cannot, in a short space of time, detect the movements going on. Or else it is because the whole sky overhead is shut off by a gray pall of cloud which may move or grow or lessen without our being soon conscious of it. Like fog and mist upon Earth's surface clouds are commonly made and dispersed through the meeting of two currents of air one warmer than the other. If when the two currents mix the air of the warmer is so much cooled down that it cannot carry all its moisture while the air of the cooler is unable to take in more moisture than it has already then a new cloud is formed or if a cloud is already there it grows bigger. But if when the two currents mix the air of the colder gains so much warmth that it can receive more vapor than it already has then any cloud floating there will be partly or wholly sucked up so as to lessen in size or to disappear. This is the manner in which clouds often vanish. Sometimes the wind carries them onward out of our sight. Quite as often they evaporate or are dried up by the air vanishing exactly as the fog cloud from the funnel of a steam engine vanishes. On a tolerably fine day when there is a fresh breeze and many small clouds lie rather low down the changes in the form and position of these clouds are astonishingly rapid. Anyone may test the truth of this for himself by watching with steady attention during 20 or 30 minutes but few people will take so much trouble. All clouds are not formed of mist or tiny water droplets. There is every reason to believe that some are made of ice. Clouds float at very different levels in the atmosphere. Careful observations have been made in several places with varying results. When at a great height it is believed they are formed not of fog or mist but of tiny interlaced ice needles. They are in short frozen. Two facts bring us to this conclusion. One is that some curious effects such as halos and mock suns are occasionally seen through high level clouds and never through low level clouds. These effects are in all probability caused by the refraction of light through ice since they could be thus produced while they could not be produced by the passing of light through mist. The other fact is that at so great a height the cold is very intense and water must almost of necessity become ice. If a fog cloud were there it probably could not remain a fog cloud but would have to become an ice cloud. On earth at a certain temperature we always have horror frost in place of dew and snow in place of rain. Just so in cloud land at a certain temperature there would be frozen clouds in place of mist clouds. Clouds as already stated commonly spring from a cold current of air meeting a warm current but this is not the only mode they are formed. A high mountain may often be seen capped by a cloud of a certain well-defined shape and this shape will perhaps remain for hours almost without change. Sometimes if the wind is high a long slender ribbon or a tail of cloud will stream persistently from the top of a lofty peak. The warm wind of the lower country carrying plenty of moisture suddenly reaches the mountain and has to rush upward. It cannot stop for the pressure of air behind forces it on as it rises it grows colder and the chill of the ice-bound peak adds further cold compelling the air to part quickly with some of the vapor so easily carried below thus a cloud is formed. We may learn something more from this fog ribbon flowing from the peak as to the nature of a cloud. If you watch carefully the slender cloud from a distance you will find it keep the same shape for a good while unchanged perhaps for hours. Then drawing nearer you will find that while the cloud as a whole remains unaltered the particles of which that cloud is made are never the same for two minutes together. The strong wind brings perpetually fresh supplies of moisture-laden air pouring up the mountainside from below and the surplus moisture is perpetually being condensed into fog by the cold peak. Yet the cloud grows no larger for the wind perpetually carries away the fog to be evaporated anew into the atmosphere. At one end of the long cloud ribbon it is born into existence out of the air and at the other end indeed along its whole length it dies out of existence into the air. The cloud as a whole remains but fast as it is formed so fast the air all around dries it up the water particles of which it is composed are whirled onward unceasingly like the waters of a river. The same thing on a small scale may be seen in the little cloud which forms outside the spout of a boiling kettle. The cloud keeps the same shape remaining intact while the particles of which it is composed are in ceaseless motion. Fresh particles rush momentally out of the spout to take their place in the cloud then in their turn they are swept along and vanish from sight. This in a greater or less degree is the character of all clouds at least of all lower level mist clouds the higher level snow clouds no doubt evaporate more slowly just as ice and snow evaporate more slowly than water. Clouds are commonly divided into different classes reckoned according to their form and appearance as seen by us from the bottom of the ocean of air. The three leading kinds are first the Mearsdale cloud known as Cirrus second the ground fog known as Stratus third the Woolpack cloud known as Cumulus but as we seldom find a perfectly pure specimen of any simple substance so we seldom come across perfectly pure specimens of these three simple types of clouds. Far more commonly we see clouds which are a mixture of two or three simple types which therefore we may call compound clouds. The Mearsdale cloud is usually regarded as a sign of coming wind. Pure Mearsdale or Cirrus is feathery and streaked and lies always at a great height. A balloon floating over four miles high seem to approach no nearer the distance Cirri than when at first left the ground. Clouds which are a mixture of the Mearsdale and ground fog lie not quite so high though still at a level which probably means that they are formed of ice not of mist. It is through them that mock suns are seen. Mackerel Sky clouds are a mixture of the Woolpack and the Mearsdale. These two lie lower than the Pure Mearsdale. It is often difficult looking up from the floor of the air ocean to decide with any certainty which class each particular cloud may belong to. Even practice size are frequently at fault. Ground fogs lie much lower in the atmosphere and much nearer to us than those of the Mearsdale type. Therefore they have not such sharply defined outlines but appear more hazy, more like masses of grey fog. Because this kind is never found at any great height and because it often forms over low lands before night to vanish in the morning much like the evening mists of marshy and damp places it has been named the ground fog. But it is distinctly a cloud far above our heads not to be confounded with mere fogs and mists which rest upon the ground. Sometimes the ground fog clouds spread over the entire sky shutting off all sunshine while the air below is foggy and dull. As a rule however they do not mean rain. Another kind of cloud which often overspreads the whole sky occasionally ushering in a storm is a mixture of the ground fog and the wool pack. The genuine rain cloud more or less a compound of three simple types is simply that which the name implies the bringer par excellence of rain. Still it must not be supposed that every cloud which pours drops upon earth is necessarily a nimbus cloud. The wool pack cloud is usually formed of rounded mountain like masses more or less white and woolly and often very beautiful in sunshine. It is commoner in summer than winter. Piled up heights of snowy brightness softened by grey shadows are visible against the blue sky on many a fine day. The base of the mountainous pile is frequently a sharp horizontal line from which the fairy like heights spring upward. This is the view which we have of wool pack clouds lying at some distance from us between the zenith and the horizon. If such a cloud mass is exactly overhead we look up at a flat grey base which cuts off the sunshine. There may be splendid piles of white cloud above but we can gain no glimpse of them. A man who would see mountain peaks must be outside and away from them not under the mountain's base. Wool pack clouds are produced by a steady upward current of warm air carrying abundance of vapor. As the air is cooled in its ascent great quantities of the vapor are fast condensed into masses of mist or cloud which collect into rounded shapes. No doubt the most wonderful views of cloud land are to be obtained from balloons. I cannot better close this chapter or introduce the next than by quoting from the vivid descriptions given by Mr. Glacier of what he and his companions saw on two occasions. The first extract relates to an ascent in fine weather the second to an ascent in wet weather. On the morning of August 21st by half past four my instruments were replaced and we again left the earth. The morning was warm but dull the sky overcast with zero stratus cloud the temperature was nearly as high as 61 degrees we at first rose very slowly by 438 we were a thousand feet high. At 441 there was a break of clouds in the east and a beautiful line of light was seen with gold and silver tints we were then still only at a thousand feet here and there dotted over the land the morning mist was sweeping. At 451 the temperature was 50 degrees scud was below us and the night cloud was in a transition state forming into the cumulus at the same level as we were about 3500 feet black clouds were above and mist was creeping along the ground. At 457 we were in cloud surrounded on every side by white mist the temperatures of the air and dew point were alike as both the dry and wet bulb read 39 and 3 quarters degrees the light rapidly increased and gradually reemerged from the dense cloud into a basin surrounded with immense mountains of cloud rising far above us and shortly afterward we were looking into deep ravines bounded with beautiful curved lines the sky immediately overhead was blue dotted with cirrus clouds as we ascended the tops of the mountain-like clouds became silvery and golden at 51 we were level with them and the sun appeared flooding with golden light almost all the space we could see for many degrees both right and left tinting with orange and silver all the remaining space around us it was a glorious sight indeed at this time we were about 8000 feet high and the temperature had increased from 38.5 degrees in the cloud to 41 degrees we still ascended rather more quickly as the sun's rays fell upon the balloon opening up to us ravines of wonderful extent and presenting to our view a mighty sea of clouds here arose shining masses of silvery heaps their large masses of cloud in mountain chains rising perpendicularly from the plane dark on one side, silvery and bright on the other with summits of dazzling whiteness some there were of a pyramidal form a large portion undilatory or wavy in some places subsiding into hollows and in one place having every appearance of a huge lake nor was the scene wanting in light and shade each large mass of cloud cast behind in its shadow and this circumstance added to the very many tints formed a scene at once most beautiful and sublime so much for cloud land in fair weather now for the contrast in a June ascent from Wolverton with concomitants of rain and high wind we were released by the simultaneous yielding of men and in a minute were 4000 feet above the earth at this elevation we were chilled by the clouds which we entered but cheerfully looked forward to emerging on the other side into the region of pure sky and brilliant sunshine on the contrary all was gray colorless and gloomy at 9000 feet the air was filled with a moaning or sighing like the wind previous to a storm this was our first experience of the sound and we listened to identify it with the cordage of our machine working in the air but it was the sound of conflicting currents meeting and opposing each other in the wilderness of space now we were two miles high with faint gleams of the sun expecting him momentarily to appear instead we entered a fog and then into a fine and wetting rain afterwards a dry fog and then again a wet fog and that was again repeated then we were mocked by gleams of sun and found that we had ascended three miles high at 17000 feet there was no change at four miles high dense clouds were still above us and for a distance of two or three thousand feet we were free from fog to our surprise at this elevation more than four miles above the earth there were dark masses of clouds two layers one above another with fringed edges unmistakable nimby without doubt clouds of rain at 23000 feet Mr. Coxwell who had been examining his ballast bags decided that we must not only descend but descend at once to my great regret I was therefore compelled to content myself with a searching look of general observation but one momentary glance was sufficient to impress it forever on my mind and were I an artist the impression was so vivid that I could portray it in all its details above below all around the sky was nearly covered with dark clouds of stratus character with sear eye above and faint blue sky between not the blue of the morning or of a dry atmosphere but is seen when the air is murky and the clouds confused the sense of storm and adverse weather generally which gave character to the scene marked it forever as a memorable experience among many others as we passed down on our descent at a height of 14000 feet we encountered a snowstorm extending through nearly 5000 feet there were no flakes only speculae and hexagonal crystals of distinct and well-known forms below the snow and almost 10,000 feet from the earth we entered again an opaque atmosphere which continued till we reached the ground this summer afternoon had exhibited many vicissitudes of weather and offered to the observer a fine and comprehensive study of meteorological influences at work removed from the immediate surface of the earth End of Chapter 17 Chapter 18 of The Ocean of Air Meteorology for Beginners This is a LibriVox recording All LibriVox recordings are in the public domain For more information or to volunteer please visit LibriVox.org The Ocean of Air Meteorology for Beginners by Agnes G. Byrne Chapter 18 Rain, Snow and Hail Thus we see that the cloud like a mist or fog is expressed by a cold air current out of a warm one to hang suspended as a countless multitude of minute floating water drops sometimes through a change of wind or temperature these drops are drawn anew into the air and the cloud vanishes but sometimes they go on increasing more and more vapor is turned into mist and the cloud spreads over a wider and wider extent then the tiny drops get more and more crowded together till they begin to run into one another and so bigger drops are formed which again join and make still larger ones now though the air can hold up very tiny drops of water it cannot for any length of time support large and heavy drops naturally when they get beyond a certain size they can be buoyed up no longer but must fold the ground at the shower of rain it should have ever be understood that this certain size is by no means always the same size moving air can support a heavier weight than still air a gale of wind could keep floating above us drops which a gentle breeze would have no power to hold up this may help to explain the difference in the sizes of rain drops there are often strong winds blowing in the higher regions of the atmosphere when we have a calm below sometimes the same thing is seen in a fog which after all is merely a cloud resting on the ground the tiny water drops become so abundant that they run together and form bigger drops which hold to the ground like rain but all this is a very partial explanation there is much that is mysterious in the formation of rain drops and other forces such as electricity have a hand in the matter rain briefly is caused as mist and fog are caused by the meeting of two currents of air one warm one cold the moisture of the warm air is condensed into a cloud and in many cases rain follows rain is also caused by the rushing of warm lowland air up a mountainside some of the heaviest rains known in any part of the world are on mountains especially on mountains which lie not far from the sea the air over the ocean gets thoroughly soaked with vapor which while warm it can well carry then suddenly it comes against the mountain range and has to pour upward losing heat as it does so becoming fast holder it can no longer contain its supplies of hidden moisture then clouds of floating mist are formed and torrents of rain are poured down air harrying up a mountainside loses heat in two ways the coldness of the mountain takes effect chilling down the warm air also in rising to a higher level it expands becomes more thin or rare spreading out its particles over a larger space because of less than pressure this expansion of air or of any gas always means increase of coldness heat being given out in the act of expansion increase of coldness means lessened power to carry moisture which means nearer approach to saturation and therefore increased dampness therefore too it means often heavy downfalls of rain so no wonder high mountains in hot countries are often famous for the deluges with which they are favored the amount of rain which falls in different places varies extremely taking all the year round they have a moderately abundant supply in the British Isles but nothing like what is known in tropical countries the fall of rain in London except on rare occasions seldom exceeds an inch footnote measured by means of a rain gauge a gallon of water weighs 10 pounds and if spread out in a layer one inch thick will only cover an area of two square feet an inch of rain gives 100 tons of water per acre or 60,000 tons per square mile scott and a footnote in 24 hours but in other places this amount is enormously increased a joyous in France 30 inches have been known to come down in less than 24 hours if the whole amount which falls on the east coast of England during a year could be carefully collected on the level surface no running away or sinking into the earth or drying up being a load we should have at the year's end a layer of water about 20 inches in depth if the same were done on the west coast of Ireland or Scotland the supply of water at the year's end would be 6 or 8 feet deep so even in the British Isles there is a very marked diversity between different places but if this same course were followed at a certain hill station in India named Cherapongi a much greater result would be obtained in the shape of a large lake at the very least between 30 and 40 feet in depth Cherapongi enjoys the far from delightful eminence of being one of the wettest spots on the face of the earth sometimes in cold weather we have snow instead of rain snow is not frozen rain it falls directly as snow from snow clouds in winter the frozen clouds lie at a much lower level than in summer snow clouds like mist clouds when they grow too heavy have to part with some of their substance which falls to the earth in white flakes instead of water drops much more than mere weight however is involved in the fall of snow snowflakes are made of the most exquisite ice crystals the minute needles groups themselves in a beautiful star-like shapes always six-pointed many substances when changed from a liquid into a solid will crystallize into delicate and beautiful forms and it is notably so with water when under free conditions water freezes into snow the snow crystals are always formed of six-pointed stars or plates that it is so we can see with the microscope why it should be so we do not know if very large flakes come down they are caused by the union of smaller flakes the drier the air the smaller and harder will be as a rule the flakes for they cannot stick together unless a little damp snow in England comes fitfully and seldom lasts long skaters have scarcely time to learn the use of their skates before the opportunity is gone yet even in England we have exceptional winters of really hard frosts and deep snows towards the end of the seventh century for instance the Thames was completely frozen over for nearly three months so that according to an old chronicle it became a small city with booths, coffee houses taverns, glass houses printing, bull baiting shops of all sorts and whole streets made on it food was scarce prices rose and the birds of the air died numerously another such winter seems to have recurred about 25 years later and again after an interval of nearly 50 years each differing from the others in details but all alike in severity in the present century the mightiest snowstorm yet known was that of December 1836 many lives were lost and business generally came to a standstill for nearly a week the actual snowfall was said to be from 4 to 9 feet in depth and snowdrifts were piled to a height of 20, 30, 40 even 50 feet after all the most excessive of English snowfalls are but a child's play seen side by side with that awful visitation to which parts of America are subject to blizzard to constitute a true blizzard writes Miss Gordon Cumming the whole atmosphere must be full of the finest most cutting ice dust sharp as powdered glass Ningoldus very small three-cornered frozen snowflakes driven with appalling swiftness by a rushing mighty wind while a sudden fall of the temperature from comparative warmth to 30 or 40 degrees below zero produces an intensity of cold which is altogether unbearable imagine who deem ourselves frozen should the thermometer fall two or three degrees below zero the blast sweeps on with irresistible velocity so densely charged with pulverized snow and ice as fine as flower that it obscures the air with what is described as white darkness rendering large objects totally invisible at the distance of two or three yards and accompanied by such a roaring and tumult that the human voice can scarcely make it so heard within a few feet the luckless traveler who is caught in such a blast runs every risk of suffocation the action of the lungs being stopped by the swiftness as well as the intense cold of the wind while the ice dust penetrates the thickest closing is more choking than the sand of the semen with the anguish of suffocation the victims of the blizzard seem occasionally to become insane before dying in some cases tearing out their closes as if thus to gain relief one of the worst blizzards ever known raged through several of the states on and after the 11th of January 1888 an ice-laden hurricane awful in its power within 24 hours the temperature dropped above to 28 below zero and in a single hour blue sky was replaced by a wild storm of powdered snow people were taken utterly unawares children died by the roadside on their way home from school farmers died in the fields before they could get to their houses a woman stepping outside her front door to watch for her husband perished there and then probably so numbed and bewildered as to have neither power nor sense to turn back some even under shelter succumbed to the overwhelming cold and were literally frozen to death but the greater number at least of those exposed to the blast seemed to have died of suffocation fighting for breath often bearing their throats in the terrible struggle for air others again were found dead stripped of their clothes with their piecemeal as if flung away one by one under the influence of madness such dying madness in its victims is a well-known occasional feature of the blizzard this great storm lasted unbroken for 60 hours 3 long days and nights of horror of a white darkness of a ceaseless hurricane roar of fearful cold of a stifling rush of ice and snow after such an account one can hardly say much about English winters another form of frozen water descending from the clouds is hail and hail may fall in summer as well as winter hail stones are not made like snowflakes of delicate ice needles but neither are they shapeless lumps of ice if the formation of snowflakes is mysterious the make of hail stones is not less so they too are usually crystallized in beautiful shapes so quite differently from snowflakes the manner of crystallization is often so complicated as to render it almost impossible that they should have sprung into being instantaneously or as the hail stones fell from the clouds probably each hail stone begins with the freezing of a drop of water through a sudden rush of ice-cold air around this central ice particle other particles of ice form in succession taking curious shapes according to certain laws of crystallization all this must occupy time it could hardly be accomplished in a single second possibly not in many seconds yet how heavy growing hail stones not like than feathery like snowflakes but solid and firm can be born up in the air while their formation goes on no easy problem in a high wind doubtless they could be carried aloft much longer than in still weather and hail is usually accompanied by a gale moving air as already said in connection with raindrops can bear a far heavier weight than air in repose still there is much in the story of hail stones which we are as yet unable to explain hail stones as large as peas are common and they have been known to fall in Britain fully the size of marbles in the Orkneys they have been picked up big enough to rival a goose egg and even this has been exceeded elsewhere though many stories of broad ignanian hail stones might be dismissed as mythical the true individual crystallized hail stone is never very big when great stones fall they are merely rough masses of small stones glued together in the course of their descent by natural stickiness probably resulting from some degree of damp one of the heaviest falls of hail ever known in England was during the thunderstorm of August 9th 1843 it was exceedingly violent in the neighborhood of Cambridge and scarcely less so in Oxfordshire an extraordinary darkness of the atmosphere with clouds hanging so low as almost to rest upon the housetops dazzling flashes of lightning and one long continued unceasing roar of thunder where enough in themselves to be impressive but to them was added a deluge of hail stones which lasted more than 20 minutes the scene brought Mr. Glacier afterward was positively terrific on the fright of many of the inhabitants of the town and a footnote was in no small degree increased by the crash of broken windows and the inundation of their houses during the whole of this time it was impossible for the eye to penetrate many yards through the storm the hail fell with such wonderful closeness and there was such a peculiar mistiness rising from the earth that a complete barrier was opposed to the power of vision we are almost afraid to speak of hail stones or rather blocks of ice but we are certainly not exaggerating in the least degree when we say that very many of them were as large as ordinary walnuts some indeed far exceeded this size one that was picked up measured three and a half inches in circumference and several have been described to us as being about as big as a bullet's egg three hours after the storm was over unmelted hail stones lay in piles one gentleman finding his horse unable to pull the carriage through them stepped out to clear way and sank up to his knees in Cambridge alone where the brunt of the storm fell damage was done to at least the extent of 25,000 pounds glass was shivered window frames were dashed in fruit was cut up, birds were killed crops were utterly destroyed in a single half hour the standing corn was stripped, laid flat and literally cut up into little pieces nothing of it remained for use well for us that such outbursts are rare in England while on the subject of ice a passing mention may be made of frozen waterfalls often seen in other countries though not so common in England the mere fact of a severe frost over the plains does not ensure the freezing of waterfalls upon the higher ground they usually occupy sheltered spots where the radiation of heat from the earth is less rapid than elsewhere and a slight or short frost has little power to chain the falling water but if the frost is severe and lasts long enough especially if it is accompanied by a dry and bitter wind the stream is at length more or less first the spray at the bottom hardens into a snow-like mass then the trickling water on either side becomes solid each struggling drop is turned to ice where it rests and fantastic forms grow slowly into shape including hollow icicles through which the ice cold water still flows some of the great Norway waterfalls gained such a mass of frozen spray below in the course of the winter that summer cannot entirely do away with it even in England we say something of this the horde raw force in Wensley dale a waterfall 100 feet high built up in 1881 a cone of ice at its base no less than 30 feet high the entire waterfall being that winter frozen for the first time since 7040 end of chapter 18 chapter 19 of the ocean of air meteorology for beginners this is a LibriVox recording all LibriVox recordings are in the public domain for more information auto volunteer please visit LibriVox.org the ocean of air meteorology for beginners by Agnes G. Byrne chapter 19 the nature of wind the ocean of air is never at rest stagnation of our atmosphere is a thing unknown streams of air particles flow hither and thither forming winds from the north and east winds from the west and south winds from every interrining point some are variable hour by hour some are changeless for months some are hundreds of miles in extent some may be measured by yards some are great rivers of air as well as tiny brooks of air in the air ocean just as there are vast streams besides little currents in the water ocean our next step is to think about the various movements of air commonly called wind those movements which all together make up the worldwide circulation of the atmosphere in some parts of the south and east of England you may note the act of wind interning the sails of a windmill a pyramid shaped tower has a large light structure facing to one side formed of four huge spreading arms or sails each some 30 or 40 feet in length small crossbars partly fill up the skeleton shapes of these sails and more or less of a canvas covering over spreads the crossbars the sails are set at such an angle or slope that when the wind blows against them they move round under its pressure and thus work the grinding mill was in the action of the wind upon a windmill sail is much the same as upon the sail of a boat a kind of slanting push sending it onward wind is nothing more nor less than moving air but why should air move why should not the whole ocean of air remain at rest we can picture to ourselves a great ocean of air in perpetual sublime repose a world without winds an atmosphere without circulation such a world with such a belt of air might not impossibly exist under certain conditions but it would have to be a very different world from ours a reposeful a stagnant world not spinning upon its axis day and night it would have to be a world without varieties of heat and cold, of climate and weather the moment one part of the atmosphere is warmer than another part a disturbing element comes in and the air begins to stir wind commonly springs from what is called difference of pressure if in one place the air is warm in one light in another cold and heavy the heavier air must always flow towards the lighter air to keep up the balance of the atmosphere just as water always moves to preserve a level surface so air always moves to preserve an even balance moreover one current of air always causes other currents that is a fact worth remembering everything that happens in nature as well as in our lives is always caused by other things going before and always helps to cause other things coming after no one thing in nature or in life stands alone uncaused and uncausing a current springs from different causes but once said going it is certain to cause other currents so even if we had an ocean of stagnant air to start with it could not remain stagnant so long as a single disturbing element was there to upset its balance each slight disturbance would draw countless other disturbances in its train moving bodies in the air ocean are one sorts of disturbance and a very frequent source we are not commonly conscious of this not a hand can move or can fly without causing air pressure and consequent little air breezes but the fact does not become apparent to us unless the moving body is large the compression of air tolerably great and the wind resulting somewhat strong we all know from experience what a wind is caused by the rush of an express train through a station when we are standing on the platform dust and sticks are carried along by the whirl of moving air the same thing is seen on a much greater scale with an avalanche avalanches are of different kinds sometimes in summer on the higher mountains they consist of solid blocks of ice dashing down steep slopes in spring they are more usually huge masses of soft coherent snow lighting downward with frightful rapidity overwhelming whole villages in the valleys below another kind quite as dangerous is the drift avalanche of winter it consists of loose dry powdery snow first said going perhaps by a strong wind in the descent it gathers volume and speed leaping from precipice to precipice till the tremendous compression of air caused by its downward rush sets in motion a far more agile than that which began its own career the mere wind from such an avalanche has not only lifted a strong man bodily from a ledge and born him some distance but has leveled trees and shattered whole houses with its hurricane blast a mighty snowmass will often break loose from some frowning snow ridge above and will dash downward leaping from ridge to ridge to precipice at the first concussion the whole mass is broken into fragments and soon only a cloud of white dust is seen to flash like lightning from point to point roar following roar till the last leap is taken though behind any human being so unfortunate as to stand in the path of such an avalanche or within reach of its rushing wind but to come back to a very everyday matter what is it that causes an ordinary drought in a room or public building the droughts in St. Paul's Cathedral are pretty well known to Londoners if no disturbing elements existed the mass of air within our great cathedral might remain calm and move less but this is a state of things seldom if ever possible certainly never possible and a crowd of human beings is gathered under the dome the heat given out by their bodies and the hot air pouring from their lungs warms the atmosphere in and near the center then the warmed air rises and streams of cold air pour centerwards from the side aisles to supplies a place of that which passes upwards thus a drought is created and old ladies draw their shawls quietly round them and old gentlemen cast savage looks at the vergers but neither old ladies nor old gentlemen blame themselves as being in part the physical cause it often happens that on the seacoast especially in hot countries land and sea breezes regularly alternate all day long a fresh breeze from the sea will blow in upon the land in the evenings and after a pause a land breeze will for hours blow steadily out to sea in the morning another pause is followed again by the sea breeze the direct cause of these breezes is the rapid heating and cooling of the ground land grows warm much faster than water and also loses its warmth much faster water heats slowly and once heated it cools slowly today the ground becomes warmer and warmer under the burning sun and helps to heat the layers of air above the cooler and heavier sea air has to flow in upon the warm and light land air after sundown the reverse happens the ground cools fast helping to cool the air above whilst the sea keeps warm the air over the land becoming the coldest and heaviest has to flow towards the more warm light air over the water some explain this by saying that the cold air flows towards the warm air because the letter being light has a tendency to rise and to leave a space which must be filled another explanation is as follows the warm air is enlarged by heat and expands or swells upward rising like a blister on the outer surface of the air ocean the upper layers of air being piled too high have to slide away to a lower level over the colder air then the added weight and pressure over the colder region forces the air below to flow towards the region of warm and light air it is in fact readjustment of the balance all around the earth near the equator are hot countries lands when the sun is down warming soil, sea and atmosphere this causes on a large scale what has just been described on a small scale the air being peculiarly laden with moisture in those regions is peculiarly susceptible to the heat of the sun vast masses of air already warm are more and more heated by the sun's rays and by the burning earth below till they are far lighter and larger than air elsewhere then the cooler heavier smaller air comes pouring in from north and south to restore the balance of the atmosphere I use the words larger and smaller with purpose for literally the air of lower levels does expand or grow bigger with heat and does shrink or grow smaller with cold a mass of heated air weighing one pound truly larger than a mass of cold air weighing one pound as one pound of heated iron is bigger than one pound of cold iron the air in the tropics near the equator grows hot, expands, gets large and light this means that the mass of the atmosphere thereabouts occupies more space than if cold its particles being more widely separated the result if we could see it doubtless and actual swelling upward of the outer surface of the air ocean the rising of a huge blister or air wave pushed outward by the enlarging of the air below now the air ocean like the water ocean flows hither and thither in a perpetual struggle to keep itself even like the water ocean it might surge out at its upper surface could not the word surface is of course used with a reservation the atmosphere probably thinks away so gradually that no man if able to examine for himself would be able to say where it ceased but since the air ocean does not extend through space it must stop somewhere and a footnote in great waves but those waves are sure to fall back and flatten themselves out the superfluous air piling itself above has to flow away in mighty currents towards the north and south as these streams pour off they cause extra weight and pressure where they go and other currents pour in below just as we saw was the case in St. Paul's Cathedral we may be quite sure of one thing in a matter of air circulation whatever quantity of air flows towards one particular place the same quantity of air must flow away from that place also whatever quantity of air flows away from one particular place the same quantity of air must flow towards that place no part of earth can be left with less air or more air than any other part so a perpetual struggle to make things equal is carried on over the whole world air rushing hither and thither to restore the balance which as soon as it is restored is directly upset anew all around the earth to the north of the equator except we're disturbed by other influences lies a broad belt of winds always glowing towards the equator from the northeast known as the trade winds the belt reaches from near the equator to about 30 degrees north latitude under streams of air continue day and night, night and day with more or less intensity throughout the year exactly the same kind of trade wind belt is found also to the south of the equator only as these winds too blow towards the equator they come from the southeast the northern belt is known as the zone of the northeast trades and the southern belt as the zone is known as the southeast trades in bygone years the trades were a great perplexity to sailors and often a serious difficulty it was all very well so long as a ship wanted to sail in the direction wither the trade blew the mariner might lash his helm and go to sleep if he choose leaving the lookout to a girl or a child for a long run through the open sea he could be absolutely secure with a good fresh breeze always from the same point never ceasing and never growing too strong but if we wish to sail in the teeth of the trade that was another matter wishing for a change of wind was useless work for the wind never did change now that men understand the extent and limits of the trades knowing where they are constant where they change with the season to land influences check or draw them aside the sailor can make use of or avoid the trades as he will moreover the widespread use of steam makes the direction of winds less vitally important matters than of old in a large proportion of cases end of chapter 19 Chapter 20 The Circulation of Air All around the world on and near the equator lies a remarkable band of calm air The Northern Trades are one of the world's most beautiful cities and the world's most beautiful cities and the world's most beautiful cities and the world's the northern trades meet there the southern trades and each tends to counteract the other the belt of calms known as the daldrums has to be passed by every ship going from one hemisphere to the other sailing vessels have often been kept there for weeks unable to advance like that of the ancient mariner as idle as a painted ship upon a painted ocean in olden days before steam power was known passengers to Australia or South America were often long delayed the wind is scant rain is abundant sometimes to a tremendous extent old sailors tell of such deluges combined with dead calms that fresh water has absolutely been ladled up from the surface of the still ocean the trade wind belts have been already described as currents of air pouring constantly from north and south towards the equator these are on the surface of earth low down in the ocean of air high up in the atmosphere above the trades are to exactly opposite currents of air known as anti-trades pouring away from the equator towards the north and south they too have been mentioned earlier as the result of the heated air over the equator as there are upper and under currents of water in the ocean so there are upper and under currents of air in the atmosphere the wind may blow where we stand straight from the north but there is no proof that higher up the wind does not blow direct to the north careful watching of clouds shows often that the upper layers are traveling in a perfectly different direction from the lower layers before a thunderstorm when attention is drawn to the sky we often hear it remarked that the cloud has come up against the wind the cloud has of course done nothing of the kind a cloud can no more float in the air against the wind a cloud of wood can float in a river against the stream the cloud is merely born towards us by a different current of air from that which at the moment we feel the same fact of differing air currents though less sharply marked than in stormy weather may be seen on many a fine and breezy day but people do not commonly observe the anti-trades lie very high up so high that no mountain climber has walked out of the lower trades into the anti-trades towards the outer verge of the trade belt however the anti-trades are found descending to lower levels and even without this there are abundant proofs of their existence beyond the trade belts to north and south lie two more so called calm belts that to the south is named sometimes the comms of Capricorn that to the north the comms of Cancer some know the latter as the horse latitudes a very obvious connection exists between sea horses mare's tails and high winds which is rather at variance with one's notions of calmness the cancer comms are however of a most variable nature much broken by severe gales and heavy storms there is a marked difference between the calm belt of the equator and the two calm belts of the tropics the calm belt of the equator has incessant currents of air pouring in below from north and south while incessant currents of air flow away above so the tendency of air there must be to rise steadily upward in the so called comms of Cancer and Capricorn regions really of unsettled winds and violent storms alternating with weary some comms things are just the other way streams of air pour in above from north and south while streams below flow out towards north and south so the tendency of the air there must be to sink steadily downward from pressure from above which indeed has been noted as markedly shown by the barometer there is also a considerable difference in the condition of air currents flowing from the equator to north and south and air currents flowing from north or south to the equator the equator is the largest circle of latitude and all other such circles from the equator to the poles row smaller and smaller if a current of air pours from the north pole to the equator it runs in an ever widening bed this may be seen by following the course of two longitude lines on the globe beginning at the pole and ending at the equator but an air current traveling the other way moves in an ever narrowing bed and in the first case it can spread itself out in the second it has less and less room regions north of the cancer calm and south of the capricorn calm are no longer characterized by such persistent winds as the trades and anti trades still the great circulation of the atmosphere goes on only in a more irregular mode something of a rough plan may still be detected the winds of the region between the horse latitudes and the north pole may seem to wander here and there without aim or rule yet on the whole there is a general tendency of air currents below to flow towards the pole and of air currents above to flow away from the pole in the northern hemisphere these winds have little fair play being constantly interfered with by the rapid heating and cooling of great continents but in the southern hemisphere when much ocean and little land are found westerly winds circling around and towards the south pole have full swing not only are they almost as persistent as the trades but far stronger becoming a time so violent as to have gained for that region the expressive cognomen of the roaring forties these winds are westerly not as one might expect southerly and northerly they do not travel towards the poles from do south and do north but rather from a westerly direction one reason for this is not hard to find if the whole surrounding atmosphere poured straight in upon either pole the piling up of air above the poles would be tremendous a general influx of air from all sides could scarcely be balanced by any amount of flowing away above such a state of things is not possible the winds nearing the poles must with lessening space play round and about must approach rather by circling than by direct lines each air current fitting in as it can with other currents another reason for these westering gales of low altitude is connected with the daily whirl of our earth upon its axis in an earlier chapter we saw how the atmosphere revolves with the earth from west to east each part of it at the same speed as the ground on which it rests we saw how the air of the equator as a steady speed of 1000 miles or so an hour while at the poles it scarcely moves at all and all the way between the poles the equator it moves like the ground at different rates now when air comes from the north towards the equator it partakes of the slow northern whirl and as the earth below rushes faster and faster it lags behind seeming to come not from the north but from the north east when on the contrary air flows from the equator towards the north it partakes of the rapid tropical whirl and as the ground beneath moves more and more slowly it out races the earth appearing to come not from the south but from the southwest thus the trades blowing equator words which would be northerly winds become north easterly and those winds blowing pole words which would be southerly become westerly we have now some idea what may be called the rough draft or general outline of atmospheric circulation briefly it is as follows one a belt of comms on and near the equator winds flowing in below and flowing out above two on either side a belt of steady winds the trades below flowing towards the equator the anti trades above flowing away from the equator three outside the trade belts to the north a belt of cancer comes to the south a belt of capricorn comms the winds in both cases flowing in above from north and south flowing out below towards north and south four beyond the comms of cancer to the north and beyond the comms of capricorn to the south a belt of variable winds below more or less westerly and tending towards the pole above tending away from the pole five at the north pole and at the south pole the comparative comm winds mainly flowing in below and flowing out above thus a continuous circulation of air is kept up over the whole earth no part of the atmosphere is ever addressed but a perpetual interchange of air currents goes on everywhere one may in imagination follow the progress of an air particle starting from the equator performing the grand tour at the equator it rises upward into higher regions of the atmosphere as part of an antitrade it journeys northward passing over the contrary blowing trade below in the comms of cancer it descends to earth plays about in a storm or two and takes its choice between joining the stream of air which returns to the equator and joining the stream of air which more fitfully finds its way towards the north pole having been carried away by the latter this wandering particle is born to and fro by many breezes visiting diverse countries yet on the whole progressing northward reaching at length the neighborhood of the pole the particle finds itself at the heart of comm though by no means a region where storms are unknown rising upward after a while it joins a high level current of air proceeding southward an infant trade wind in fact though hardly yet to be recognized as such gaining once more the comms of cancer it descends anew and this time passes out below to the southward trade at the equator again the particle works its way upward as before and joins the upper current flowing southward as part of an antitrade reaching the comms of Capricorn it descends passes then spillo through the roaring forties towards the south polar comm finds an upward path there to the outflowing currents above returns to Capricorn descends afresh joins the southeast trade and reaches the equator ready to begin the round anew this circle of possible movements for a single air particle might be almost infinitely varied by permitting it to join any of the innumerable side currents and eddies of air caused by countless land influences it should become part of a monsoon here part of a hurricane there it should unite itself to a cyclone in one place to an anticyclone in another though the broad outlines of atmospheric circulation may be sketched with a certain regularity the variations upon that rough plan are past counting in the air circulation of earth as in the blood circulation of a man's body there are little arteries as well as great arteries and hundreds of tiny streams for every big stream the wind goeth towards the south and turneth about unto the north it whirleth about continually and the wind returneth again according to his circuits what I have tried to explain in many paragraphs is given thus in five and twenty words of holy writ the circulation of air described in a nutshell end of chapter 20 recording by John Brandon