CHAPTER II.. Before Proceeding to an Examination of This Connected Atmospheric

Before proceeding to an examination of this connected atmospheric machinery, and an investigation of the particular ocean from which our rivers return, it may be well to look at the form in which they appear to return, that we may have a clear understanding of terms.

They seem to return in the form of clouds, and in storms and showers, although, in truth, they return in regular, uniform, ordinarily invisible currents, and the storms and showers are but condensations in, and discharges from portions of those currents, aided by the local moisture of evaporation.

The term storms, seems to be used by European meteorologists to denote what we term thunder showers or gusts, and tornados; while what we call storms are denominated by them regular rains. As the terms are extensively in use in this country, we must adhere to the meaning attached to them here rather than there.

Storms with us, then, are regular rains of from six to forty-eight or more hours' continuance: generally without lightning, or thunder, or gusts, and usually with wind of more or less force, from some easterly point. They are called north-east storms, or south-east storms, according to the point from which the surface winds blow. Practically we shall find that this distinction is of some importance, for the north-east storms are the longest, lasting generally twenty-four hours, or more, while the south-east ones seldom, if ever, continue as long.

These storms extend over a considerable surface, rarely less than one hundred miles in one direction or another, and sometimes fifteen hundred, or more. Distinct showers cover but a small surface, sometimes not more than forty to one hundred rods, as in the tornado, and rarely more than ten miles. Belts of showers, each new one forming a little more to the south, often, in summer, pass across the country, following each other in succession; and these belts may be of considerable width, say thirty to one hundred and fifty miles.

The clouds which constitute the storms and showers differ in appearance and character, as well in the active as in the forming state. Clouds are of distinct characters, alike, substantially, every where under like circumstances; and a distinct nomenclature has been applied to them by Dr. Howard, of London. He notes three kinds of primary clouds: viz., cirrus, stratus, and cumulus; and inasmuch as the boundary line between them is not very distinct, certain compounds of the three, viz.: cirro-stratus, cirro-cumulus, and cumulo-stratus. This nomenclature is every where received, and portions of it are of great practical importance.

The three principal descriptions of cloud, viz.: the cirrus, the stratus, and the cumulus, we have very much as they have in Europe, and doubtless as they exist every where outside of the tropics. The nimbus, another cloud described by him, is not distinct from the cumulus or stratus. An isolated, limited thunder-shower in a clear sky, presents the appearance of a nimbus, as shown in the cuts, but the basis of it is a cumulus, and it differs from an ordinary fair-weather cumulus merely in the dark and fringe-like appearance of the rain as it is falling from its lower surface, and sometimes in the existence of a stratus above and in connection with it. A similar form is often assumed by the peculiar clouds of the N. W. winds in March or November, when they assume the form of squalls, and drop flurries of snow. The nimbus, therefore, is not a distinct cloud, but an appearance which the cumulus, stratus, or cirro-stratus has in a stormy or showery state, and does not deserve a distinct name. It is but a cumulus, or a stratus, or cirro-stratus dissolving in snow or rain. It is important that this term should be abandoned. It tends to confuse and prevent a clear understanding of the difference in the character of the clouds, and in relation to which precision is both difficult and desirable.

The figures on pages 27 and 29, show the different kinds of clouds as designated by Howard. They are copied from the engravings in the sixth edition of Maury's "Sailing Directions."

Figure 5. The cirrus is indicated by 1 bird. The cirro-cumulus by 2 " The cirro-stratus by 3 " The cumulo-stratus by 4 "

Figure 6. The cirrus by 1 " The cumulus by 3 " The stratus by 2 " The nimbus by 4 "

How far these representations correspond with the actual appearance of the different compound forms in England, I can not say. But although they convey a general idea, they are not sufficiently accurate for practical illustration or observation here. Indeed Howard himself has omitted from his last edition his plate of the clouds, assigning as a reason, "that the real student will acquire his knowledge in a more solid manner by the observation of nature, without the aid of drawings, and that the more superficial are liable to be led into error by them." The collection of forms in the cuts does not contain some very important ones, and contains some which are not distinct forms; but they may aid us somewhat in this inquiry, and, therefore, I have copied them. It is well, also, for the reader to have the generally received description before him.

But for the purpose of practical illustration hereafter, and greater precision, I shall follow a somewhat different order in describing them, and introduce two forms of scud quite as important, practically, as any other.

First, then, commencing at the earth, we have what may be properly termed fog, or low fog. This forms, in still clear weather, in the valleys, and over the surface of the rivers and other bodies of water, during the night, and most frequently the latter part of it, and is at its acme at sunrise, or soon after, limiting vision horizontally and perpendicularly, and dissolving away during the forenoon. It is rarely more than from two to four hundred feet in height at its upper surface, and often much less, and is composed of vesicular condensed vapor, sometimes sufficiently dense to fall in mist, and is doubtless in composition substantially what the clouds are in the other strata of the atmosphere, as observed by us, or passed through by aeronauts. I have never seen it carried up to any considerable height into the other strata by any of the supposed ascending currents, to form permanent clouds, and shall have occasion to allude to the fact in another connection. It disappears usually before mid-day, and has, when thus formed, no connection with any clouds which furnish rain.

To this Dr. Howard originally gave the name of stratus, and so it is represented upon the cut; but the latter term may be with greater propriety applied to the smooth uniform cloud in the superior strata from which the rain or snow is known to fall, and I shall retain and so apply it.

The next in order, ascending, is high fog. This is usually from one to two thousand feet in height at its lower surface. It forms, like low fog, during the night and in still weather; and is rarely, if ever, connected with clouds which furnish rain. It breaks away and disappears between ten and twelve in the forenoon, usually passing off to the eastward. This fog is most commonly seen in summer and autumn, particularly the latter, and unless distinguished from cloud will deceive the weather-watcher. It is readily distinguishable. Although often very dense, obscuring the light of the sun as perfectly as the clouds of a north-east storm, it differs from them. It forms in still clear weather, is present only in the morning, is perfectly uniform, and, before its dissolution commences, without breaks, or light and shade, or apparent motion, and unaccompanied by scud or surface wind. The storm clouds are never entirely uniform, or without spots of light and shade, by which their nature can be discerned, and rarely, when as dense as high fog, without scud running under them and surface winds.

There is another fog still, connected with rain storms, but it does not often precede them; occurring at all seasons, but most commonly in connection with the warm S. E. thaws and rains of winter and spring; and which usually comes on after the rain has commenced and continued for awhile, and the easterly wind has abated; occupying probably the entire space from the earth to the inferior surface of the rain clouds or stratus. Practically this does not require any further notice. It is an incident of the storm. When formed it remains while the storm clouds remain, and passes off with them. It is sometimes exceedingly dense in February and March, when it accompanies a thaw, and if there is a considerable depth of snow, it has the credit of aiding essentially in its dissolution.

Mingled with the smoke of London, it produced there the memorable dark day of the 24th of February, 1832, and at various other times has produced others of like character. (See Howard's Climate of London, vol. iii. pp. 36, 207, 303.) These fogs have been so dense there that every kind of locomotion was dangerous, even with lanterns, at mid-day.

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 passing 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.

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 pass with considerable rapidity to the S. E. Sometimes they are quite large, approaching the cumulus in form, and white, with dark under surfaces, and at others, in the month of November particularly, are entirely dark, and assume the character of squalls and drop flurries of snow; and then resemble the nimbus of Howard. They assume at different times and in different seasons, different shapes like those of the scud, the cumulus, or the stratus.

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.

The cumuli appear in isolated clouds of every size, or in vast clouds composed of aggregated masses, 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 altitude 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 space 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 cumulus 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 cumuli. 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 cumulus is the principal 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 cumuli.

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 illustrations 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 masses 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.

The annexed cut shows the forming stratus, light and thin, passing 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-cumulus 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 cumulus, and does not appear in the same stratum. It was probably called cumulus because its small masses are distinct, as are those of the ordinary cumulus. 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 cumulo-stratus is seldom seen in our climate, as it is represented in the cut. Stratus condensation above, and in connection with cumulus 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 principal strata and their modifications, located with sufficient accuracy for illustration. 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-cumulus. | | Cirro-stratus. | | | Primary | { Cumulus extending up | stratum. | { in violent showers. | | | | | |Stratus. | |------------------------------------------------| | | 1-1/2 miles. Scud & |N. W. scud. { Cumulus Storm scud. | cumulus |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 assistance of this table of elevations, and a careful observation, the reader can soon become familiar with the forms of clouds and their relative situations.