All around the earth within the tropics, over the land and over the sea, there is a belt of almost daily rains, varying in width, north and south, in different sections, but averaging about five hundred miles. This belt of daily rains is formed at and by the meeting of N. E. and S. E. trades, and travels north and south with them, as they do with the sun, _encircling the globe_. By this narrow belt a portion of the earth"s surface, an average of some 35 of lat.i.tude, is supplied with moisture.
Wherever it is situated at any given period, the tropical rainy season exists; and when it is absent in its northern or southern transit, the dry season prevails. Southern Mexico is within the range of this moving belt, and in its course to the northward with the sun, in our summer from May to October, it arrives over, and covers that country with a rainy season.
When the sun returns to the south, taking with it the trades and this belt of tropical rains, that portion of Mexico is without rain, and dry, and so continues until the rainy belt returns in the following year. While the belt is over Southern Mexico it is nearly all _precipitation_, and there is little _evaporation_; while that belt is _absent_ it is all _evaporation_, with little or no _rain_. Surely this is not consistent with the prevailing belief of simple evaporation, ascent to a colder stratum, commingling, and condensation, and rain. Southern Mexico at least is not supplied by mere evaporation from its surface, and must therefore form an exception to that belief, and to the Huttonian theory.
But we shall recur again to the peculiarity of distribution within the tropics.
Turn now for a brief s.p.a.ce to Northern Mexico, Southern New Mexico, and Southern California. In Northern Mexico, Southern New Mexico, Utah, and California, between the parallels of 28 and 32, and particularly west of the mountain ranges, we find an almost rainless region, sterile and worthless, resembling that which is found upon nearly the same parallels of north lat.i.tude in Northern Africa, Egypt, Arabia, Beloochistan, Afghanistan, and North-western India; and in corresponding lat.i.tudes south of the Equator, in Peru, a portion of Southern Africa, and the northern and middle portions of New Holland. Why Northern Mexico and the other countries named are thus sterile and comparatively rainless, we shall see hereafter, when we examine critically the machinery of distribution as it operates within the tropics. It is the fact that it is thus sterile and rainless to which we desire to call attention in this place.
Mr. Bartlett thus describes it:
"On leaving the head waters of the Concho, nature a.s.sumes a new aspect. Here shrubs and trees disappear, except the th.o.r.n.y chaparral of the deserts; the water-courses all cease, nor does any stream intervene until the Rio Grande is reached, three hundred and fifty miles distant, except the muddy Pecos, which, rising in the Rocky Mountains, near Santa Fe, crosses the great desert plain west of the Llano Estacado, or Staked Plain.
"From the Rio Grande to the waters of the Pacific, pursuing a westerly course along the 32d parallel, near El Paso Del Norte, there is no stream of a higher grade than a small creek. I know of none but the San Pedro and the Santa Cruz--the latter but a rivulet, losing itself in the sands near the Gila--the other but a diminutive stream, scarcely reaching that river. At the head-waters of the Concho, therefore, begins that great desert region, which, with no interruption save a limited valley or bottom-land along the Rio Grande, and lesser ones near the small courses mentioned, extends over a district embracing sixteen degrees of longitude, or about a thousand miles, and is wholly unfit for agriculture. It is a desolate, barren waste, which can never be rendered useful for man or beast, save for a public highway."--_Bartlett"s Personal Narrative_, vol. i. p. 138.
Turning now to Central and Upper California, and Utah, and Southern Oregon, we find still another peculiarity. Like Southern Mexico, they have a rainy and dry season, but at a different period, and for a different reason. The dry season of California, etc., is the summer of the northern hemisphere, and her rainy season the winter. _California_ is, therefore, _dry_ when Southern _Mexico_ is _wet_, and _vice versa_. The belt of rains which supplies California with moisture during her rainy seasons is the belt of _extra-tropical_ rains, which extends from the northern limit of the north-east trades to the poles, encircling the earth. The southern edge of this extra-tropical belt is _carried up_ on the western coast of America, and in that portion of the continent in _summer_, when the sun and trades, and the inter-tropical rainy belt travel to the north, and uncover California, etc., leaving them without rain for a period of about six months.
[Ill.u.s.tration: Fig. 3. IN SUMMER.]
As the sun, with the trades, travels south, the southern edge of the belt of extra-tropical rain follows, and covers California, etc., again extending gradually from the north to the south, and thus their wet season returns. The annexed diagrams by the shading will show the situation of the rainy belts which cover Mexico, Utah, New Mexico, and California in summer and winter, and that the belts of rains are entirely distinct and different in character.
[Ill.u.s.tration: Fig. 4. IN WINTER.]
Here again in this section of the continent, as in Mexico, evaporation is going on for six months of the year, and were it not for the return of the belt of rains from the north, in the fall, would go on for the entire year without precipitation; and for the other six months precipitation is vastly in excess. Nor can this be reconciled with, or explained by, the Huttonian or any other received theory of rain. Here again it is obvious that evaporation alone, however great or long continued, will not furnish the evaporating section with rain.
The northern portion of the continent lies beneath the zone of extra-tropical rains, and north of the northern limit of the N. E.
trades--is never uncovered from it, and has no distinct rainy or dry season, although more rain falls at certain periods, and in certain localities, than at others. The climate of that part of Oregon which lies upon the Pacific, and the character of its rains, resemble those of North-western Europe, and will be further explained hereafter.
Coming to the portion of the continent which we occupy, the 5th section, we find it different still--a most favored region. Portions of it--Eastern Texas, for instance--are upon the same parallels of lat.i.tude as the rainless regions of Northern Mexico, etc. Eastern Texas, however, is not rainless. Other portions are upon the same parallels as California, etc., yet have no distinct rainy and dry season. We repeat, this section is a most favored region--without a parallel upon any portion of the earth"s surface, except, in degree, in China and some other portions of Eastern Asia.
It is not only without a distinct rainy and dry season, but it is watered by an average, annually, of more than forty inches of rain, while Europe, although bounded on three sides by seas and oceans, and apparently much more favorably situated, receives annually an average of only about twenty-five--if we except Norway, and one or two other places, where the fall is excessive. The distribution of this supply of moisture over the United States is, in other respects, wonderful. Iowa, in the interior of the continent, far away from the great oceans, on the east or west, or the Gulf of Mexico on the south, receives fifty inches; some ten or fifteen inches more than fall upon the slope east of the Alleghanies, and contiguous to the great Atlantic (from which all our storms are, erroneously, supposed to be derived), and the average over the entire great interior valley is about forty-five inches, falling at all seasons of the year.
Observe, then, by way of recapitulation: Southern Mexico has a rainy season furnished by the belt of _inter_-tropical rains, which _travels up over it from the south_ in summer. California has a rainy season, which is furnished by the _extra_-tropical belt of rains, which travels _down from the north_, and covers it in winter. Northern Mexico and the adjoining regions west of the 100th meridian are between the limits of the two, and neither travels far enough to reach them, except for brief and uncertain periods; they are comparatively rainless; while the eastern portion of the continent, _in all lat.i.tudes_, unlike the others, is without a distinctly marked dry season, or a rainless region, and with the exception of occasional droughts, is abundantly supplied with rain at all seasons of the year.
And now, what is the explanation of all this? What produces the extra-tropical belt of regular rains surrounding the earth, north of the parallel of 30 north, in some places, and 35 in others, extending to the pole, with its southern edge traveling up ten or more degrees in summer, leaving large portions of the earth subject to a dry season; and back again in the winter to give them a rainy one? What produces the narrow belt of inter-tropical rains, encircling the earth; traveling up and down every year over an average of 35 of lat.i.tude, supplying every portion of it alternately with rain? And what connects the two together over the eastern portion of North America, so as to leave no distinctly marked wet and dry season, and no rainless and sterile portion there? Are all these the result of simple evaporation, ascent to a colder region, condensation, and descent again? Demonstrably not. Of the forty inches which fall annually upon the middle and eastern portions of the United States, an average probably of one-half or twenty inches, runs off by the rivers to the ocean, or is carried away eastward by the westerly and north-westerly evaporating winds. The same is true, in degree, of the rain which falls upon the other portions. Evaporation, therefore, could not keep up the supply. From whence, then, does it come? this twenty inches, thus lost by the rivers and winds, and with such wonderful regularity every year.
"All the rivers run into the sea, yet the sea is not full. _Note the place whence the rivers come, hither they return again._"
But how is it that they thus return with such wonderful regularity, in a narrow traveling belt of daily rains within the tropics, and a movable belt of irregular rains without the tropics, extending to the poles, leaving a s.p.a.ce on each side of the equator encircling the earth in like manner (except at two points, _viz._, Eastern Asia and Eastern North America), from which they do not go, and to which they do not return, and which is almost entirely unfurnished with rain? And all this without any relation, whatever, to the contiguity of the oceans? Obviously this is not the work of mere evaporation, or of the accidental or irregular commingling of winds with different dew points, or quant.i.ties of moisture in solution, or accidental, irregular changes of barometric pressure. _It is one vast, wonderful, connected, and regular system--co-extensive with the globe--necessary to the return of moisture from the oceans upon the most inconsiderable portion of it, and to the condensation of the local moisture of evaporation; and by it the waters are returned from the oceans as regularly and bountifully upon the far interior of the great continents in the same lat.i.tudes, as upon the "isles which rest in their bosoms."_
CHAPTER II.
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, pa.s.s 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 const.i.tute 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 circ.u.mstances; 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 c.u.mulus; and inasmuch as the boundary line between them is not very distinct, certain compounds of the three, _viz._: cirro-stratus, cirro-c.u.mulus, and c.u.mulo-stratus. This nomenclature is every where received, and portions of it are of great practical importance.
The three princ.i.p.al descriptions of cloud, _viz._: the cirrus, the stratus, and the c.u.mulus, 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 c.u.mulus 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 c.u.mulus, and it differs from an ordinary fair-weather c.u.mulus 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 a.s.sumed by the peculiar clouds of the N. W. winds in March or November, when they a.s.sume the form of _squalls_, and drop flurries of snow. The nimbus, therefore, is not a distinct cloud, but an appearance which the c.u.mulus, stratus, or cirro-stratus has in a stormy or showery state, and does not deserve a distinct name. It is but a c.u.mulus, 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."
[Ill.u.s.tration: Fig. 5.]
[Ill.u.s.tration: Fig. 6.]
Figure 5.
The cirrus is indicated by 1 bird.
The cirro-c.u.mulus by 2 "
The cirro-stratus by 3 "
The c.u.mulo-stratus by 4 "
Figure 6.
The cirrus by 1 "
The c.u.mulus 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 ill.u.s.tration or observation here_. Indeed Howard himself has omitted from his last edition his plate of the clouds, a.s.signing 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_ ill.u.s.tration 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 pa.s.sed 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 pa.s.sing 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 s.p.a.ce 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 pa.s.ses 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_.