V.

_CLOUDS._

From every natural fact invisible relations radiate, the apprehension

of which imparts a measure of delight; and there is a store of pleasure

of this kind ever at hand for those who have the capacity to turn

natural appearances to account. It is pleasant, for example, to lie

on one’s back upon a dry green slope and watch the clouds forming

and disappearing in the blue heaven. A few days back the firmament

was mottled with floating cumuli, from the fringes of which light of

dazzling whiteness was reflected downwards, while the chief mass of

the clouds lay in dark shadow. From the edge of one large cloud-field

stretched small streamers, which, when attentively observed, were

seen to disappear gradually, and finally to leave no trace upon the

blue sky. On the opposite fringe of the same cloud, and beyond it,

small patches of milky mist would appear, and curdle up, so as to form

little cloudlets as dense apparently as the large mass beside which

they were formed. The counter processes of production and consumption

were evidently going on at opposite sides of the cloud. Even in the

midst of the serene firmament, where a moment previously the space

seemed absolutely void, white cloud-patches were formed, their sudden

appearance exciting that kind of surprise which might be supposed to

accompany the observation of a direct creative act.

These clouds were really the indicators of what was going on in the

unseen air. Without them no motion was visible; but their appearance

and disappearance proved not only the existence of motion, but also

the want of homogeneity in the atmosphere. Though we did not see them,

currents were mingling, possessing different temperatures and carrying

different loads of invisible watery vapour. We know that clouds are

not true vapour, but vapour precipitated by cold to water. We know

also that the amount of water which the air can hold in the invisible

state depends upon its temperature; the higher the temperature of the

air, the more water will it be able to take up. But, when a portion

of warm air, carrying its invisible charge, is invaded by a current

of low temperature, the chilled vapour is precipitated, and a cloud

is the consequence. In this way two parcels of moist air, each of

which taken singly may be perfectly transparent, can produce by their

mixture an opaque cloud. In the same way a body of clear humid air,

when it strikes the cold summit of a mountain, may render that mountain

‘cloud-capped.’

An illustration of this process, which occurred some years ago in a

Swedish ball-room, is recounted by Professor Dove. The weather was

clear and cold, and the ball-room was clear and warm. A lady fainted,

and air was thought necessary to her restoration. A military officer

present tried to open the window, but it was frozen fast. He broke the

window with his sword, the cold air entered, and _it snowed in the

room_. A minute before this all was clear, the warm air sustaining

a large amount of moisture in a transparent condition. When the

colder air entered, the vapour was first condensed and then frozen.

The admission of cool air even into our London ball-rooms produces

mistiness. Mountain-chains are very effective in precipitating the

vapour of our south-westerly winds; and this sometimes to such an

extent as to produce totally different climates on the two sides of

the same mountain-group. This is very strikingly illustrated by the

observations of Dr. Lloyd on the rainfall of Ireland. Stations situated

on the south-west side of a mountain-range showed a quantity of rain

far in excess of that observed upon the north-east side. The winds in

passing over the mountains were drained of their moisture, and were

afterwards comparatively dry.

Two or three years ago I had an opportunity of witnessing a singular

case of condensation at Mortain in Normandy. The tourist will perhaps

remember a little chapel perched upon the highest summit in the

neighbourhood. A friend and I chanced to be at this point near the hour

of sunset. The air was cloudless, and the sun flooded the hillsides and

valleys with golden light. We watched him as he gradually approached

the crest of a hill, behind which he finally disappeared. Up to this

point a sunny landscape of exquisite beauty was spread before us, the

atmosphere being very transparent; but now the air seemed suddenly to

curdle into mist. Five minutes after the sun had departed, a dense

fog filled the valleys and drifted in fleecy masses up the sides of

the hills. In an incredibly short time we found ourselves enveloped

in local clouds so dense as to render our retreat a matter of some

difficulty.

In this case, before the sun had disappeared the air was evidently

nearly saturated with transparent vapour. But why did the vapour curdle

up so suddenly when the sun departed? Was it because the withdrawal of

his beams rendered the _air_ of the valleys colder, and thus caused the

precipitation of the moisture diffused through the air? No. We must

look for an explanation to a more direct action of the sun upon the

atmospheric moisture. Let me explain. The beams which reach us from the

sun are of a very composite character. A sheaf of white sunbeams is

composed of an infinitude of coloured rays, the resultant effect of all

upon the eye being the impression of whiteness. But though the colours,

and shades of colour, which enter into the composition of a sunbeam are

infinite, for the sake of convenience we divide them into seven, which

are known as the prismatic colours.

The beams of the sun, however, produce _heat_ as well as light, and

there are different _qualities_ of heat in the sunbeam as well as

different qualities of light--nay, there are copious rays of heat in

a sunbeam which give no light at all, some of which never even reach

the retina at all, but are totally absorbed by the humours of the eye.

Now, the same substance may permit rays of heat of a certain quality

to pass freely through it, while it may effectually stop rays of heat

of another quality. But in all cases the heat stopped is expended in

heating the body which stops it. Now, water possesses this _selecting

power_ in an eminent degree. It allows the blue rays of the solar beam

to pass through it with facility, but it slightly intercepts the red

rays, and absorbs with exceeding energy the _obscure rays_; and those

are the precise rays which possess the most intense heating power.

We see here at once the powerful antagonism of the sun to the

formation of visible fog, and we see, also, how the withdrawal of his

beams may be followed by sudden condensation, even before the air

has had any time to cool. As long as the solar beams swept through

the valleys of Mortain, every particle of water that came in their

way was reduced to transparent vapour by the heat which the particle

itself absorbed; or, to speak more strictly, in the presence of this

antagonism precipitation could not at all occur, and the atmosphere

remained consequently clear.[43] But the moment the sun withdrew, the

vapour followed, without opposition, its own tendency to condense, and

its sudden curdling up was the consequence.

[43] At this time I was brooding over experiments on the absorption of

radiant heat by aqueous vapour.

With regard to the _air_, its temperature may not only have remained

sensibly unchanged for some time after the setting of the sun, but it

may have actually become warmer through the heat set free by the act of

condensation. It was not, therefore, the action of cold air upon the

vapour which produced the effect, but it was the withdrawal of that

solar energy which water has the power to absorb, and by absorbing to

become dissipated in true vapour.

I once stood with a friend upon a mountain which commands a view of

the glacier of the Rhone from its origin to its end. The day had

been one of cloudless splendour, and there was something awful in the

darkness of the firmament. This deepening of the blue is believed by

those who know the mountains to be an indication of a humid atmosphere.

The transparency, however, was wonderful. The summits of Mont Cervin

and the Weisshorn stood out in clear definition, while the mighty mass

of the Finsteraarhorn rose with perfect sharpness of outline close at

hand. As long as the sun was high there was no trace of fog in the

valleys, but as he sloped to the west the shadow of the Finsteraarhorn

crept over the snow-fields at its base. A dim sea of fog began to form,

which after a time rose to a considerable height, and then rolled down

like a river along the flanks of the mountain. On entering the valley

of the Rhone, it crossed a precipitous barrier, down which it poured

like a cataract; but long before it reached the bottom it escaped from

the shadow in which it had been engendered, and was hit once more by

the direct beams of the sun. Its utter dissipation was the consequence,

and though the billows of fog rolled on incessantly from behind, the

cloud-river made no progress, but disappeared, as if by magic, where

the sunbeams played upon it. The conditions were analogous to those

which hold in the case of a glacier. Here the ice-river is incessantly

nourished by the mountain snow: it moves down its valley, but does

not advance in front. At a certain point the consumption by melting is

equal to the supply, and here the glacier ceases. In the case before

us the cloud-river, nourished by the incessant condensation of the

atmospheric vapour, moved down its valley, but ceased at the point

where the dissipating action of the sunbeams equalled the supply from

the cloud-generator behind.