24

FOG AND CLOUD.

[CHAP. rapidly with the crushed ice, the vessel is made much colder than before; and the vapour of the air will be condensed on the outside, not in minute liquid drops, but in fine white solid filaments, which are almost exactly the same thing as the snow that lies on the tops of the Himálaya mountains, and from which, as I dare say my readers know, they take their name Himálaya, "the abode of snow."

We are now prepared to understand how fog, clouds, rain, snow, and hail are formed, and in part, why we often find the grass and leaves quite wet in the early morning, though no rain has fallen during the night. In all these cases the vapour of the air has been condensed by cold.

A fog is produced by the cooling of the air that rests directly on the ground. In the cold weather it may often be seen to form very soon after sunset. The vapour in the air begins to condense as it cools, and is converted into water-drops so small that for a time they remain floating in the air, and cannot be seen apart from each other. But if a perfectly dry cloth be hung up in such a fog it will soon become damp. A cloud is much the same as a fog, but formed high up in the air, and is very variable in shape, since the substance of the cloud is continually changing. When the sun shines on one of those white isolated clouds, rounded on the top and flat below, that are so common about noonday in fine weather, the upper surface of the cloud is evaporating under the sun's rays, while more vapour rising from the earth is condensed and added at the bottom; the cloud seems to be the same, because it remains in the same place, but in reality the condensed vapour that composes it is constantly changing. Now if a cloud of this kind grows very large and thick, and unites with other clouds round about it and higher up in the atmosphere, which of course can take place only by more vapour being condensed from the air, it forms at last a rain-cloud. The

In villages and among houses, a great deal of smoke is generally mixed with the fog; but this is not the case when it forms over an open plain, such as the Calcutta maidán, or over tanks, jhíls, and marshy places, far away from houses.

11.]

RAIN, HAIL, AND SNOW.

25

very minute water-drops that compose the cloud become larger, then unite with one another, and finally fall to the earth as rain-drops. It seems at first hard to believe that all the water that falls in a heavy shower of rain should have existed in the air, only a short time before, as invisible vapour, but there is no doubt that it is so. One may learn much by watching the clouds for an hour or two together, and noticing their changes of form and size; just before one of those thunderstorms so common in March and April, which we call north-westers, new layers of cloud may sometimes be seen to form so rapidly, as entirely to obscure the sky overhead in the course of half an hour. In February and March it sometimes happens that, together with the rain, lumps of ice fall. These are termed hailstones, not because they are really stones, but simply because they are solid, and falling from a great height in the air, they are, when large, as destructive to trees and crops as if they were really stones. They are very large raindrops that come from some cloud high overhead, where it is so cold that they are frozen into ice. Snow is also a kind of frozen rain, but it is very light, and there is this difference in its mode of formation, that it is changed at once from invisible vapour into little frozen needles; just as, in the experiment with the ice and salt described a short time ago, the vapour condensed on the surface of a very cold vessel becomes at once a white frozen layer of a snowlike substance, without passing through the condition of water. We never see snow fall on the plains of India (except very rarely in the northern part of the Punjáb), nor even on the hills south of the Himalaya, because it is never cold enough, but on the higher peaks of the Himálaya it falls abundantly, and this it is that makes them so white. Snow is very light; as it falls it looks much like flocks of cotton wool; and where it falls there it remains, until it melts or is drifted into heaps by the wind. In the winter-time in Europe, it sometimes accumulates in this manner till it forms a layer over the ground, several feet thick, and in severe winters houses are sometimes buried in

26

DEW, HOAR-FROST, AND WINDS.

[CHAP.

the snow-drifts. On high mountains it collects in the same way in hollows, forming beds many hundreds of feet thick ; what becomes of it then we shall see later on, when we have to speak of glaciers.1

There is one other form in which the vapour of the air condenses, and which is familiar to us all. Often, after a fine clear night in the cold weather, we find the grass and trees quite wet, and large drops of water resting on the leaves. This is called dew. It is the vapour of the air condensed on the cold surface of the leaves. A cloth left out on the grass on such a night will become as wet as if it had been dipped into water. When it is so cold that the dew freezes as it forms, it is called hoar-frost. This is often seen in the Upper Provinces of India, on the Nilgírí hills, and even no further off than Házáribágh, after a very cold clear night in December or January.

We see then that fog, clouds, rain, hail, snow, dew, and hoar-frost are all formed in the same way, viz., by the cooling of the air, which then gives out in a liquid or solid form a part of the vapour that it contains. The vapour comes originally from the surface of seas, rivers, tanks, and even the land if it is wet, and the chief cause of its evaporation is the heat of the sun. It is, then, to this genial heat that we are indebted for those refreshing rains that cool the air and enable our trees and food crops to grow.

But this is only a part of the work done by the sun's heat in our atmosphere. It is the sun that sets the air in motion, and causes those winds, by which vapour is carried from the surface of the ocean and distributed over the fields and forests in rain; by which therefore rivers are fed, and the land made habitable and covered with verdure. To understand how all this is brought about, we must go back to certain simple rudimentary facts which we may gather from every-day experience. In order to learn how the air is affected by heat, a good way is to watch what takes place

1 Chapter VII.

round about an ordinary fire. If we make a large fire of twigs, dead leaves or straw in the open air, and if there be no wind blowing at the time, the smoke goes straight upwards; but if any burning leaves or twigs happen to lie a little on one side, the flame and smoke from them are drawn towards the rest of the burning mass, showing that currents of air are drawn in towards the fire at the bottom. This air is heated by the flame, and then rushes upwards, carrying with it the smoke, bits of ash, and similar light bodies. It rises for the same reason that a piece of bamboo, if thrown into the water, at once rises to the surface. Just as the bamboo is lighter than the water, so the hot air is lighter than the cold air around. Cold smoke is not light and does not rise, as may be seen any evening in the cold weather, when the smoke that is produced by the fires of the village hangs low down about the houses, and gradually settles to the ground. Now if a fire be very large, such as may be seen when a village is burning, a strong wind blows in from all sides towards the flames; and when a large forest is burning (as sometimes occurs in the great forests of America), and in the conflagrations of great cities, such as that of Londou in 1666, which burnt for four days, or that of Chicago in the United States a few years ago, the wind that is produced around, blows with the force of a hurricane, rushing in towards the flames.

The heat of the sun produces effects similar to these. It is true that it does not heat the air so intensely as a great conflagration will do, but the tracts that are affected by it are much more extensive, and the winds therefore, though not so violent as around a burning city or forest, prevail over very much larger spaces. But it may be asked, since the sun is shining at the same time over half the earth, how is it that one place is so much hotter than another. The reasons are chiefly twofold. First these places that have the sun immediately overhead are hotter than those where it does not rise so high; just as, at the same place, it is hotter at noon when the sun is highest, than in the morning and evening when it is low. And, secondly, land is

28

HEATING AND COOLING.

[CHAP.

much more quickly heated than water, so that on the open sea at midday and even on and near the sea-shore, it is much cooler, in April and May, than in the interior of Bengal; and it is hotter still in Central India and the Punjab, which are still further from the sea.

On the other hand, at night time, when the sun is below the horizon, and both land and water, as well as the air above them, are cooling down, giving out their heat to the starry space, the land cools faster than the sea. And in the same way, in the cold weather months, when the earth parts with more heat than it receives from the sun, the land becomes colder than the water; and places far from the sea --such as the Punjáb and upper Asám-are cooler than Bengal, which lies near the sea.

We are now prepared to understand why the intense heat of April and May is followed by the strong steady wind. from the sea that brings the grateful rains of June and July. Up to the setting in of the rains, India is very much hotter than the great sea that lies to the southward. In Bengal indeed we have winds from the south and south-east even before the regular rains set in, and these bring us some showers; but when, in June and July, the heat of Central India, the North-West Provinces and the Punjáb has become very great indeed, the whole body of the lower part of the atmosphere from beyond the Equator moves northwards towards India, and brings all the vapour that has been evaporated by the heat of the sun from many thousand square miles of ocean. This is our south-west monsoon.

Something yet remains to be explained. We have learned why the winds bring us the vapour, that when condensed produces our rains; but we have also learned that the air must be cooled in order that this vapour may be condensed; and it is not clear how this is effected; since we have seen that it is the great heat of India that produces the winds. Can we have great heat producing a wind, and at the same time this very wind cooled down so as to condense its vapour in cloud and rain? How, it will be asked, can one explanation be reconciled with the other?