CHAPTER XVIII.. 373. _what Are Clouds?_

373. What are clouds?

Clouds are volumes of vapour, usually elevated to a considerable height.

374. Whence do clouds arise?

From the evaporation of water at the earth's surface.

375. Why do we not see them ascend?

We do, sometimes, in the form of what we call mists, but generally the vapours that rise and contribute to the formation of clouds are so thin that they are invisible.

376. Why, if they are invisible when they rise, do they became visible when they have ascended?

Because the vapours become cooled in passing through the air, and form a denser body.

377. Why, when they are condensed, do they not follow the course of gravitation, and descend?

Because the vapours form into minute vesicles, which we may call vapour bubbles, and these, being warmed by the sun, are specifically lighter than the air.

Because, also, the lower parts of clouds do partially descend, but again becoming more rarefied by meeting with a warmer atmosphere, they again ascend, and are thus poised upon the air.

Because, also, there is always a degree of atmospheric motion upward, caused by the convection of heat from the earth's surface. And, although there must also be downward movements of the air to supply the place of that which has ascended, still the heat of the ascending air, combined with its upward movement, expands and floats the vapour of the clouds.

378. At what height do clouds usually fly?

They fly at every degree of altitude; but clouds of specific character are said to fly at given altitudes, or to occupy certain ranges of altitude. We will give their probable altitudes when speaking of the specific clouds.

379. How many descriptions of clouds are there?

There are seven.

1. The Cirrus (Fig. 5), estimated range of altitude from 10,000 to 24,000 feet.

2. The Cumulus (Fig. 7), from 3,000 to 10,000 feet.

3. The Stratus, an extended continuous level sheet of cloud, increasing from beneath. They fly very low.

4. The Nimbus (Fig. 10), 1,500 to 5,000 feet.

5. The Cirro-cumulus (Fig. 4), from 3,000 to 20,000 feet.

6. The Cirro-stratus (Fig. 6), from 5,000 to 10,000 feet.

7. The Cumulo-stratus (Fig. 9), from 3,000 to 10,000 feet.

The estimated heights given must be looked upon as very conjectural, although they have been derived from the best existing authorities. It is sufficient to know that the range of the altitude of the various clouds is from that of the Nimbus, or thunder cloud, 1,500 feet, to that of the Cirrus, 24,000 feet, the others being intermediate. The first three of the clouds above enumerated constitute what are called the primary forms. The remaining four are called secondary forms, because they arise, as their names generally indicate, out of combinations of the primary forms. Although, from the frequent mingling of clouds, it is not always practicable to identify them by the adopted classification, still, as there is generally a prevalence of one type of cloud over another, the observer would be able to distinguish a "Cirrus sky," or "Cirro-cumulus sky," &c. Upon some occasions the typical characters of the clouds are beautifully defined; and the contemplation of their forms, and the laws of their formation, affords infinite pleasure to the observer. The advantages of scientific knowledge are such, that whether you look downwards, to the earth, or upwards to the sky, you have still the writing of God to read.

380. What produces the various shapes of clouds?

1. The state of the atmosphere.

2. The electrical condition of the clouds.

3. The movements of the atmosphere.

4. The season of the year.

381. What are the dimensions of clouds?

A single cloud has been estimated to have as many as twenty square miles of surface, and to be above a mile in thickness, while others are no larger than a house, or a man's hand.

382. How are clouds affected by winds?

If cold winds blow upon the clouds, the cold condenses the vapour, turning the clouds into rain. But if warm dry winds blow upon the clouds, they rarefy the vapour to a greater degree, and temporarily disperse the clouds.

383. How do winds affect the shapes of clouds?

When winds are mild and gentle, the clouds break into small patches, and rise to a considerable height. But when the winds are cold and blustering, the clouds fly low, and roll along in heavy masses.

384. Why are east winds usually dry?

Because in coming towards England they pass over vast continents of land, and comparatively little ocean. Hence they are not loaded with vapours.

385. Why do west winds generally bring rain?

Because they come across the Atlantic, and are heavily charged with vapour.

386. Why are north winds generally cold and dry?

Because they come from the arctic ocean, over vast areas of ice and snow.

387. Why are south winds warm and rainy?

Because they come from the southern regions, heated by the hot earth and sands, and as they cross the sea they absorb a large amount of vapour.

388. Why are clouds said to indicate the changes of the weather?

Because, as it is the state of the clouds that, to a great extent, determines the state of the weather, the formation of the clouds must predicate approaching changes.

389. What do cirrus clouds foretell?

Cirrus clouds foretell fine weather, when they fly high, and are thin and light.

They foretell light rain when, after a long continuance of fine weather, they form fleecy lines stretched across the sky.

They foretell a gale of wind when, for some successive days, they gather in the same quarter of the heavens, as if denoting the point from which to expect the coming gale. (Fig. 5).

390. What do cumulus clouds foretell?

Cumulus clouds, when they are well defined, and advance with the wind, foretell fine weather.

When they are thin and dull, and float against the wind, or in opposition to the lower currents, they foretell rain.

When they increase in size, and become dull and grey at sunset, they predict a thunder-storm. (Fig. 7.)

391. What do stratus clouds foretell?

Stratus clouds foretell damp and cheerless weather.

392. What do nimbus clouds foretell?

Nimbus clouds foretell rain, storm, and thunder. (Fig. 10.)

393. What do cirro-cumulus clouds foretell?

Cirro-cumulus clouds, in summer, foretell increasing heat attended by mild rain, and a south wind; but in winter they commonly precede the breaking up of a frost, and the setting in of foggy and wet weather. (Fig. 4.)

394. What do cirro-stratus clouds foretell?

Cirro-stratus clouds foretell rain or snow, according to the season of the year.

These clouds extend in long horizontal streaks, thinning away at their base, and in parts becoming wavy or patchy.

When they are thus defined in the heavens they are a certain indication of bad weather. (Fig. 6.)

395. What do cumulo-stratus clouds foretell?

Cumulo-stratus clouds usually foretell a change of weather--from rain to fine, or from fine to rain. (Fig. 9.)