Clouds, in any one of the preceding forms, at the same degree of elevation, or two or more of these forms at different elevations, may increase and become so dense as completely to obscure the sky; this, to an inexperienced observer, would seem to indicate the speedy commencement of rain. But Mr. Howard is of opinion that clouds, while in any of the states above described, never let fall rain.
Before rain the clouds always undergo a change of appearance, sufficiently remarkable to give them a distinct character. This appearance, when the rain happens overhead, is but imperfectly seen; but from the observations of aeronauts, it appears that whenever a fall of rain occurs, and the sky is at the same time entirely overcast with clouds, there will be found to exist another stratum of clouds at a certain elevation above the former. So, also, when the sky is entirely overcast and rain is altogether or generally absent, the aeronaut, upon traversing the canopy immediately above him, is sure to enter upon an upper hemisphere either perfectly cloudless or nearly so. These remarks were, we believe, first made by Mr. M. Mason, and he states that they have been verified during many hundred ascents.
In October, 1837, two ascents were made by Mr. Mason, which well ill.u.s.trate what has been said. On the 12th, "the sky was completely overspread with clouds, and torrents of rain fell incessantly during the whole of the day. Upon quitting the earth, the balloon was almost immediately enveloped in the clouds, through which it continued to work its way upwards for a few seconds. Upon emerging at the other side of this dense canopy, a vacant s.p.a.ce, of some thousand feet in breadth, intervened, above which lay another stratum of a similar form and observing a similar character. As the rain, however, still continued to pour from this second layer of clouds, to preserve the correctness of the observation, a third layer should, by right, have existed at a still further elevation; which, accordingly, proved to be the case. On the subsequent occasion of the ascent of the same balloon, (October 17th,) an exactly similar condition of the atmosphere, with respect to clouds, prevailed; unaccompanied, however, with the slightest appearance of rain.
No sooner had the balloon pa.s.sed the layer of clouds immediately above the surface of the earth, than, as was antic.i.p.ated, not a single cloud was to be found in the firmament beyond; an unbroken expanse of clear blue sky everywhere embracing the frothy plain that completely intercepted all view of the world beneath."
Mr. Howard had not the advantages of a balloon to a.s.sist his observations. He has noticed that during rain and before the arrival of the denser and lower clouds, or through their interstices, there exists, at a greater height, a thin light veil or a hazy appearance. When this has considerably increased, the lower clouds are seen to spread till they unite in all points and form one uniform sheet. The rain then commences, and the lower clouds arriving from the windward, move under this sheet and are successively lost in it. When the latter cease to arrive, or when the sheet breaks, letting through the sun-beams, every one"s experience teaches him to expect that the rain will abate or leave off.
But there often follows an immediate and great addition to the quant.i.ty of cloud. At the same time the darkness becomes less, because the arrangement, which now returns, gives free pa.s.sage to the rays of light; the lower broken clouds rise into c.u.muli, and the upper sheets put on the various forms of the c.u.mulo-stratus, sometimes pa.s.sing to the cirro-c.u.mulus.
The various phenomena of the rain-cloud are best seen in a distant shower. If the c.u.mulus be the only cloud at first visible, its upper part is seen to become tufted with cirri. Several adjacent clouds also approach and unite at its side. The cirri increase, extending upwards and sideways, after which the shower is seen to commence. At other times, the cirro-stratus is first formed above the c.u.mulus, and their sudden union is attended with the production of cirri and rain. In either case the cirri spring up in proportion to the quant.i.ty of rain falling, and give the cloud a character by which it is easily known at great distances, and which has long been called by the name of _nimbus_.
When one of these arrives hastily with the wind, it brings but little rain, and frequently some hail or driven snow.
Since rain may be produced and continue to fall from the slightest obscuration of the sky by the nimbus, while a c.u.mulus or a c.u.mulo-stratus, of a very dark and threatening aspect, pa.s.ses on without discharging any until some change of state takes place; it would seem as if nature had destined the latter as reservoirs, in which water is collected from extensive regions of the air for occasionally irrigating particular spots in dry seasons; and by means of which it is arrested, at times, in its descent in wet ones.
Although the nimbus is one of the least beautiful of clouds, it is, nevertheless, now and then adorned by the splendid colouring of the rainbow, which can only be seen in perfection when the dark surface of this cloud forms for it a background.
The small ragged clouds which are sometimes seen sailing rapidly through the air, are called _scud_. They consist of portions of a rain-cloud, probably broken up by the wind, and are dark or light according as the sun shines upon them. They are the usual harbingers of rain, and, as such, are called by various names, such as _messengers_, _carriers_, and _water-waggons_.
In attempting to explain the production of clouds and rain, it is necessary to observe that the subject is beset with difficulties-the discussion of which does not belong to this little volume; but the following notice of Dr. Hutton"s theory may not be out of place.
It has been already stated, that the air supplies itself with moisture from the surface of the waters of the earth. This it continues to do at all temperatures, until it is so charged with vapour that it cannot contain any more. The air is then said to be _saturated_. Now, the quant.i.ty of moisture which a given bulk of air can contain, depends entirely upon the temperature of the air for the time being. The higher the temperature of the air the greater will be the quant.i.ty of vapour contained in it; and, although it may be perfectly invisible to the eye, on account of the elasticity which the heat imparts to it, yet it can easily be made visible by subtracting a portion of the heat. If, for example, a gla.s.s of cold water be suddenly brought into a warm room, moisture from the air will be condensed upon the outside of the gla.s.s in the form of dew. A similar change is supposed to take place when two currents of air having different temperatures, but both saturated with vapour, are mingled together; an excess of vapour is set free, which forms a cloud or falls down as rain. If the currents continue to mingle uniformly, "the clouds soon spread in all directions, so as to occupy the whole horizon; while the additional moisture, incessantly brought by the warmer current, keeps up a constant supply for condensation, and produces a great and continued deposition of moisture in the form of rain. By degrees, the currents completely intermingle, and acquire a uniform temperature; condensation then ceases; the clouds are re-dissolved; and the whole face of nature, after being cooled and refreshed by the necessary rain, is again enlivened by the sunshine, thus rendered still more agreeable by its contrast with the previous gloom."
If the cloud, produced by the mingling of two differently heated currents of moist air, happen to form in the upper regions of the sky, it may be heavier than its own bulk of air, and will consequently begin to sink.
Should the atmosphere near the earth be less dense than the cloud, the latter will continue to descend till it touches the ground, where it forms a mist. If the vapour has been condensed rapidly and abundantly, the watery particles will form rain, hail, or snow, according to the temperature of the air through which they pa.s.s. But it may happen that the cloud, in descending, arrives in a warmer region than that in which it was formed: in this case, the condensed moisture may again become vapour, and ascend again to a region where condensation may again take place.
Mr. Daniell"s explanation of the formation of rain differs from the above in some of its particulars, which are not sufficiently elementary to be given here; but it may be instructive to give a few of Mr. Howard"s ill.u.s.trations respecting the formation of the various clouds. If hot water be exposed to cool air, it _steams_-that is, the vapour given off from the surface is condensed in mixing with the air; and the water thus produced appears in visible particles, the heat of the vapour pa.s.sing into the air. This effect may be seen about sunrise, in summer, on the surface of ponds warmed by the sun of the previous day, and also with water newly pumped from a well. But the small cloud formed in these instances usually disappears almost as soon as formed, the air being too dry to allow it to remain. But in the wide regions of the atmosphere the case is different, on account of the vast supply of vapour, and the ascent and descent of the cloud to regions which allow it to remain tolerably permanent. In the fine evenings of autumn, and occasionally at other seasons, mists appear suddenly in the valleys, gradually filling these low places, and even rising to a certain height, forming a foggy atmosphere for the following day. These collections of visible vapour resting on the earth, and often cut off so as to form a level surface above, so nearly resemble a sheet of water, as to have been occasionally mistaken for an inundation, the occurrence of the previous night. Such is the origin and appearance of the _stratus_: it const.i.tutes the fog of the morning, and sometimes, as at the approach of a long frost, occupies the lower atmosphere for several days. But the sun, we will suppose, has broken through and dissipated this obscurity, and cleared the lower air.
On looking up to the blue sky, we see some few spots showing the first formation of a cloud there: these little collections increase in number, and become clouds, heaped, as it were, on a level base, and presenting their rounded forms upwards; in which state they are carried along in the breeze, remaining distinct from each other in the sky. This is the _c.u.mulus_, or _heap_.
By and by, if the clouds continue to form, and enough vapour is supplied from above, these heaps are seen to grow over their base like a mushroom or cauliflower. Perhaps a flat top is seen forming separately, and this afterwards joins the simple heap of cloud; or the flat forms and the heaps become mixed irregularly among each other, occupying the s.p.a.ces everywhere, till the sky becomes overcast, and presents the usual appearance of dense clouds. This is the _c.u.mulo-stratus_, or _heaped and flat cloud_. It is not productive of rain, and it forms, both in summer and in winter, the common scenery of a full sky.
On examining minutely the higher regions of the air, especially after the sky has been clear for some time, the spectator will probably see the cirrus descending from above in the form of _threads_ or _locks_ and _feathers_, which go on increasing until they fill the sky. They are more commonly seen above the two former kinds, which float upon the clear air below. On continuing to watch the cirri, they will be seen to pa.s.s to the intermediate form of cirro-c.u.mulus, consisting of smaller rounded clouds attached to each other, or simply collected together in a flat aggregate, and forming the mottled or dappled sky.
The c.u.mulo-stratus is more dense and continuous in its structure; thick in the middle, and thinned off towards the edges. Over-head it is a mere bed of haze, more or less dense. In the horizon, when seen sideways, it often resembles shoals of fish, as already noticed; but it is liable to put on the most ragged and patchy appearances, making a very ugly sky.
The nimbus, or rain cloud, is seen to the greatest advantage in profile, in the horizon, and at a great distance, when it often resembles a lofty tower raised by its greater height to a conspicuous place among the dark threatening clouds, and catching the sun"s last rays upon its broad summit and sides. In its nearer approach, it may always be known by being connected below with an obscurity caused by the rain it lets fall, and which reaches down to the horizon.
In ascending from the lower valleys to the tops of lofty mountains, clouds may be traced through six modifications, the cirrus being seen from the loftiest summits, while the other forms are only skirting the sides of the mountains. Mr. Mason remarks, that clouds occasionally lie so low, that before the balloon seems to have entirely quitted the earth, it has been received between their limits, and entirely enveloped within their watery folds. Clouds, on the contrary, are sometimes at such a height, that the balloon either never comes into contact with them at all, or, if it pa.s.ses through one layer, the aeronaut continues to behold another occupying a still remoter region of the skies above.
As a general rule, it is stated that the natural region of clouds is a stratum of the atmosphere lying between the level of the first thousand feet, and that of one removed about ten thousand feet above it. Of course it is not supposed but that clouds are occasionally found on both sides of the bounds here a.s.signed to them; the mist occupies the lowest valleys, while, on the other hand, long after the aeronaut has attained the height of ten thousand feet, some faint indications of clouds may still be seen partially obscuring the dark blue vault above him. As he continues to ascend, the blue of the sky increases in intensity; and should a layer of clouds shut out all view of the earth, "above and all around him extends a firmament dyed in purple of the intensest hue; and from the apparent regularity of the horizontal plane on which it rests, bearing the resemblance of a large inverted bowl of dark blue porcelain standing upon a rich Mosaic floor or tesselated pavement. Ascending still higher, the colour of the sky, especially about the zenith, is to be compared with the deepest shade of Prussian blue."
[Picture: Various forms of hail-stones]
CHAPTER IV.
ON HAIL-THE HAIL-STORMS OF FRANCE-DISASTROUS EFFECTS OF HAIL-THE HAIL-STORMS OF SOUTH AMERICA-THEIR SURPRISING EFFECTS-ORIGIN AND NATURE OF HAIL-PERIODICAL FALLS OF HAIL-HAIL CLOUDS-HAILSTONES-THEIR VARIOUS FORMS-EXTRAORDINARY SIZE OF HAILSTONES.
As hail seems to be nothing more than frozen rain, it is necessary to collect a few particulars respecting it in this place.
Great Britain is essentially a rain country; but there are some parts of the world which have obtained the unhappy distinction of being _hail_ countries: such, for example, as some of the most beautiful provinces of France, which are frequently devastated by hail-storms. One of the most tremendous hail-storms on record is that which occurred in that country in July 1788. This fearful storm was ushered in by a dreadful and almost total darkness which suddenly overspread the whole country. In a single hour the whole face of nature was so entirely changed, that no person who had slept during the tempest could have believed himself in the same part of the world when he awoke. Instead of the smiling bloom of summer, and the rich prospects of a forward autumn, which were just before spread over the face of that fertile and beautiful country, it now presented the dreary aspect of an arctic winter. The soil was changed into a mora.s.s; the standing corn beaten into a quagmire; the vines were broken to pieces, and their branches bruised in the same manner; the fruit-trees of every kind were demolished, and the hail lay unmelted in heaps like rocks of solid ice. Even the robust forest trees were incapable of withstanding the fury of the tempest; and a large wood of chesnut trees, in particular, was so much damaged, that it presented, after the storm, little more than bare and naked trunks. The vines were so miserably hacked and battered, that four years were estimated as the shortest period in which they could become again in any degree productive. Of the sixty-six parishes included in the district of Pontoise, forty-three were entirely desolated; while, of the remaining twenty-three, some lost two-thirds, and others above half their harvest.
This storm began in the south, and proceeded in two parallel bands from the south-west to the north-east; the extent of one of them being 175 leagues, and of the other 200; thus traversing nearly the whole length of that great kingdom, and even a portion of the Low countries. The mean breadth of the eastern portion was four leagues, and of the western two: and, what is very remarkable, the interval between the two bands, amounting to five leagues, was deluged with heavy rain. The largest of the hail-stones weighed half a pound each.
The progress of this storm, which was from south to north, was at the rate of 16 leagues an hour; and the velocity of the two bands was precisely the same. The continuance of the hail was limited to seven or eight minutes, at each of the princ.i.p.al stations marked.
There are instances, however, on record, in which hail has produced even more tremendous results than those above recorded. In some parts of South America hail-stones are sometimes so large and so hard, and fall with such violence, that large animals are killed by them. Mr. Darwin, encamping at the foot of the Sierra Tapalguen, says:-"One of the men had already found thirteen deer lying dead, and I saw their fresh hides.
Another of the party, a few minutes after my arrival, brought in seven more. Now I well know that one man without dogs could hardly have killed seven deer in a week. The men believed they had seen about fifteen dead ostriches, (part of one of which we had for dinner;) and they said that several were running about evidently blind in one eye. Numbers of small birds, as ducks, hawks, and partridges, were killed. I saw one of the latter with a black mark on its back, as if it had been struck with a paving-stone. A fence of thistle-stalks round the hovel was nearly broken down; and my informer, putting his head out to see what was the matter, received a severe cut, and now wears a bandage. The storm was said to have been of limited extent: we certainly saw, from our last night"s bivouac, a dense cloud and lightning in this direction. It is marvellous how such strong animals as deer could thus have been killed; but, I have no doubt, from the evidence I have given, that the story is not in the least exaggerated." Dr. Malcolmson informed Mr. Darwin, that he witnessed, in 1831, in India, a hail-storm, which killed numbers of large birds, and much injured the cattle. These hail-stones were flat; one was ten inches in circ.u.mference; and another weighed two ounces.
They ploughed up a gravel-walk like musket-b.a.l.l.s, and pa.s.sed through gla.s.s windows, making round holes, but not cracking them.
There is much in the origin and formation of hail that cannot well be explained. Volta regarded the formation of small flakes of ice, the kernels of future hail-stones, in the month of July, during the hottest hours of the day, as one of the most difficult phenomena in nature to explain. It is difficult to account for the comparative scarcity of hail-showers in winter; as also, for the great size which hailstones are often known to attain.
It appears from certain resemblances in the descents of rain, snow, and hail, that they have a common origin, their different formations being explained by difference of temperature. Howard has observed a huge nimbus affording hard s...o...b..a.l.l.s and distinct flakes of snow at the same time. Hail and rain are by no means uncommon from the same cloud. The size of a cloud may be such, or clouds may exist in different elevations, which in an upper region produce hail, in a lower region snow, and at a still lower elevation rain. Rain may also form in an upper region of the sky, and descend into a colder stratum of the atmosphere, and be frozen into hail. Hail generally precedes storms of rain.
Change of wind and the action of opposite currents, so necessary for the production of rain, are also frequent during hail-storms. While clouds are agitated with the most rapid motions, rain generally falls in greatest abundance; and if the agitation be very great it generally hails. Before the descent of hail a noise is heard, a particular kind of crackling, which has been compared to the emptying of a bag of walnuts.
The descent of hail in some countries appears to occur at particular periods. In the central parts of France, Italy, and Spain, it usually hails most abundantly during the warmest hours of the day in spring and summer, and in Europe generally it falls princ.i.p.ally during the day; but there are examples recorded of great hail-storms which have taken place during the night. Near the equator, it seldom hails in places situated at a lower level than 350 fathoms, for, although the hail may be formed, the warmth of the regions prevents it from falling in that state.
The appearance of hail clouds seems to be distinguished from other stormy clouds by a very remarkable shadowing. Their edges present a mult.i.tude of indentations, and their surfaces disclose here and there immense irregular projections. Arago has seen hail-clouds cover with a thick veil the whole extent of a valley, at a time when the neighbouring hills enjoyed a fine sky and an agreeable temperature.
Hailstones of similar forms are produced at similar levels. They are smaller on the tops of mountains than in the neighbouring plains. If the temperature or the wind alter, the figures of the hailstones become immediately changed. Hailstones of the form of a six-sided pyramid have been known to change, on the wind changing to the north-east, to convex lenses, so transparent and nicely formed, that they magnified objects without distorting them. Some hailstones are globular, others elongated, and others armed with different points.
In the centres of hailstones small flakes of spungy snow are frequently found, and this usually is the only opaque point in them. Sometimes the surface is covered with dust, like fine flour, and is something between hail and snow. This never falls during summer in southerly countries.
In the Andes hailstones from five to seven lines in diameter are sometimes formed of layers of different degrees of transparency, so as to permit rings of ice to be separated from them with a very slight blow.
In Orkney, hailstones have fallen as finely polished as marbles, of a greyish white colour, not unlike fragments of light-coloured marble.
Hailstones are often so hard and elastic, that those which fall on the stones rebound without breaking to the height of several yards; and they have been known to be projected from a cloud almost horizontally, and with such velocity as to pierce gla.s.s windows with a clear round hole.
On the 7th May, 1822, some remarkable hailstones fell at Bonn, on the Rhine. Their general size was about an inch and a half in diameter, and their weight 300 grains. When picked up whole, which was not always the case, their general outline was elliptical, with a white, or nearly opaque spot in the centre, about which were arranged concentric layers, increasing in transparency to the outside. Some of them exhibited a beautiful star-like and fibrous arrangement, the result of rows of air bubbles dispersed in different radii. The figures at the head of this chapter show the external and internal appearances of these hailstones.
The smaller figures represent pyramidal hail, common in France, and occasionally in Great Britain.
Brown hailstones have been noticed. Humboldt saw hail fall of the colour of blood.
On the 15th July, 1808, Howard noticed, in Gloucestershire, hailstones from three to nine inches in circ.u.mference; appearing like fragments of a vast plate of ice which had been broken in its descent to the earth.
On the 4th June, 1814, Dr. Crookshank noticed, in North America, hailstones of from thirteen to fifteen inches in circ.u.mference. They seemed to consist of numerous smaller stones fused together.
On the 24th July, 1818, during a storm in Orkney, Mr. Neill picked up hailstones weighing from four ounces to nearly half a pound.
[Picture: Rain gauges]