CHAPTER XVIII.

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_.

[Verse: "With clouds he covereth the light, and commandeth it not to shine by the cloud that cometh betwixt."--JOB XXXVI.]

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.

[Verse: "Who giveth rain upon the earth, and sendeth waters upon the fields."--JOB V.]

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_.

[Verse: "Behold, he withholdeth the waters, and they dry up; also he sendeth them out, and they overturn the earth."--JOB XII.]

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_.

[Verse: "Terrors are turned upon me: they pursue my soul as the wind; and my welfare passeth away as a cloud."--JOB XXX.]

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.)

[Verse: "When he made a decree for the rain, and a way for the lightning and the thunder."--JOB XXVIII.]

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.)

[Verse: "Behold, I will put a fleece of wool in the floor; and if the dew be on the fleece only, and it be dry upon all the earth beside, then shall I know that thou wilt save Israel." * * *]