The Philosophy of the Weather Part 3

The next in order, ascending, are the storm scud, which float in the north-east or easterly, south-east or southerly wind, before and during storms.

These, as the reader will hereafter see, are, _practically_, very important forms of cloud condensation--although they have found no place in any practical or scientific description given of the clouds, and are not upon the cuts. They are patches of foggy seeming clouds of all sizes, more or less connected together by thin portions of similar condensation, often pa.s.sing to the westward, south-westward, north-westward, or northward with great rapidity. Their average height is about half a mile, but they often run much lower. They are usually of an "ashy gray" color.

The annexed cut shows one phase of them, from among many taken by daguerreotype. The arrows pointing to the west show the scud distinguished from the smooth partially formed stratus above. This view was taken a few hours prior to the setting in of a heavy S. E. rain storm. It is a northerly view.

[Ill.u.s.tration: Fig. 7.]

At about the same height, but in a _different state of the atmosphere_, float the peculiar fair-weather clouds of the N. W. wind. They usually form in a clear sky, and pa.s.s with considerable rapidity to the S. E.

Sometimes they are quite large, approaching the c.u.mulus in form, and white, with dark under surfaces, and at others, in the month of November particularly, are entirely dark, and a.s.sume the character of squalls and drop flurries of snow; and then resemble the nimbus of Howard. They a.s.sume at different times and in different seasons, different shapes like those of the scud, the c.u.mulus, or the stratus.

[Ill.u.s.tration: Fig. 8.]

They form and float in the peculiar N. W. current which is usually a fair-weather wind, and are never connected with storms. In mild weather they are usually white, and in cold weather sometimes very black, and at all times differ _in color_ from the ashy gray scud of the storm. This variety is not represented upon the general cuts. The annexed diagram shows one phase of them, but they are readily observable at all seasons of the year, when the N. W. wind is prevailing; differing in appearance according to the season. Let these, as well as the storm scud, be carefully observed and studied by the reader, and let no opportunity to familiarize himself with their appearance be lost. A brief glance at each recurrence of easterly or north-westerly wind will suffice.

[Ill.u.s.tration: SUMMER c.u.mULI.]

The _c.u.muli_ appear in isolated clouds of every size, or in vast clouds composed of aggregated ma.s.ses, as the peculiar cloud of the thunder shower. They form as low down as the scud or fair-weather cloud of the N.

W. wind, which, for convenience, I will call N. W. _scud_; and often in violent showers, and particularly in hail storms, extend up as far as the density of the atmosphere will permit them to form. Professor Espy thinks he has measured their tops at an alt.i.tude of ten miles. Others have estimated their height, when most largely developed, at twelve miles; but it is very doubtful whether the atmosphere can contain the moisture necessary to form so dense a cloud at that elevation. It is their immense height, however, whether it be six, or eight, or ten miles, together with the sudden and violent electric action, condensing suddenly all the moisture contained in the atmosphere within the s.p.a.ce occupied by the cloud, which produces such sudden and heavy falls of rain or hail. As the rain drops or hail, when formed at such an elevation, in falling through the partially condensed vapor of the cloud must necessarily enlarge by accretion from the particles with which they come in contact, and probably also by attraction, their size when they reach the earth, though frequently very considerable, is not a matter of astonishment. The c.u.mulus is represented in the general plate with sufficient accuracy to show its peculiar character.

In summer, when the air is calm, the weather warm, and no storm is approaching, there is always, in the day time, a tendency to the formation of c.u.muli. This tendency exhibits itself about ten o'clock in the forenoon, and they gradually form and enlarge until about two in the afternoon; and after that, if they do not continue to enlarge and form showers, they melt away and disappear before nightfall. Sometimes in July and August the atmosphere will be studded with them at mid-day, floating about three-quarters of a mile from the earth (in a level country), gently and slowly away to the eastward. At times it may seem as if they must coalesce and form showers, yet they frequently do not, but gradually melt away, as before stated.

The c.u.mulus is the princ.i.p.al cloud of the tropics, and is not often seen with us except in summer, or when our weather is tropical in character.

The engraving on the preceding page, shows a phase of these fair-weather summer c.u.muli.

The last in order occupying (with their compounds) the higher portions of the atmosphere, are the cirrus and stratus. The cirrus is often the skeleton of the other, and precedes it in formation.

These are the proper clouds of the storm, in our sense of the term. While, however, the cirrus remains a cirrus, it furnishes no rain. When it extends and expands, and its threads widen and coalesce into cirro-stratus and stratus, or it induces a layer of stratus below it, the rain forms.

The following is Dr. Howard's description of cirrus: "Parallel, flexuous or diverging fibers, extensible by increase in any or in all directions.

Clouds in this modification appear to have the least density, the greatest elevation, and the greatest variety of extent and direction. They are the earliest appearance after serene weather. They are first indicated by a few threads penciled, as it were, on the sky. These increase in length, and new ones are in the mean time added to them. Often the first-formed threads serve as stems to support numerous branches, which in their turn, give rise to others."

The ill.u.s.trations in the general cut are imperfect, and do not represent the delicate fibers of the cloud, for it is a difficult cloud to daguerreotype or engrave, but the representation is sufficiently accurate to give the reader a general idea of the different varieties, and enable him to discover them readily by observation. They are the most elevated forms, always of a light color, and often illuminated about sunset by the rays of the sun shining upon their inferior surface; the sun, however, often illuminates, in like manner, the dense forms of cirro-stratus, and the latter, from their greater density, are susceptible of a brighter and more vivid illumination.

The stratus is a smooth, uniform cloud--the true rain cloud of the storm; often forming without much cirrus above, or connected with it. It may be seen in its partially formed state in the bank in the west, at nightfall, or in the circle around the moon in the night. When it becomes sufficiently condensed, rain always falls from it, but in moderation. If there be large ma.s.ses of scud running beneath it for its drops to fall through (especially as is sometimes the case, in two or more currents), the rain may be very heavy. But more of this hereafter.

[Ill.u.s.tration: Fig. 10.]

The annexed cut shows the forming stratus, light and thin, pa.s.sing to the east, as indicated by the short arrows just before a storm, while the scud beneath is running to the west.

It was copied from a daguerreotype view, facing northwardly.

Intermediate between the fibrous, tufted, cirrus, and the smooth uniform stratus, there is a variety of forms partaking more or less of the character of one or the other, and termed _cirro-stratus_. No single correct representation of cirro-stratus as a distinct cloud, can be given--but several varieties will be hereafter alluded to, under the head of prognostics. Several modifications are represented with tolerable accuracy upon the cuts.

The cirro-c.u.mulus is a collection in patches of very small distinct heaps of white clouds; they are called fleecy clouds, from their resemblance to a collection of fleeces of wool, and are imperfectly represented on the general cut. They do not appear often, and are usually _fair-weather clouds_.

This form has none of the characteristics of the c.u.mulus, and does not appear in the same stratum. It was probably called c.u.mulus because its small ma.s.ses are distinct, as are those of the ordinary c.u.mulus. It occurs in the same stratum as cirro-stratus, and properly belongs to that modification. I retain the name inasmuch as the cloud is of some practical importance.

The c.u.mulo-stratus is seldom seen in our climate, as it is represented in the cut. Stratus condensation _above_, and in connection with c.u.mulus condensation, is not uncommon, but that precise form is rare.

This, too, is practically of no consequence, and I shall take no further notice of it.

Recapitulating, I give (in a tabular form) the three princ.i.p.al strata and their modifications, located with sufficient accuracy for ill.u.s.tration.

The clouds which are found in an upper or lower portion of a stratum are so represented by the location of their names; those which appear at all heights in the stratum, with the names across. The elevation is the average one--although there is no limit to the cirrus above, except the absence of sufficient moisture. It was seen by Guy Lussac, and has been by other aeronauts, at an elevation of five miles, or more, when too delicate to be visible below.

+------------------------------------------------+ 3 miles.

Cirrus. Cirro-c.u.mulus. Cirro-stratus. Primary { c.u.mulus extending up stratum. { in violent showers. Stratus. ------------------------------------------------ 1-1/2 miles.

Scud & N. W. scud. { c.u.mulus Storm scud. c.u.mulus Fair-weather { ordinarily and stratum. { its base always. ------------------------------------------------ 1/2 mile.

Fog High fog. Storm fog. stratum. Low fog at the surface of the earth. +------------------------------------------------+

With the a.s.sistance of this table of elevations, and a careful observation, the reader can soon become familiar with the forms of clouds and their relative situations.

CHAPTER III.

Having thus taken a brief view of the different clouds, let us return to the inquiry, from what ocean, and by what machinery, _our_ "rivers return."

Not wholly or mainly from the North Atlantic, although it lies adjacent to us, and they often _seem_ to do so; for, first, all storms, showers, and clouds, which furnish, _independently_, any appreciable quant.i.ty of rain to the United States, and even adjacent to the Atlantic, or indeed to the Atlantic itself, come from a westerly point, and pa.s.s to the eastward.

_This is a general, uniform, and invariable law, although there is in different places, and in the same place at different times, some variation in their direction; ranging in storms from W. by S. to S. S. W., and in showers between S. W. and N. W., to the opposite easterly points of the compa.s.s; the most general direction, east of the Alleghanies, being from W. S. W. to E. N. E._

But do we not see, you inquire--at least those of us who live east of the Alleghanies--that when it rains, the wind is from the eastward; and that the _clouds_ follow the wind from the east to the west? You do indeed, generally, in all considerable storms, observe that the wind blows from some easterly point, and that _seeming_ clouds are blown by it to the westward; but what you see, and call clouds, are not the clouds which furnish the rain. Far above the seeming clouds you notice, directly over your head when it rains or snows, are the rain or snow clouds, dense and dark, pa.s.sing to the eastward, how strong soever the wind may blow from the quarter to which they tend, or any other quarter, between you and them. What you see below them are _scud_. So the sailors call them, and so I have termed them. It is a "dictionary name," and a good one, expressive of a distinction between them and _clouds_. They are thin, and the sun shines through them, although with some difficulty, when the rain clouds above are absent or broken. _This east wind and the scud are not the storm, or essential parts of it._ Storms occasionally exist, particularly in April, without either. They are but _incidents_, _useful_, but not _necessary incidents_, as all surface winds are.

If you could see a section of the storm, you would see the rain cloud above, moving to the east, and the scud beneath running to the west, as indicated by the arrows in the cut on page 40. Opportunities frequently occur when these appearances may be seen. Storms are sometimes very long, a thousand miles, perhaps, from W. S. W. to E. N. E., and not more than one to three hundred miles wide from S. E. to N. W., and their sides, particularly the northern ones, regular, and without extensive partial condensation. Then the storm cloud above, moving to the eastward, and the scud running under to the westward, may be seen as in the cut.

So they may be seen before, at the commencement, and at the conclusion of easterly storms, in a majority of cases, and the reader is desired to notice them particularly as opportunities occur.

The term _running_, too, is a very expressive one, used by sailors as applicable to _scud_. For while the forming or formed storm clouds may be moving moderately along, at the rate of twelve to fifteen or twenty miles an hour, from about W. S. W. to E. N. E., the scud may be running under them in a different direction--opposite, or diagonal, or both--at the rate of twenty, fifty, sixty, and, in hurricanes, even ninety miles an hour.

You have doubtless seen these scud running from N. E. to S. W., and without dropping any moisture, a day or sometimes two days, before the storm coming from the S. W. reached the place where you were; and then, sometimes the storm cloud slipped by to the southward, and the expected storm at that point proved "a dry northeaster." Sometimes the condensation, although sufficiently dense to influence and attract the surface atmosphere, and create an easterly wind and scud, does not become sufficiently dense to drop rain, and then, too, we have a dry northeaster, which may melt away or increase to a storm after it has pa.s.sed over us. _I have never seen, except, perhaps, in a single instance, one of these ma.s.ses of scud, however dense, which had not a rain (stratus) cloud above it, drop moisture enough to make the eaves run._ So you see it may be true, and if you will examine carefully, you may satisfy yourself that it is true, that the storms all move from a westerly point to the eastward, notwithstanding the wind under them is blowing, and the scud under them are running to the westward.

There are many other methods by which the reader may determine this matter himself. He may catch an opportunity for a view, when there is a break in the stratus cloud above, and the sun or moon, no longer obscured by the _storm cloud_, shines through the scud beneath. Then he may see they are moving in different directions. _The upper cloud, if there be any of it left, always to the eastward._

Again, we may see the storm approach from the westward, as it often does, before the wind commences to blow, and the scud to run from the eastward; particularly snow storms in winter, and the gentle showers and storms of spring.

Again, thunder storms, we know, come from the westward, and apparently against an east wind. It is sometimes said they approach from the east, but it is a mistake. During thirty years attentive observation in different localities, I have never seen an instance. They sometimes _form_ over us, or just east of us, or one may form at the east and another at the west, and as they _spread out in forming_, one may seem to be coming from the east, or there may be an easterly current, with dense flocculent scud at the under surface of the shower cloud running westward, but they finally pa.s.s off to the eastward, and never to the westward. It is possible that a _patch of scud_ may become sufficiently _dense_ and _electrified_ to make a _shower_, but I have never observed one. Such an _apparent_ instance may be found recorded in "Sillman's Journal," vol.

x.x.xix. page 57. I have seen the scud a.s.sume a distinct c.u.mulus form, but never to become sufficiently dense to make a thunder shower.

Thunder and lightning sometimes attend portions of regular storms in spring and autumn, but the thunder is always heard first in the west, and last in the east.

Again, there are admitted facts with which you are conversant, which prove this proposition. When it has been raining all day, and just at night the storm has nearly all pa.s.sed over to the eastward, and the sun shines under the western edge of it, and "_sets clear_," as it is termed--you say that "_it will be clear the next day_." Why? Because the storm will not pa.s.s to the westward, covering the sun and continuing, how strong soever the wind may be from the east; and because it is pa.s.sing, and will continue to pa.s.s off to the eastward, leaving the sky clear. _The easterly wind will stop as soon as the storm clouds have pa.s.sed, and it will fall calm, or the wind will "come out" from the westward._