Whether this be so or not, the apparent origin of the cloud on the peak, and radiation from it, distinguish it from the drift-cloud of level country, which arranges itself at the horizon in broken masses, such as Fig. 89, showing no point of origin; and I do not know how far they are vertical cliffs or horizontally extended fields. They are apt to be very precipitous in aspect, breaking into fragments with an apparently concentric motion, as in the figure; but of this motion also—whether vertical or horizontal—I can say nothing positive.
§ 12. The absolute scale of such clouds may be seen, or at least demonstrated, more clearly in Fig. 88, which is a rough note of an effect of sky behind the tower of Berne Cathedral. It was made from the mound beside the railroad bridge. The Cathedral tower is half-a-mile distant. The great Eiger of Grindelwald is seen just on the right of it. This mountain is distant from the tower thirty-four miles as the crow flies, and ten thousand feet above it in height. The drift-cloud behind it, therefore, being in full light, and showing no overhanging surfaces, must rise at least twenty thousand feet into the air.
§ 13. The extreme whiteness of the volume of vapor in this case (not, I fear, very intelligible in the woodcut[3]) may be partly owing to recent rain, which, by its evaporation, gives a peculiar density and brightness to some forms of clearing cloud. In order to understand this, we must consider another set of facts. When weather is thoroughly wet among hills, we ought no more to accuse the mountains of forming the clouds, than we do the plains in similar circumstances. The unbroken mist buries the mountains to their bases; but that is not their fault. It may be just as wet and just as cloudy elsewhere. (This is not true of Scottish mountain, by the way.) But when the wet weather is breaking, and the clouds pass, perhaps, in great measure, away from the plains leaving large spaces of blue sky, the mountains begin to shape clouds for themselves. The fallen moisture evaporates from the plain invisibly; but not so from the hill-side. There, what quantity of rain has not gone down in the torrents, ascends again to heaven instantly in white clouds. The storm passes as if it had tormented the crags, and the strong mountains smoke like tired horses.
§ 14. Here is another question for us of some interest. Why does the much greater quantity of moisture lying on the horizontal fields send up no visible vapor, and the less quantity left on the rocks glorify itself into a magnificent wreath of soaring snow?
First, for the very reason, that it is less in quantity, and more distributed; as a wet cloth smokes when you put it near the fire, but a basin of water not.
| Fig. 88. |
| Fig. 89. |
| To face page 128. |
| Fig. 90. |
The previous heat of the crags, noticed in the first volume, p. 249, is only a part of the cause. It operates only locally, and on remains of sudden showers. But after any number of days and nights of rain, and in all places exposed to returning sunshine and breezes, the distribution of the moisture tells. So soon as the rain has ceased, all water that can run off is of course gone from the steep hill-sides; there remains only the thin adherent film of moisture to be dried; but that film is spread over a complex texture—all manner of crannies, and bosses, and projections, and filaments of moss and lichen, exposing a vast extent of drying surface to the air. And the evaporation is rapid in proportion.
§ 15. Its rapidity, however, observe, does not account for its visibility, and this is one of the questions I cannot clearly solve, unless I were sure of the nature of the vesicular vapor. When our breath becomes visible on a frosty day, it is easily enough understood that the moisture which was invisible, carried by the warm air from the lungs, becomes visible when condensed or precipitated by the surrounding chill; but one does not see why air passing over a moist surface quite as cold as itself should take up one particle of water more than it can conveniently—that is to say, invisibly—carry. Whenever you see vapor, you may not inaccurately consider the air as having got more than it can properly hold, and dropping some. Now it is easily understood how it should take up much in the lungs, and let some of it fall when it is pinched by the frost outside; but why should it overload itself there on the hills, when it is at perfect liberty to fly away as soon as it likes, and come back for more? I do not see my way well in this. I do not see it clearly, even through the wet cloth. I shall leave all the embarrassment of the matter, however, to my reader, contenting myself, as usual, with the actual fact, that the hill-side air does behave in this covetous and unreasonable manner; and that, in consequence, when the weather is breaking (and sometimes, provokingly, when it is not), phantom clouds form and rise in sudden crowds of wild and spectral imagery along all the far succession of the hill-slopes and ravines.