The Philosophy of the Weather Part 13

"In the first two series, viz.: August 17th and 26th, this peculiar interruption of the progress of temperature is strikingly coincident with a _large_ and _rapid fall_ in the temperature of the _dew-point_. The same is exhibited in a less marked manner on November 10th. On October 21st a dense cloud existed at a height of about 3,000 feet; the temperature decreased uniformly from the earth up to the _lower_ surface of the cloud. When a slight rise commenced, the rise continuing through the cloud, and to about 600 feet above its upper surface, when the regular descending progression was resumed. At a short distance above the cloud, the dew-point fell considerably, but the rate of diminution of temperature does not appear to have been affected in this instance in the same manner as in the other series; the phenomenon so strikingly shown in the other three cases being perhaps modified by the existence of moisture in a _condensed_ or vesicular form.

"It would appear, on the whole, that about the princ.i.p.al plane of condensation heat is developed in the atmosphere, which has the effect of raising the temperature of the higher air above what it would have been had the rate of decrease continued uniformly from the earth upward."

These gentlemen do not adopt the absurd explanation of the French philosophers; they account for the phenomenon by supposing heat to be _developed_ at that particular part of the atmosphere; but they are equally wide of the mark. They found the excess of heat there to the extent of 7 to 12, and on days when there was no condensation, or other a.s.signable cause for its _development_.

The temperature of the counter-trade partakes, doubtless, of the temperature of the adjoining strata at its upper and lower portion, and has never been found much, if any, higher than 60 at the center. Nor could it be expected. The trade, in its upward curving course, within the tropics, attains a considerable alt.i.tude where the atmosphere is comparatively cold, and necessarily loses a portion of its heat there, and during its northern flow. Probably its central summer range, in the lat.i.tude of Paris, is not far from 55, and with us 60.

The contrast between the trade and the surrounding atmosphere, in winter, is much more striking, and this has been observed particularly upon the Brocken of the Alps, and in the polar regions.

"In all seasons the temperature is higher on the Brocken, on a serene, than on a cloudy day, and, in the month of January, _the serene days were warmer than at Berlin_." (Kamtz's Meteorology, by Walker, p. 217.--Note.)

As the portion of the counter-trade, which does not become depolarized--in diminished volume--progresses toward the polar regions, it settles nearer the earth, and within the Arctic circle is found but little way above it.

Thus, in December, 1821, Parry, at Winter Island, in lat.i.tude 66 11', flew a kite, with a thermometer attached, to the height of 379 feet, and found that the temperature, instead of falling 1-1/4, the usual ratio of decrease, rose 3/4 of a degree.

The same thing was observed at Spitzbergen, in lat.i.tude 77 30' north, and at Bosekop, lat.i.tude 69 58', by a scientific commission, and by means of kites, confined balloons, and the ascent of elevations.

"In winter the temperature goes on increasing with the height, up to a certain limit, which is variable, according to the different atmospheric circ.u.mstances, the influence of which is not yet very exactly known. The hour of the day appears to be indifferent, since there exists no thermometric diurnal variation in the strata of the surface. The mean of thirty-six experiments, made with kites, or with captive balloons, at Bosekop, lat.i.tude 69 58' north, has given a mean rate of increase of 1 6' for the first hundred meters.[6]

Beyond this limit, and even beyond the first 60 or 80 meters, the temperature again becomes decreasing, at first very slowly, but afterward the decrease is accelerated. The observations that have been made on the flanks, or on the summits, of mountains, during the same expeditions, entirely confirm these results. The cooling influence of a soil, that radiates its own heat for several weeks, without receiving any thing on the part of the sun, in compensation of its losses, the influence of _counter-currents from above_, coming from the west and the south-west, with a high temperature, account for this anomaly, which, in winter, represents the normal state of the most northern parts of the European continent." (Walker's Kamtz, p. 515.--Note.)

Mr. Walker is the only author, so far as I know, who has suspected the true cause of the phenomenon, viz.: "currents from above coming from the west and south-west, with a high temperature;" but the caloric theory "sticks like a burr," and he adheres also to the idea that a snow-clad surface, in the absence of the sun, can aid, by radiation, in warming the atmosphere for a distance of several hundred yards above it, increasing the warmth as the distance from the earth increases!

This contrast between the counter-trade and the adjacent atmosphere, in winter, in lat.i.tudes as low as that of the Brocken, is probably heightened by the increased warmth of the former, at that season. The S. E. trades then form under a vertical sun, and the difference of temperature can not be less than from 6 to 8. Not unfrequently in winter and spring the rain will fall with a temperature of 50 to 55, when the atmosphere near the earth is 10 or 20 or more, below those points; and it is frozen to every object upon which it falls. The trade stratum, from which it descends, is not warmed by "radiation" or by ascending currents from a snow-clad surface, and during a cloudy day; nor by a "development of heat" at that particular alt.i.tude, but it has brought its heat from the South Atlantic, and imparts it to the rain which forms within it. There is every reason to believe that the counter-trade flows north in a regular descending plane, not materially differing from that of the line of perpetual snow. The descent of the latter is well ascertained to be from about 16,000 feet at the equator, to _the surface_ at the poles. The plane of the counter-trade is probably much the same, varying over different localities, from the varied action between it and the earth which we are considering; and probably both correspond with the increase of magnetic intensity.

Lieutenant Maury, in an able and original article upon the circulation of the atmosphere, conceives the bands of comparative calms at the northern limits of the trades, which he appropriately terms the "_Calms of Cancer_," to be nodes in the circulation of the atmosphere, and that the upper or counter-trade here decends and becomes a surface wind from the S.

W., as the N. E. trade is a surface wind; and that an upper current from the poles approaches and descends at the same node, to make the N. E.

trade. But it is evident he adopted that conclusion too hastily, as he obviously did the conclusion that the calms of the horse lat.i.tudes were a type of all. We have seen that the latter are increased by a diversion of the counter-trade, and that they are avoided by making easting. So it may be observed that our upper current is a S. W. current, and no northerly upper current is visible, or exists over the country, however it may be in western Europe and the North Pacific, on the west of the magnetic poles, where cold, dry northerly and north-easterly winds are found. The origin and progress of storms withal demonstrates that no such node can exist.

Two points have been made in relation to the course of the counter-trade in the tropics, and are relied upon to show its progress there to the N.

E., which deserve consideration.

In the first place, it is well known that "rain dust" falls in considerable quant.i.ties on the western coast of Africa, particularly about the Cape de Verde Islands, and also upon the Mediterranean and south-western Europe, where it is termed "sirocco dust."

"This dust," says Lieutenant Maury, "when subjected to microscopic examination, is found to consist of infusoria and organisms, whose _habitat_ (place of abode) is not Africa, but South America, and in the S. E. trade-wind region of South America. Professor Ehrenberg has examined specimens of sea dust, from the Cape de Verdes and the regions thereabout, from Malta, Genoa, Lyons, and the Tyrol, and he has found such a similarity among them as would not have been more striking had these specimens been all taken from the same pile.

"South American forms he recognizes in all of them; indeed, they are the prevailing form in every specimen he has examined.

"It may, I think, be now regarded as an established fact, that there is a perpetual upper current of air from South America to north Africa, and that the volume of air in these upper currents, which flows to the northward, is nearly equal to the volume which flows to the southward with the N. E. trade-winds, there can be no doubt,"

etc.

Now, it is doubtless true that this dust is transported in a counter-trade, and that such dust is found in South America, and is taken up there by sand-spouts, like those of the ocean in form and action. Both Humboldt and Gibbon have graphically described them. Yet I do not think the point well taken. South-eastward of the Cape de Verdes, where the surface-trades--which, becoming counter-trades, pa.s.s over these islands, and, recurving, pa.s.s over the Mediterranean and south-western Europe--should originate, there is a vast extent of unexplored continent in the same lat.i.tude as the portion of South America where the dust is found; and the same dry seasons, and the same spouts, in all probability, exist in both. Until it be shown that such forms have no "_habitat_" in central and southern and unexplored Africa, upon the same lat.i.tudes as in South America, it may fairly be presumed that the dust is taken up there.

Indeed, the _curve_ upon which this dust is found to fall, in the greatest quant.i.ties, is very remarkable, and corresponds remarkably with the _law of curvature_ of the counter-trade we have considered, and with the progress of a storm upon that coast, and over the Mediterranean, investigated by Colonel Reid. (See Reid, on Storms and Variable Winds, p.

276.) This _curve clearly indicates the origin of the dust in South Africa_.

The second point is, that ashes from the volcanos of Mexico and Central America have fallen to the north-east of the place where they were ejected. Mr. Redfield has grouped these instances of volcanic eruption usually cited, and I copy from him:

"We learn from Humboldt, that in the great eruption of Jorullo, a volcano of southern Mexico, which is 2,100 feet above the sea, in lat.i.tude 18 45', longitude 161 30', the roofs of the houses in Queretaro, more than 150 miles north, 37 east from the volcano, were covered with the volcanic dust. In January, 1845, an eruption took place in the volcano of Cosiguina, on the Pacific coast of Central America, in lat.i.tude 13 north, and having an elevation of 3,800 feet, the ashes from which fell on the island of Jamaica, distant 730 miles north, 60 east from the volcano. The elevated currents by which volcanic ashes are thus transported are seldom or never of a transient or fortuitous character; and these results, therefore, afford us one of the best indications of their general course. Thus, the progress of the higher portion of the trade-wind was marked by the eruption of Tuxtla, lat.i.tude 18 30', longitude 95, which covered the houses in Vera Cruz with ashes, at the distance of 80 miles north, 55 west, and also at Perote, 160 miles north, 60 west.

The ashes from the volcano, at St. Vincent, which fell at Barbadoes, and east of that island, in 1812, mark the course of a current from the westward, which appears there at times, in the region of clouds, and may, perhaps, be connected with the permanent winds on the Pacific coast of Mexico."

As to one of the instances cited in the foregoing paragraph, that of Tuxtla, it may be laid out of the case--the direction conforming substantially to the a.s.sumed course of the counter-trade at that point.

St. Vincent lies W. N. W., or nearly so, of Barbadoes, and a N. W. or westerly surface-wind, prior to, and during storms, is common in the West Indies as the N. E. is here--both alike, blowing in opposition to the progressive course of the storm. There is nothing strange or peculiar, therefore, respecting that instance, or the existence of variable and especially S. W. currents, between the trades, with occasional partial condensation.

The falling of the ashes from Cosiguina, upon Jamaica, has long and often been cited, as proof that in the West Indies the prevailing upper currents run from the S. W. But it has been ascertained that, _during the same eruption, ashes fell 700 miles to the westward, on the deck of the Conway_, a vessel then upon the Pacific Ocean. That case, therefore, does not prove the absence of the S. E. counter-trade at the time, but only the presence of another, and a different current above or below it--and it may have been either, and transient.

So of the Jorullo instance. Investigation would probably have shown that ashes fell to the N. W., and that they were carried N. E. by a transient S. W. wind produced by the existence of a storm to the eastward, or one of those states of partial condensation of the counter-trade which often produce currents at greater distances without a storm. Not one of these cases disproves the existence of a S. E. counter-trade, and the invariable N. W. progression of the storms of those lat.i.tudes demonstrates it.

Occasional anomalous currents, depending upon storm action at considerable distance, are found in our atmosphere, and doubtless are there also. Thus, although the N. W. wind is almost invariably a surface wind, I have, in a few instances, seen a N. W. set at a considerable elevation, converging toward a peculiarly stormy state of atmosphere far south of us, about the period of the spring equinox. And so in one or two instances I think I have seen light cirro-stratus clouds _above_ the counter-trade, when it ran very low, setting from the N. E., although the usual and almost invariable location of the N. E. wind is below the counter-trade and the stratus clouds of the storm. Aeronauts, too, have found these secondary currents beneath a serene and cloudless sky. Indeed, the S. E.

counter-trade doubtless often induces a thin secondary current of S. W.

wind between itself and the surface-trade, in the same manner that similar currents are induced with us, and every where.

A question arises here of considerable interest, which, I confess, I can not answer to my own satisfaction. It is, whether there be, or not, _an eastern progression of the body of the atmosphere above the machinery of distribution_. I have thought there was, and that in set fair weather I had seen a peculiar kind of cirro-c.u.mulus cloud, in patches, the small c.u.muli very distinct and rounded, moving due east, which indicated such a current. But I am not satisfied, from my own observation, that it is so, nor is it easy to determine the question. The moisture of evaporation rarely, if ever, ascends to any considerable elevation, and the upper strata must be very dry. Hence, condensation, if it takes place, is thin, and perhaps often undiscernable. Investigations upon mountains prove little, for the winds of the inferior strata rush up their sides and over them. It is an open question, and future observation may solve it. The prevailing opinion seems to be that there is. If the theory of Oersted, in relation to the circular currents of a magnet, be true, there should be such a progression produced by opposite secondary currents, unless, indeed, it be also true that those currents are inoperative at so great a distance, or their influence barely suffices to retain the attenuated atmosphere in its place. Perhaps the investigations of Ampere conflict with it. But it is worth while, I think, for philosophers to inquire whether the transverse position of the needle upon the wire is not the effect of the central _longitudinal_ currents, conforming to the circular currents of the wire, and whether it is not owing to the production of the same currents in a globe by the circular currents of Ampere, that the globe is magnetized, and the needles made to dip.

CHAPTER VIII.

It is exceedingly desirable, in a practical point of view, to understand the precise character of the reciprocal action which takes place between the earth and the counter-trade, and produces the varied phenomena which mark our climate. We have seen that the same laws, other things being equal, operate every where, and that a.n.a.logies may be sought in the character of those phenomena elsewhere, under the same, or different, modifying circ.u.mstances. Looking, therefore, at the magneto-electric movable machinery as a whole, and its influence upon the atmospheric circulation and conditions, we find many facts which point to a primary action in the counter-trade, and others that point as significantly to a primary local-inducing-action in the earth. Let us briefly review those to which we have alluded, and advert to some others, and see what solution of the question they will justify:

The belt of inter-tropical rains appears to be, in width, and amount of precipitation, and annual travel north and south, proportionate to the volume of trades which blow into it, the quant.i.ty of moisture they contain, and the elevation of the surface over which they meet.

South America is the most thoroughly-watered country within the tropics, except, perhaps, portions of Hindoostan, Burmah, Siam, etc., on south-eastern Asia. The contrast between both, and Africa, as far as explored, and as shown by its rivers, is most obvious. The Amazon, alone, delivers more water to the ocean than all the rivers of Africa.

Of the width of the belt of rains over Africa, in the interior, we know little. Its northern extension is less, by from 7 to 10, than the same belt over South America, the West Indies, and Mexico. Probably its southern is also. Upon South America, the southern edge is carried down to Cochabamba, in lat.i.tude 18, and probably to 25, to the northern edge of the coast-desert of Peru, while it is rarely, if ever, found over the Atlantic below 7, a difference of 12 to 20. Over South America, too, the quant.i.ty of water which falls is also vastly in excess of that which falls upon the Atlantic. The main cause of these differences is obvious.

The N. E. counter-trades which blow over Africa, originate on a surface which is rainless, as eastern Sahara, Egypt, Arabia, etc., or subject to a dry season by the northern ascent of the southern line of the extra-tropical belt, as the Barbary States, Syria, Persia, etc., and their supply of moisture is necessarily scanty. On the south, the S. E. trades originate, in part, upon the eastern portion of southern Africa, and, in part, upon the Indian Ocean, and from the latter source, and a portion of the Mediterranean, doubtless most of the water which falls upon Central Africa, is derived.

The N. E. and S. E. trades which blow into the inter-tropical belt upon the eastern portion of the Atlantic, originate upon similar surfaces, and with like effect. Thus, the S. E. trades, in summer, are from the Southern portion of Africa, and the N. E., in part, from the Mediterranean; and, in winter, the N. E. from the deserts, Senegambia, Nigritia, etc., and the S.

E., owing to the narrowing of the African continent, mainly from the South Atlantic and Indian Oceans. Going west, the belt widens, and its range increases until the Andes are reached; but under their lee, on the western side, a totally different state of things is found, and the belt of the coast becomes broken and irregular, as we have seen in the citation from Maury.

The width, extension, and excessive precipitation of the belt, over South America, follow the same law. The South Atlantic widens out by the trending of the coast to the S. W., and furnishes a large area for the un.o.bstructed formation and evaporative action of the S. E. trades. So the trending of the coast to the N. W., from 5 south to the northward, opens a large area for a like formation and action of the N. E. trades. No correspondingly favorable circ.u.mstances exist any where, except, perhaps, around Hindoostan, and there the fall of rain is very excessive in some places, as on the Ka.s.saya hills, to the extent of 400 inches per annum. In addition to this, the magnetic line of no variation, and of greater intensity, which runs from our magnetic pole, obliquely, S. S. E., to its opposite and corresponding pole in the southern hemisphere, enters the Atlantic on the coast of North Carolina, and traverses it, and the eastern portion of South America, through the whole trade-wind region. The table-lands, and slopes, and high mountain peaks, meet the trades successively, as they go west, and the latter wrench from them, to an unusual extent, their moisture; depressing the line of perpetual snow, by an increase of quant.i.ty on the eastern sides, several thousand feet, as it is for a like cause depressed on the southern side of the Himmalayas. On the eastern slopes and tops of the Andes, as we have seen, and owing to their elevation, falls the moisture which, according to the working of the machinery, and the law of curvature, should bless the coast line of Peru and northern Chili, the eastern Pacific, northern Mexico, California, Utah, and New Mexico; and, while the Andes stand, the curse of comparative aridity must rest upon them all.

Southern Chili, and western Patagonia are supplied by the N. E. trades, which originate in the West Indies, the Gulf of Mexico, and the Caribbean Sea, and the Pacific, off Central America, in the neighborhood of the Bay of Panama. But there, again, the same effect of elevation is seen. The mountain slopes of southern Chili and Patagonia are abundantly supplied, and their mountain ranges are drenched with rain, while eastern Patagonia and southern Buenos Ayres, under their lee, are comparatively dry. So the S. E. trades, which originate off the western coast of South America, curve in upon, and aided by the oceanic currents, supply, abundantly, the N. W. coast of this continent, north of California; and there, too, the coast, and its elevated ranges, receive, as we have seen, a very large proportionate supply of their moisture. Substantially, the same state of things, as far as circ.u.mstances permit, is reproduced upon Malaysia, Hindoostan, etc., and the interposition of arid New Holland upon the evaporating trade-surface may be distinctly traced upon south-western Asia. Deserts abound there; the Caspian Sea receives the drainage of a very large surface, without an outlet; their southern line of extra-tropical rains is carried up very far in summer, and their dry season is intensely hot. (See an article in the American Journal of Science, for July, 1846, by Azariah Smith.)

Another fact in this connection is worthy of a moment's consideration. The magnetic equator, as sought by the dipping needle, is not coincident with the geographical one. Humboldt found it, on the Andes, at 7 1' south, and it has been found still lower in the Atlantic. Over Africa it rises above the geographical equator, and descends again on the Indian Ocean. About midway the Pacific, it becomes coincident with the equator of the earth again. (See diagram, on page 83.) Perhaps it is not known, with certainty, why this is so. The south pole may be situated nearer the geographical pole than the north one--but this is not believed to be so, nor could it make the difference. The greatest southern depression of the magnetic equator is found where the lines of greatest intensity, and of no variation, are found; and at the more intense of these lines exists the greatest depression. From this, I think, it may be inferred that the needle is affected by the greater magnetic intensity of the northern hemisphere, to which it may yet appear the obliquity of the earth's axis is owing. However this may be, or whatever the cause, no marked effect is produced upon the trades. The S. E. trades, by reason of the greater extent of ocean-surface on which they originate, are every where the most extensive, regular, and forcible. The south polar waters, from which they rise, are every where trenching upon, and overriding, the north polar ones; and thus, by a most beneficent provision, the greater portion of the habitable surface is placed in the northern hemisphere, and the princ.i.p.al portion of the southern is left open to an extensive, active evaporative action, which supplies the northern habitable surface with a large excess of the needed moisture.

The condensation, and consequent precipitation, which takes place at the pa.s.sing of the trades, as we have already said, over the ocean and lowlands, takes place mainly in the day-time. Upon the table-lands and mountain-ranges, it often continues during the evening and night. The morning, and early part of the day, however, in tropical countries, are generally fair at all elevations.

Storms also originate in the equatorial belt, and issuing forth in great volume and with great intensity of action, find their way up even within the Arctic circle. Those which pa.s.s over this continent, or the northern Atlantic, generally originate in the West Indies, some of them over the Caribbean Sea, some over the islands, and some over the open ocean to the east of them; and, nearly all the most violent, during the months of August, September, and October. It would seem most probable that the primary action in such cases was in the trades themselves, but it is by no means certain that such is the case. This is the cla.s.s of storms of which Mr. Redfield has industriously investigated some twenty or more; Mr. Espy some, and Lieutenant Porter two. Their course, when very violent, is often more directly north than that of storms, however violent, which originate north of the calms of Cancer, owing, perhaps, to their greater paramagnetic character. This course I have myself observed, in several instances, about the period of the autumnal equinox--never, however, more southerly than from S. W. to N. E., on the parallel of 41, except in three, and, perhaps, four, instances, when it has been S. W. by S. to N.

E. by N. I know of no cla.s.s of storms in relation to which the evidence of primary action in the counter-trade is stronger than in those of the cla.s.s which originate on the ocean east of the Windward Islands. But it is not satisfactory as to them. Doubtless the conflict of polarities between the pa.s.sing trades is sufficient to produce the showers and rains which are ordinarily found over the ocean and lowlands, in the equatorial belt; but it is doubtful whether it is sufficient to produce such extensive, long-continued, and violent action, as that which characterizes the hurricane autumnal gales.

They occur, too, at the time when the whole machinery of distribution has reversed its course, and is rapidly pursuing its journey south. It is a period of great magnetic disturbance, over both land and sea; of more active gales and local-increased precipitation. At the Magnetic Observatory of Toronto, Canada West, these disturbances are carefully and systematically observed, and their maxima, or periods of greatest disturbance occur in April and September. (See Silliman's Journal, new series, vol. xvii. p. 145.)

The tendency to volcanic action is not as great at the autumnal, as at the vernal equinox, for the reason that most of the volcanic action of the western hemisphere develops itself now upon South rather than North America. But both exist, and are active, and what are improperly termed equinoctial storms, and gales, and rains, are proverbial during, or just subsequent to, both periods with us--as they are when the same change, called the breaking up of the monsoons, takes place in the line of magnetic intensity, over southern and eastern Asia. A volume might be filled with extracts, showing, at least, most remarkable coincidences between violent volcanic action and great atmospheric disturbance. Perhaps the increased fall of rain at and after the equinoxes, in the northern hemisphere, and in certain localities subject to volcanic activity, is as strikingly ill.u.s.trated by the register, kept by Mr. Johnson, on the volcanic Island of Kauai, one of the Hawaiian group, already alluded to, as in any other case, although it is by no means a singular one. The greatest fall of rain, in any month except April and October, was eight inches. In April, the fall was fourteen inches, in October, eighteen inches. Neither the equatorial, nor extra-tropical belt, were over the island during those months; but they were the N. E. trades, and the result was owing solely to the interposition of high volcanic mountains, _in a state of disturbance_, into, or near, the strata of the counter-trade. Mr.

Dobson, in stating a theory to which we shall hereafter advert, advances the following proposition:

"7. _Cyclones (hurricanes) begin in the immediate neighborhood of active volcanoes._ The Mauritius cyclones begin near Java; the West Indian, near the volcanic series of the Caribbean Islands; those of the Bay of Bengal, near the volcanic islands on its eastern sh.o.r.es; the typhoons of the China Sea, near the Philippine Islands, etc."

The peculiar stormy state of the atmosphere, over the Gulf Stream, to which I have alluded, certainly affords no evidence of primary atmospheric action. It is a body of south polar water, pursuing its way under the guidance of magnetism--maintaining its polarity--arched somewhat like the roof of a house, by the outward pressure of a cold north polar current which it has met to the east of the Banks of Newfoundland, and forced to take an in-sh.o.r.e course to the southward, and the bodies of water which the rivers discharge, and a conflict with the north polar surface-winds which sweep over it, and fogs, and thunder, and rain, are a matter of course. Dr. Kane met a portion of this singular current in Baffin's Bay, north of 75, which had preserved its characteristics and a considerable proportionate excess of heat, although it probably had been around Greenland, or found its way to the west, toward the magnetic pole, through some of its northern fiords or straits. (Grinnel Expedition, p. 120.)