Fall (hydrology)

“ The fresh fall of snow, which had come over night, was exceedingly slight. It had come down softly, floatingly, with all the winds of the prairies hushed, every flake consisting of one or two large, flat crystals only, which, on account of the nearly saturated air, had gone on growing by condensation till they touched the ground. Such a condition of the atmosphere never holds out in a prolonged snowfall, may it come down ever so soft-footedly; the first half hour exhausts the moisture content of the air. ”

“ As these various falls are in one vertical plane, the effect of the whole from the opposite side of the valley is nearly as grand, and perhaps even more picturesque, than it would be if the descent was made in one sheet from the top to the bottom. The mass of water in the 1,500 feet fall is too great to allow of its being entirely broken up into spray, but it widens very much as it descends, and as the sheet vibrates backward and forward with the varying pressure of the wind, which acts with immense force on this long column of water, the effect is indescribably grand. ”

“ But many of the rainmakers believe that dry ice makes more rain fall or causes it to fall sooner than 228 it would otherwise. Thus, as the cloud moves along, the rainmaker may be able to cause a shower in a certain place, whereas the cloud might have moved far away before it began to rain. In this story the important point is that some of the experimenters believe that dry ice or some other chemical will cause the rain to fall but will make it much less likely that nature’s process will develop to the point of producing hail. ”

“ A method has been devised, also by Professor Russell, for determining the maximum fall of temperature at the center of the cold wave. The maximum fall and extent of fall being known, from suitably prepared tables, the area of twenty-degree fall can be derived. Previously prepared pieces of card-board are laid in the proper position on a map of suitable scale, and lines drawn around them. ”

“ Over small areas and in exceptional cases as many feet have been known to fall in that time, as fresh, vapor-bearing winds steadily blew into a storm center, rose, discharged their burdens as they cooled with ascent, and then flowed away, again to be charged with moisture when they came into contact with wet surfaces. It is impossible to drown the entire earth with rainfall, no matter how long continued. ”

“ The force of these movements is measured partly by the force of latent heat set free, and partly by the mechanical power of the rain-fall, a very small fraction of which constitutes the water-power of all our rivers. Such a fruitful source of disturbance, generated by so slight an accident as the upward movement of the saturated air, expanded by its own agency to so great an extent, so sudden and discontinuous in its action, so obscure in its origin, and so distinct in its effects,—such a phenomenon defies the powers of mathematical prediction, and rouses all the winds to sedition. ”

“ Even below the temperature at which water freezes, it still evaporates; and thus, when a fall of snow is succeeded by a continued frost, the snow gradually disappears from the fields without having melted, evaporating while yet solid. From the surface of all the water of the globe, therefore, there is continually ascending a stream of watery vapour; but as the proportion of sea is so much greater than that of land, we may look upon the ocean as being the source of the watery vapour of the air upon the large scale. ”

“ Obviously percolation will vary considerably with the seasons, and a succession of wet periods or a continuous downpour will increase the quantity of infiltrating water; but the effect of a fall may not be experienced until some time after it occurs, as in districts or rivers that are fed with water from the thawing of snow upon surrounding or distant hills water reaches the lower tracts of country in hot and sunny, and therefore generally dry weather, when evaporation is the greatest, and not in winter. ”

“ And this is what ought to be expected; for, though the storms themselves are essentially anomalous, yet many of the causes which cooperate to induce them are constant or periodic, while others are subject to but slight perturbations. It is obvious that no more moisture can be precipitated than has been evaporated, and that the winds only gain suddenly by the fall of rain the forces which they have lost at their leisure in the absorption of moisture. ”

“ The falling rain—and also the hail which occasionally attends a thunder-storm—cools the air through which it passes and the cold air sinks to the earth with a considerable velocity. As it reaches the earth the down-rush plows underneath the warm, moist air in front of the storm, lifting it and thereby aiding the updraught. As the cold air spreads over the ground its velocity is great enough to raise clouds of loose dust that almost always precede the fall of rain. ”

“ In the other case, they fall as drops of rain through the air. Rain, therefore, is thus a further stage in the condensation of the aqueous vapor of the atmosphere. The minute particles of the cloud, as condensation proceeds, gather more moisture round them, until at last they form drops of water, too heavy to hang any longer suspended in the air. These then fall to the earth as rain-drops. ”

“ It is known that upon the thawing of a heavy fall of snow, and of quickly succeeding and separate falls of snow, percolation is great; also after heavy and continuous rains, especially if the strata dip towards a river, and in the case of springs whose yield depends more upon percolation than the amount of rainfall, a wet winter will cause an increased flow some time after, when the earth may become saturated. ”

“ But when the temperature [Pg 41] and pressure are not suitable for the formation of what we commonly know as the rain, the fine drops fall into the air under the cloud, where they immediately evaporate from their dust free-surfaces, if the air is dry and warm. This is, in other words, the decay of clouds.
It is a curious fact that objects in a fog may not be wetted, when the number of water-particles is great. It seems that these water-particles all evaporate so quickly that even one’s hand or face is not sensible of being wetted. The particles are minutely small ”

“ The crackling sound so commonly heard along the outer edge of the ice exposed to the action of the waves, is a consequence of the penetration of its pores by the sea-water, which is then immediately frozen, and disruption follows at once. But disruption on a far grander scale is due to a cause the very opposite of this, the sudden contraction and splitting of the ice, even in the great ice-fields, which is produced usually in winter by the sudden fall of the temperature. ”

“ The ether evaporates the water, the wind carries it about and fills the air with dust, and the dust and vapor again fall to the earth as rain, and the falling becomes a process of disintegration in part, but mainly an agency for the transportation of material of the rocky cellate by sheets of water into streams and by streams into the larger bodies. Then gravity acts as a process, throwing the load of transportation to the bottom in assorted layers. ”

“ The condensed aqueous vapors raised in the atmosphere by the sun and wind, converted into clouds, which fall in rain, snow, hail, or mist: their falling is occasioned by their own weight in a collision produced by contrary currents of wind, from the clouds passing into a colder part of the air, or by electricity. If the vapors are more copious, and rise a little higher, they form a mist or fog, which is visible to the eye; higher still they produce rain. Hence we may account for the changes of the weather: why a cold summer is always a wet one—a warm, a dry one. ”

“ The limestone edge of both falls is wearing away in the center, the Canadian fall now being V-shaped, and the American fall showing the same tendency, although its process of recession has begun more recently. For some distance below the falls there is smooth current, the mass of water which pours over the precipice sinking and only coming to the surface two miles below, where the rapids, more magnificent and wilder than those above the falls, begin, and culminate in the rapids of the Upper Whirlpool. ”

“ Even when there is no sensible breeze, the unequal heating of the surface causes ascending and descending currents, which lose or gain heat by expansion or contraction. More commonly winds are experienced which, by impinging on the inclined surfaces, force bodies of air to higher elevations, and thereby directly cause a fall of temperature. ”

“ When fogs spread, as a rule they do so in air already saturated with moisture. What really spreads, is the cold air which by mixing with, and thereby cooling, the warmer, moisture-laden atmosphere causes the condensation. That is why our fall mists mostly are formed in an exceedingly slight but still noticeable breeze. But in the case of these northern mist pools, whenever the conditions are favourable for their formation, the moisture of the upper air seems to be pretty well condensed as dew It is only in the hollows of the ground that it remains suspended in this curious way. ”

“ The waves caused by such falls in a river differ much from the waves of the sea. The water of an ocean wave merely rises and falls; the form only passes on, and form chases form unceasingly. A body floating on such waves merely rises and sinks--does not progress unless impelled by wind or some other power. But here the water of the wave passes on while the form remains. The waters plunge down ten or twenty feet to the foot of a fall, spring up again in a great wave, then down and up in a series of billows that gradually disappear in the more quiet waters below ”

“ At the foot of the Falls, where the water seethes and mounts, both in the form of vapor and liquid globules, an eminence is gradually formed, rising constantly in tapering shape, until it reaches a considerable altitude, sometimes one-fourth or one-third the height of the Fall itself. ”

“ We find the water-fall amongst the most ancient contrivances resorted to by miners, for urging a current of air through underground workings Bellows, and circular blowing machines, were also used, at an early period for the same purpose, but rarifaction by fire, which is by far the most efficacious and certain mode, seems to have been the latest in being discovered. The water-fall is effected by pouring a stream of water down one of the pits, A or B, which, carrying a column of air with it, forces the current through the workings, and up the opposite pit. ”

“ Under the pressure of its own weight, and owing to the peculiar property of regelation which solid water possesses, the new fall attaches itself to the old crusts, and the conditions become, comparatively speaking, safe. ”

“ The downward currents are sometimes shown by a curling over of the edges of the upper disc, but the phenomenon is not often seen, as the descending movement is generally enough to dry up the cloud particles.
The rapid rising of damp air drawn from the ground brings about rapid condensation and heavy rain. The large size of thunder-drops is almost certainly due to the fact that it is only the larger drops which can fall in the teeth of the strong updraught. ”

“ Though the weight of the air is diminished by the fall of rain, yet the bulk is increased by the expansive force of the latent heat which the condensed vapors set free. Thus the rainy air expands upwards and flows outwards, and no longer able to balance the pressure of the surrounding air, it is carried still higher by inblowing winds, which rise in turn and continue the process, often extending the storm over vast areas. ”

“ But at sunset the clouds gathered again, bringing an earlier night, and the snow began to fall straight and steadily from a sky without wind, in a soft universal diffusion more confusing than the gusts and eddies of the morning. It seemed to be a part of the thickening darkness, to be the winter night itself descending on us layer by layer. ”

“ Then they bow like willows, their leaves streaming forward all in one direction, and, when the sun shines upon them at the required angle, entire groves glow as if every leaf were burnished silver. The fall of tropic light on the royal crown of a palm is a truly glorious spectacle, the fervid sun-flood breaking upon the glossy leaves in long lance-rays, like mountain water among boulders. ”

“ The broad expanse of bright green water falls perpendicularly 144 to 150 feet into the abyss below, which is entirely concealed by the vapour, the whole torrent of falling water being completely dissolved into foam and mist in the midway of its descent. Below the fall, and before its surface is quite calm, it recovers its green colour, which is, of course, totally lost in the rapidity of its descent. To make my description more clear, I subjoin a little plan of the cataracts drawn by Mr. ”

“ Friend of this worthless heart! but happier thoughts
Spring like unbidden violets from the sod,
Where patiently thou tak'st
Thy sweet and sure repose.
The shadows fall more soothing: the soft air
Is full of cheering whispers like thine own ”

“ Oppressive dusk was drooping as Mr. Tuke came in sight of the lonely tavern on the downs. The inconsistent moodiness of autumn had fallen into another humour as the day declined. The steely thrust of wind at night—a morning cold and fair and pure, such as ever seems an earnest of weeks of serene tranquillity—and then, as noon ticked into evening, a dull fall in the barometer, and gathering battalions of clouds rolling to the front, with a noise of rumbling in them like the labouring wheels of gun-carriages—such was the record of twenty-four hours. ”