Chapter XIII: Part 13

Whoever wishes to be weatherwise, and who has time to study the weather charts published daily, may easily acquire such knowledge of local characteristics as will enable him to forecast fairly accurately. Cirrus clouds, as a rule, are reliable guides; they form, as we have said, in parallel threads, from the position and movements of which forecasts may be made. Should the threads appear on, and parallel to, the west horizon, and moving from a northerly point, a depression is approaching from the west, but, although causing some bad weather, it will probably pass to the north of the observer. Should the lines appear parallel to the southwest or south-southwest horizon, and be moving from a northwesterly point, the depression will very likely pass over the observer and occasion very bad weather. These are two of many possible prognostics. Weather forecasting is much helped by a study of the daily weather charts. Again, weather is often very local, and to predict with fair accuracy a knowledge of local conditions is necessary.

WINDS
--WILLIAM HUGHES

Among the secondary causes affecting climate, probably none is of greater importance than the direction of prevailing winds. The currents of air are warm or cold, wet or dry, according as they have had their origin in warm or cold latitudes, and have traversed inland tracts, or the expanse of ocean, in their advancing course. With us, and in the northern half of the globe in general, north and east winds are cold and dry, while south and west winds are warm, and often accompanied by moisture. Within the Southern Hemisphere these conditions are reversed, and the hottest currents of air come from a northwardly direction. The prevailing winds of western Europe are from the west and southwest; and it is to this fact that we must mainly ascribe the high winter temperature, as well as the comparative freedom from extremes of heat and cold which distinguishes the countries of western Europe. The same cause explains the abundant moisture which belongs to those regions in general, and which distinguishes the western shores of our own islands in a remarkable degree. Such winds have traversed the immense expanse of the Atlantic, and come to the western seaboard of Europe laden with the moist vapors gathered on their course. These vapors, condensed upon the high grounds which line the western side of the British Islands, or, further to the northward, upon the long chain of the Scandinavian Mountains, fall to the earth in copious torrents of rain. In the process of condensation, a vast quantity of latent heat is disengaged, and the temperature is correspondingly raised. Warmth and moisture are, indeed, speaking generally, concomitant conditions of European climate, and are especially so in the case of western Europe.

Even in the case of lands which nearly approach the tropic, the influence of prevailing winds in raising or lowering the temperature is strikingly seen. At New Orleans, bordering on the Mexican Gulf, and throughout the adjacent portions of the United States, the winters are often of excessive severity. Cold winds, generated in the higher latitudes of the New World, and blowing for weeks in succession from the northern quarter of the sky, are the cause of this. The generally level interior of the North American Continent--a vast lowland plain, bounded only to the east and west by the Alleghanies and the Rocky Mountains--presents no obstacle to the advance of these cold northerly blasts. The middle and eastwardly parts of North America are subject to like influences, in this regard, to the plains of eastern Europe. To the westward of the Rocky Mountains, on the other hand, the conditions affecting climate present greater analogy to those that belong to western Europe.

In the case of many countries, some local wind, of occasional prevalence, forms a marked characteristic of climate. The most remarkable of these local winds are the simoon, the sirocco, the föhn, the harmattan, and the mistral.

The often-described _simoon_ of the desert is an intensely heated and dry wind, which raises the temperature like the blast of a furnace, and fills the air with particles of sand, of suffocating quality. The same wind is known in the deserts of Turkestan as the _tebbad_ (fever-wind), the terrible conditions of which are thus described by the pen of a traveler. “The kervanbashi (_leader of the caravan_) and his people drew our attention to a cloud of dust that was approaching, and told us to lose no time in dismounting from the camels. These poor brutes knew well enough that it was the _tebbad_ that was hurrying on; uttering a loud cry they fell on their knees, stretched their long necks along the ground, and strove to bury their heads in the sand. We intrenched ourselves behind them, lying there as behind a wall; and scarcely had we, in our turn, knelt under their cover, than the wind rushed over us with a dull, clattering sound, leaving us, in its passage, covered with a crust of sand two fingers thick. The first particles that touched me seemed to burn like a rain of flakes of fire. Had we encountered it when we were deeper in the desert we should all have perished. I had not time to make observations upon the disposition to fever and vomiting caused by the wind itself, but the air became heavier and more oppressive than before.”

The _sirocco_ of the Mediterranean coasts is the hot wind of the African desert, tempered, before reaching the coasts of southern Europe, by its passage across the great expanse of inland waters. The enervating influences of this wind are well known to the resident on the shores of Sicily, the Italian mainland, or the islands of the Archipelago. The same wind, when it reaches the high mountain regions of the Apennines and the Alps, is known as the föhn.

The _föhn_, or warm south wind, is an important agent in modifying the climate of the higher Alpine region, where its prevalence for a few days in succession causes the snow-line to recede, and is often accompanied by inundations occasioned by the suddenly melted snows. Its absence during a longer period than usual is attended, on the other hand, by a prolongation of the glaciers into a lower region of the mountain valleys. The Swiss peasants have a saying, when they talk of the melting of the snow, that the sun could do nothing without the föhn.

The _harmattan_ of Senegambia and Guinea is a cold and intensely dry wind, which blows from the northeast during the months of December and January.

The _mistral_ of southern France possesses similar qualities to the last-named wind, and blows, for days together, down the valley of the Rhone.

Winds transport particles of dust, and, with them, the minuter forms of vegetable and animal life, to vast distances. The phenomena known to sailors as _red fogs_ and _sea-dust_ are evidence of this. In the Mediterranean, and also in the neighborhood of the Cape Verde Islands, showers of dust, of brick-red or cinnamon color, are sometimes experienced in such quantity as to cover the sails and rigging hundreds of miles away from land. Among this sea-dust, examination with the microscope has detected infusoria and other organisms native to the tropical regions of South America.

The prevailing currents of the atmosphere, or _winds_, constitute an important feature in the climate of any country, and it belongs to Physical Geography to explain the prevalent winds which distinguish great regions of the globe. Such explanation is more easily made in regard to the warmer latitudes of the earth, where alone the direction of the wind is constant, than might be at first supposed by those whose personal experience is limited to such countries as Britain, and other temperate lands, where the variable condition of the atmosphere is the well-known subject of common observation and remark. But within those parts of the globe which experience a vertical sun, and for a few degrees beyond the exact line which marks the limit of the sun’s vertical influence on either side of the equator, the conditions either of perennial calm, or of currents of air that constantly blow in one given direction, are the uniform characteristics of climate.

Throughout a zone of a few degrees in breadth, which extends round the globe in the neighborhood of the equator, and the limits of which undergo a certain amount of variation, dependent on the sun’s passage of the equinox, the variation of temperature throughout the year is confined within very narrow limits, and the result is a general prevalence of calms--that is, of undisturbed atmosphere. Wind is air set in motion, mainly by the existence of different conditions of temperature between adjacent bodies of air--of colder and denser air pressing against warmer and lighter air, and taking the place which is left vacant by the latter, as it rises into the higher regions of the entire aerial sea. Between the heated air of the tropics in general, and the comparatively cooler air of the regions lying some distance north and south of the tropics, for example, there is a very manifest difference as to temperature, as well as in regard to other conditions; but for a few degrees in the immediate neighborhood of the equator there is no such obvious difference, and, consequently, nothing to occasion disturbance (temperature alone being considered) in the general equilibrium of the atmosphere. Hence the prevalence of calms in that region. Within the parallels of 8° or 10° on either side of the line, the angle at which the solar rays reach the earth is at no time more than a few degrees from the perpendicular, for the equator divides the total amount of angular difference which is involved in the entire yearly path of the sun.

The average breadth of the calm latitudes--or the _Zone of Calms_, as it is the custom, in books and maps, to term it--may be stated at about six or seven degrees. The mid-line of this zone does not coincide with the equator, for the reason that the equator does not represent the line of the earth’s highest temperature, owing to the preponderance of land in the Northern Hemisphere. Hence the Zone of Calms is, for the most part, to the northward of the equator--extending, with varying seasonal limits, from about the first to the seventh or eighth parallel of north latitude. But the limits oscillate with the sun’s passage of the equinox and consequent place in the heavens vertically over either side of the equator.

The calm latitudes are the dread of the mariner, whose ship is often delayed for weeks together within their limits. The wearisome and tantalizing nature of this delay can, perhaps, only be adequately appreciated by those who have experienced the monotony attendant on a calm in mid-ocean, when, with a still and glassy sea around, a glittering atmosphere, and a burning sun overhead, the sails hang idly by the yards, and the vessel makes no appreciable progress.

Between the oscillating limit of the Zone of Calms and the parallel of 28° in the Northern Hemisphere, on one side of the globe, and between the correspondent limit and the parallel of 25° south latitude, on the opposite hemisphere, there prevail through above two-thirds of the earth’s circumference, steady winds, blowing with almost undeviating uniformity from the eastward. These are the trade-winds. More precisely, _the trade-wind of the Northern Hemisphere is a wind blowing from the northeastward_--that is, a _northeast wind_. _The trade-wind of the Southern Hemisphere blows from the southeastward_, and is _a southeast wind_.

The trade-wind belts stretch round more than two-thirds of the earth’s surface. They comprehend (within the latitudinal limits already defined) the Atlantic and Pacific Oceans, with the countries that lie adjacent to those vast areas of water. In the Pacific, however, their limits are less distinctly marked, and their influence less powerful, to the southward of the equator than to the north of that line. Over the Indian Ocean and its shores, the atmospheric currents follow, during portions of the year, an opposite course.

The trade-winds of the Atlantic and Pacific--blowing constantly, and with almost undeviating steadiness, from the eastward--regulate the course of the mariner across those oceans. They, of course, facilitate the passage of either ocean in a westerly direction--that is, from the shores of the Old World to the eastern seaboard of America, or from the western coast of the New World to the eastern shores of the Asiatic and Australian Continents. It was the trade-wind of the Northern Atlantic that carried Columbus to the westward, on the adventurous voyage which resulted in the discovery of the New World, inspiring terror in the breasts of his companions, while in the mind of the great navigator himself it strengthened the assurance of reaching land by pursuing the direction in which his vessels’ prows were turned. On a like great occasion, the trade-wind of the Pacific carried Magellan’s ship steadily forward through the ocean which he was the first to cross, and facilitated the earliest circumnavigation of the globe. On the other hand, the same winds compel the return voyage across either ocean to be made in higher latitudes, where westerly winds prevail.

The explanation of the trade-winds is found in the different measure in which the sun’s heat is experienced by regions within or nearly adjacent to the tropics, and by those of higher latitudes. They are currents of air set in motion by the differences of density consequent upon such various conditions of temperature--conditions which are of uniform prevalence, and the result of which is also constant.

To sum up, we may say that the trade-winds, like the currents of the ocean, are due, _first_, to the sun,--that is to the different measure in which the solar heat is distributed on the globe’s surface; and, _secondly_, to the earth’s axial rotation, which affects the direction of currents in the aerial ocean in manner precisely analogous to that in which it affects the like currents in the aqueous ocean. In truth, the ocean of water, and the ocean of air--in contact with one another, and possessing many properties in common--act and react upon one another, mutually imparting their respective temperatures, movements, and other conditions. This is only one among the instances of mutual harmony--one of the many mute sympathies--which abound in the natural world.

The monsoons are winds which blow over the Indian Ocean, and the countries adjacent to its waters. In general terms, it may be said that they prevail within the same latitudes as those over which the trade-winds of the Atlantic and Pacific blow. But the monsoons differ from the trade-winds of the two greater oceans in the fact that they are _periodical winds_, not perennial. The monsoon blows for half the year from one quarter of the heavens, and for the other half from an opposite quarter.

Over the northerly portion of the Indian Ocean--from the neighborhood of the equator to the shores of the Asiatic Continent, including the Malay Archipelago and the adjacent China Sea--a northeast monsoon blows during the winter months of the Northern Hemisphere; that is, from October to March, inclusive. During the summer months--April to September--and within the same limits, the southwest monsoon blows. Southward from the equator to the neighborhood of the tropic of Capricorn, the southeast monsoon blows during the winter of those latitudes (April to September): this is exchanged, during the other half of the year, for a northwest monsoon in the neighborhood of the Australian coasts, and for a northeast monsoon along the line of the African shores. The term _monsoon_--derived from a Malay word which signifies “season”--expresses the periodical nature of these winds, and indicates to how large an extent the climate of Indian seas and lands is dependent upon their periodical recurrence.

The change from the one monsoon to that from an opposite quarter is not accomplished at once. The breaking-up of the monsoon, as it is termed, is attended by thunderstorms and other meteorological phenomena, which prevail during some weeks, until the setting-in of the coming monsoon is fairly accomplished. The nature of these changes, and the general characteristics of the monsoon itself, are admirably depicted in the following passage, by a master hand:

“Meanwhile the air becomes loaded to saturation with aqueous vapor drawn up by the augmented force of evaporation acting vigorously over land and sea; the sky, instead of its brilliant blue, assumes the sullen tint of lead, and not a breath disturbs the motionless rest of the clouds that hang on the lower range of hills. At length, generally about the middle of the month, but frequently earlier, the sultry suspense is broken by the arrival of the wished-for change. The sun has by this time nearly attained his greatest northern declination, and created a torrid heat throughout the lands of southern Asia and the peninsula of India. The air, lightened by its high temperature and such watery vapor as it may contain, rises into loftier regions, and is replaced by indraughts from the neighboring sea, and thus a tendency is gradually given to the formation of a current bringing up from the south the warm humid air of the equator. The wind, therefore, which reaches Ceylon comes laden with moisture, taken up in its passage across the great Indian Ocean. As the monsoon draws near, the days become more overcast and hot, banks of clouds rise over the ocean to the west, and in the peculiar twilight the eye is attracted by the unusual whiteness of the sea-birds that sweep along the strand to seize the objects flung on shore by the rising surf.

“At last sudden lightnings flash among the hills and shoot through the clouds that overhang the sea, and with a crash of thunder the monsoon bursts over the thirsty land, not in showers or partial torrents, but in a wide deluge, that in the course of a few hours overtops the river banks and spreads in inundations over every level plain.

“All the phenomena of this explosion are stupendous: thunder, as we are accustomed to be awed by it, affords but the faintest idea of its overpowering grandeur in Ceylon, and its sublimity is infinitely increased as it is faintly heard from the shore, resounding through night and darkness over the gloomy sea. The lightning, when it touches the earth where it is covered with the descending torrent, flashes into it and disappears instantaneously; but when it strikes a drier surface, in seeking better conductors, it often opens a hollow like that formed by the explosion of a shell, and frequently leaves behind it traces of vitrification. In Ceylon, however, occurrences of this kind are rare, and accidents are seldom recorded from lightning, probably owing to the profusion of trees, and especially of cocoanut palms, which, when drenched with rain, intercept the discharge, and conduct the electric matter to the earth. The rain at these periods excites the astonishment of a European; it descends in almost continuous streams, so close and so dense that the level ground, unable to absorb it sufficiently fast, is covered with one uniform sheet of water, and down the sides of acclivities it rushes in a volume that wears channels in the surface. For hours together, the noises of the torrent as it beats upon the trees and bursts upon the roofs, flowing thence in rivulets along the ground, occasions an uproar that drowns the ordinary voice and renders sleep impossible.”

The monsoons of the Indian Ocean are not divided by any such distinctly defined belt of calms as separates the opposite trade-winds of the northern and southern Pacific and Atlantic. The southeast monsoon of the southern Indian Ocean passes gradually into the southwest monsoon, which prevails at the same time in the northern half of that ocean. Nor is the season of change from the one monsoon to the other precisely the same over all parts of that ocean. Indeed, the Indian Ocean, from the geographical conditions already adverted to, is exposed in much higher measure than either of the other oceans to the disturbing influences consequent upon proximity to land, and its winds are hence affected in a vastly greater degree by local conditions. Thus the Indian monsoon, the Arabian and East African monsoon, and the monsoon of northwestern Australia, assume in each case a direction which is dependent upon the geographical position and contour of the lands whence they derive their distinguishing names. In the Red Sea, the monsoons follow the direction of its shores, and blow, for six months of the year, alternately, up and down its long and trough-like valley, confined and guided in their passage by the mountain-chains which bound it upon either side.

We have hitherto spoken of the monsoons only in connection with the Indian Ocean. But, in truth, a monsoon, or season-wind--which is what the word monsoon means--is experienced upon a large portion of the West African coasts, and thence far out into the mid-Atlantic, within the proper region of the Atlantic trades. The evidence of this is one among the many valuable results due to the Wind and Current Charts of Maury, and the cause of it is precisely the same as that which occasions the monsoon of the Indian coasts. Between the equator and the parallel of 13° north, the intense heat of a vertical sun, acting upon the western coasts and adjacent interior of the African Continent, occasions a reversal of the ordinary wind of that region. The intensely heated atmosphere of the land, owing to superior rarity, ascends, and the cooler air of the neighboring sea sets in to fill its place. The monsoon thus generated lasts as long as the sun remains to the northward of the equator. Further to the south a like phenomenon accompanies, in those localities, the passage of the sun into south declination. The influence of these monsoons extends to a distance of a thousand miles or more from land, the entire space within which they prevail forming a cuneiform (or wedge-shaped) region in the midst of the Atlantic, the base of which rests upon the African Continent, while its apex is within ten or fifteen degrees of the mouth of the Amazon.

A similar reversal of the trade-winds of the North Pacific occurs off the western shores of Central America, capable of explanation in precisely like manner--due, that is, to the excess of heat which the summer sun brings to the adjacent lands, and the consequent rarefaction and rising of the currents of air over those lands. This, and the like instance of the West African monsoons, show in the most striking manner how powerfully the land is affected by the sun’s heat, and to how wide a distance the atmospheric movements which are generated by such influences extend over the adjacent seas. Even such limited tracts of land as the Society and Sandwich Islands have a marked influence upon the winds experienced over the surrounding waters. They interfere, says Maury, with the trade-winds of the Pacific very often, and even turn them back, for westerly and equatorial winds are common at both groups, in their winter time.

Upon the coasts of most countries that are within the warmer latitudes of the globe, there occur daily, at or shortly before the hour of early dawn, and toward the approach of sunset, breezes that blow respectively _off the shore_ or from _off the adjacent waters_. The former is known as the land-breeze; the latter as the sea-breeze.

These refreshing movements of the air are not confined to countries within, or even very near to, the tropics, though they are more powerful in the case of countries that are within the torrid zone than in the case of other lands. But they are felt upon the coasts of the Mediterranean, and in even much higher latitudes than those of the Mediterranean, during the warmer portions of the year. The hour at which they begin to be perceptible is not the same in all localities; but, speaking generally, the land-breeze begins to be felt about an hour before sunrise, and the sea-breeze toward the early evening, as the time of sunset approaches. During the midday hours the intense heat of the atmosphere, accompanied by general calm and almost perfect repose of the animal world, is painfully felt by all residents in warm countries, and the cooling sea-breeze which sets in as the sun approaches the horizon is welcomed with intense delight. To the sojourner in Indian lands, it is the signal for outdoor exercise, and is accompanied by a general reawakening of the outer world of nature. The dweller on the African or Australian coasts equally rejoices in its refreshing power. The mariner within Indian seas, frequently becalmed during the stillness of the night-watch, finds like relief in the breeze which blows off the land with the approach of early morning.

The land and sea-breezes are due to a cause strictly analogous to that which produces the monsoon of eastern seas--that is, the influence of the sun heating in various measures the lands and seas, and with them the superincumbent air. Successive movements are generated in the atmosphere according as different portions of the whole acquire, with difference of temperature, various degrees of density. During the hours of midday heat, the air over the land becomes relatively hotter, by many degrees, than the air which is above the adjacent water, for it is the well-known attribute of land to experience much greater extremes of temperature than water does. As afternoon, with its sultry temperature, advances, this continued heat occasions the land-air to form an ascending current, while the cooler (and relatively denser) air from the neighboring waters flows in to take its place. This cooling breeze is an effort of nature to restore equilibrium in the atmosphere, the heavier portions of the whole body of air assuming the place of lower strata, and the higher portions spreading over the superior regions. This effort continues until the desired balance is attained, and, with the approach of midnight, the air is again calm and settled. But during the night, while the water retains a nearly uniform temperature, the land rapidly parts with the heat, so that the air over the land becomes at length colder than that over the water. This latter, therefore, relatively the warmer of the two, tends to rise, while the cooler air of the land fills its place. A wind blowing from off the land is thus generated. In some localities this blows during great part of the night. But the period of its commencement varies in different places, and the intervals of calm between both land and sea-breezes are often of uncertain duration.

The land and sea-breezes repeat, on a scale of diurnal variation, the phenomena shown by the monsoons on a scale of yearly change. They show how readily the atmosphere yields to the slightest pressure, and how powerful an influence on the laws of climate, and, with them, on the condition of mankind, is exercised by every change, of temperature, or otherwise, to which it is subject. Similar winds--alternating from opposite quarters of the heavens--are experienced in inland districts, as on the banks of the Tapajos River, in South America.

The rotary storms which occur, at uncertain intervals, in particular latitudes, are to be included among the exceptional phenomena of atmospheric change. They prevail, however, over larger areas than was formerly supposed, and perhaps belong to a general system of atmospheric movements in which electric and magnetic influences fill an important place. The hurricanes of the West Indies, the tornadoes and cyclones of the Indian Ocean, and the typhoons of the China Sea, are winds of this description. Within the Southern Hemisphere, the direction of the rotating circle is always found to correspond to the movement of the hands of a watch (_i. e._, from west to north, east, and south): to the north of the equator, the circle of wind follows an opposite direction (or west to south, east, and north). By a knowledge of this law, combined with careful observation of the track usually taken by such storms, mariners are enabled to avoid some of the dangers incident to their occurrence. The destruction which they occasion, however, within maritime tracts exposed to their influence, as well as upon the high seas, is at times fearfully great.

Waterspouts are another form in which the rotary movements of the air are manifested. In the case of these phenomena, a taper column of cloud, descending from above, is joined by a spiral column of water which winds upward from the agitated surface of the sea, the two together forming, by their union, a continuous column which moves over the sea. Waterspouts seldom last longer than half an hour. They are more frequent near the coast than on the high seas, and more commonly seen in warm climates.

SQUALLS, WHIRLWINDS, AND TORNADOES
--SIR RALPH ABERCROMBY

If we watch the stages of gradually increasing wind, we find that as the strength rises the tendency is more and more to blow in gusts. Gradually these gusts get still more violent, and in their highest development come with a boom like the discharge of a piece of heavy ordnance. This is what sailors call “blowing in great guns,” and these are the gusts which blow sails into ribbons, and dismast ships more than any amount of steady wind. These gusts only last a few minutes, but they seem to be very closely allied to the simplest form of squalls. In a true, simple squall the wind generally need not be of the exceptional violence which causes “guns”; but after it has rather fallen a little, the blast comes on suddenly with a burst, and rain or hail, according to intensity, or other circumstances, while the whole rarely lasts more than five or ten minutes. At sea one often sees two or three squalls flying about at a time. Then we readily observe that over the squall there is firm, hard, cumulus cloud; that the disturbance only reaches a short distance above the earth’s surface; that the squall moves nearly in the same direction as the wind; and that there is little or no shift of the wind before or during the squall. We also see that the shape of the squall is merely that of an irregular patch, with a tendency rather to be longer in the direction of the wind than in any other quarter; and that the motion of the squall as a whole is much slower than that of the wind which accompanies the first blasts. If, at the same time, we watch our barometer closely, we find that if the squall is sufficiently strong, the mercury invariably rises--sometimes as much as one-tenth of an inch--and returns to its former level after the squall is over. No difference is observed in this sudden rise, whether the squall is accompanied with rain, hail, or thunder and lightning; and though we are unable exactly to explain why the wind sometimes takes this irregular method of blowing, we have still to do with a comparatively simple phenomenon.

The simplest kind of thunderstorm may more properly be described as a squall accompanied by thunder and lightning, instead of only with wind and rain. On a wild, stormy day, with common squalls, one or two of these, which are exceptionally violent, will be accompanied by one or two claps of thunder with lightning. The principal interest which attaches to this type of thunderstorm consists in the proof which is afforded that there is no essential difference between a common squall and another which may be associated with electrical discharge, except intensity. The look and motion of the clouds, and the sudden rise of the barometer, are identical in both cases. In western Europe this class of thunderstorm is much more common in winter than in summer, which is the reverse of what takes place with all other kinds of thunderstorm. So much is this the case that in Iceland there are no summer thunderstorms, but only winter ones, of this simple squall type. In Norway both types occur; and the winter ones are there found to be the most destructive, because they are lower down, and therefore the lightning is the more likely to strike buildings. In that country, however, the summer thunderstorms are not nearly so violent as in more southern latitudes.

We must now just mention a class of thunderstorms which are more complicated than a simple squall, and yet differ in many ways from line-thunderstorms. They are associated with secondary cyclones, and are much commoner in England than line-thunderstorms, but none have been tracked over a sufficiently long area to allow us to say anything about their shape or motion. All we know is, that as surely as we see a secondary on the charts in summer, so certainly will thunderstorms occur during the day, though we can not say in what portion of the small depression.

The special features of this class of thunderstorm are the calm sultry weather with which they are associated, so different from the squall of a line-thunderstorm, and the limited rotation of the surface-wind during the progress of the storm. Another very remarkable feature is that this surface circling of the wind extends only a very short distance upward, and whenever a glimpse can be caught of the drift of the upper clouds, they are found to move in the same direction throughout the whole period of the disturbance. This is the familiar class of thunderstorm which we associate with sultry weather, and with the thunder coming against the wind.

One of the first things which must strike everybody is, that even in the temperate zone some countries are far more ravaged by thunderstorms than others. For instance, France suffers more than any other part of Europe, and England the least. We may probably find at least two causes which modify the development of thunderstorms. In the first place, the geographical position of the country relative to the great seasonal areas of high and low pressure. From this point of view we can readily see that France is far more exposed to the influence of small secondaries, which come in from the Atlantic, and which die out before they reach central Europe, than any other portion of that continent.

In Great Britain, though the bulk of winter rain is cyclonic, a great deal of summer rainfall is non-isobaric; in Continental Europe a still larger proportion is of the latter character; so are most tropical rains, except the downpour of hurricanes; while the whole of the heavy rain on the equator, and all that falls in the doldrums, is also absolutely non-isobaric.

A moderate whirlwind may be two hundred feet high, and not above ten feet in diameter. The dimensions, however, are very variable, for a whirlwind may vary in intensity from a harmless eddy in a dusty road to the destructive tornado of the United States.

But by far the most striking non-isobaric rain in the world is the burst of the southwest monsoon in the Indian Ocean. The quality of the rain, if nothing else, distinguishes the monsoon from cyclonic precipitation. The rain in front of a Bengal cyclone seems to grow out of the air, while that of the monsoon falls in thunderstorms and from heavy cumulo-form clouds. The only rational suggestion which has been made to account for this burst of rain would look to a sudden inrush of damp air from the region of the doldrums as the source of the change in weather, but not of the direction of the wind, or of the shape of the isobars; for the burst is apparently almost coincident with the disappearance of the belt of high pressure to the south of the Bay of Bengal.

The word “pampero” is, unfortunately, used in a very vague manner in the Argentine Republic and neighboring states. Every southwest wind which blows from off the pampas is sometimes called a pampero; and there is a still further confusion caused by calling certain dry dust-storms _pamperos sucios_, or dry pamperos. The true pampero may be described as a southwest wind, ushered in by a sudden short squall, usually accompanied by rain and thunder, with a very peculiar form of cloud-wreath.

The barometer always falls pretty steadily for from two to four days before the pampero, and always rises for some days after the squall. Temperature is always very high before the squall, and then the sudden change of wind sends the thermometer rapidly down, sometimes as much as 33° in six hours. Thunder accompanies about three out of four pamperos; but more or less rain always falls, except in the rarest cases. The wind before this class of pampero almost invariably blows moderately or gently for some days from easterly points, and then with a sudden burst the southwest wind comes down with its full strength, and, after blowing thus from ten to thirty minutes, either ceases entirely or continues with diminished force for a certain number of hours. In all cases but one the upper wind-currents have been seen to come from the northwest before, during and after the pampero.

The general appearance of a pampero will be best understood by a description of an actual squall. “In the early morning of a day in November, the wind blew rather strongly from the northeast. The sky was cloudy, but not overcast, save in the southwest horizon. The clouds were moving very slowly from the west, or a little south of it, throwing out long streamers eastward. About 8 A. M. the threatening masses in the southwest had advanced near enough to show that at their head marched two dense and perfectly regular battalions of cloud, one behind the other, in close contact, yet not intermingling, and completely distinguished by their striking difference of color, the first being of a uniform leaden gray, while the second was as black as the smoke of a steamer. On arriving overhead, it was seen that the front, although slightly sinuous, was perfectly straight in its general direction, and that the bands were of uniform breadth. As they rushed at a great speed under the other clouds without uniting with them, preserving their own formation unbroken, their force seemed irresistible, as if they were formed of some solid material rather than vapor. The length of these wonderful clouds could not be conjectured, as they disappeared beneath the horizon at both ends, but probably at least fifty miles of them must have been visible, as the ‘Cerro’ commands a view of twenty miles of country. Their breadth was not great, as they only took a few minutes to pass overhead, and appeared to diminish from the effects of perspective to mere lines on the horizon. At the instant when the first band arrived, the wind--which was still blowing, and something more than gently, from the northeast--went round by north to southwest; at the same time a strong, cold blast fell from the leaden cloud, and continued to blow till both bands had passed.”

A whirlwind may be described as a mass of air whose height is enormously greater than its width, rotating rapidly round a more or less vertical axis.

A tornado is simply a whirlwind of exceptional violence; if it were to encounter a lake or the sea, it would be called a waterspout. Its most characteristic feature is a funnel, or spout, which is the visible manifestation of a cylinder of air that is revolving rapidly round a nearly vertical axis. This spout is propagated throughout the northern temperate zone in a northeasterly direction at a rate of about thirty miles an hour, and tears everything to pieces along its narrow path.

The diameter of the actual spout often does not exceed a few yards, and the total area of destructive wind is rarely more than three or four hundred yards across. The height of the spout is that of the lowest layer of clouds, which are then never high; and, as in thunderstorms, the upper currents are unaffected by the violent commotion below.

The spout as a whole has four distinct motions:

1. A motion of translation generally toward the northeast at a variable rate, but which may be taken to average thirty miles an hour.

2. A complex gyration. The horizontal portion of this rotation is always in a direction opposite to that of the hands of a watch--that is to say, in the same manner as an ordinary cyclone. But in addition to this there is a violent upward current in the centre of the cylinder of vapor or dust which constitutes the spout, and sometimes small clouds seem to dart down the outer sides of the funnel whenever these float in close proximity. There are, however, no authentic instances of any object being thrown to the ground by the individual effort of a downward current. The slight downward motion of a few small clouds is probably only a slight eddying of a violent uprush.

3. A swaying motion to and fro like a dangling whip, or an elephant’s trunk, though the general direction of the spout is always vertical.

4. A rising and falling motion, that is to say, that sometimes the end of the funnel rises from the surface of the ground and then descends again, and so on. Owing to this rise and fall, the general appearance of the tornado changes a good deal. When the bottom of the spout is some distance above the ground, the whole is somewhat pointed, and does comparatively little harm as it passes over any place. As the spout descends, a commotion commences on the surface of the ground. This latter gradually rises so as to meet the descending part of the spout, and then the whole takes the shape of an hour-glass. This is the most dangerous and destructive form, because the ground gets the whole force of the tornado.

The general appearance of the cloud over a tornado or whirlwind is always described as peculiarly smoky, or like the fumes of a burning haystack. The tornado is also never an isolated phenomenon; it is always associated with rain and electrical disturbance.

The destructive effects of the tornado are very curious, from the sharp and narrow belt to which the injury is confined. It appears that in the passage of some tornadoes wind-pressures of various amounts, from eighteen to a hundred and twelve pounds per square foot, have been demonstrated by destruction of bridges, brick buildings, etc. The upward pressures are sometimes as great as the horizontal, and even greater. Downward pressures or movements of wind have not been clearly proved. Upward velocities of 135 miles per hour seem not to be unusual, and horizontal velocities of eighty miles have been recorded with the anemometer. The destructive wind-velocities are confined to very small areas. A destruction of fences, trees, etc., is often visible over a path many miles long and a few hundred yards wide, but the path of greatest violence is very much narrower. The excessive cases above referred to are observed only in small isolated spots, less than a hundred feet square, unequally distributed along the middle of the track. Thus, in very large buildings, only a small part is subject to destructive winds. In different parts of this area of _maximum_ severity, the winds are simultaneously blowing in different, perhaps opposite, directions, the resultant tending not to overturn or carry off or crush in, but rather to twist round a vertical axis. Buildings are generally lifted and turned round before being torn to pieces. As the chances are very small that a building will be exposed to the violent twisting action, it is evidently the average velocity of rectilinear winds within the path of moderate destruction that it is most necessary to provide against in ordinary structures. These winds may attain a velocity of eighty miles an hour over an area of a thousand feet broad, and generally blow from the southwest; the next in frequency blow from the northwest. The time during which an object is exposed to the more destructive winds varies from six to sixty seconds. An exposed building experiences but one stroke, like the blow of a hammer, and the destruction is done. Hence, in a suspension-bridge, chimney, or other structure liable to be set into destructive rhythmic vibrations, the _maximum_ winds do not produce such vibrations. The duration of the heavy southwest or northwest winds over the area of moderate destruction is rarely over two minutes. The motion of translation of the central spout of a tornado, in which there is a strong vertical current, is, on ah average, at the rate of thirty miles an hour.

Tornadoes mostly occur on sultry days and either in the southeast or right front of cyclones, or in front of the trough of V-depressions.

The general character of all tornadoes is so similar that the description of one will do for all. We shall therefore give some of the description furnished by an eye-witness to the United States Signal Office, which is described in the reports as the “Delphos tornado”:

“On Friday morning, May 30, 1879, the weather was very pleasant, but warm, with the wind from the southeast, from which direction it had blown for several days. The ground was very dry, and no rain had fallen for a number of weeks. About 2 P. M. threatening clouds appeared very suddenly in the west (against the wind), attended in a few minutes by light rain, the wind still in the southeast It stopped in about five minutes, and then commenced again, wind still the same, accompanied by hail, which was thick and small at first, but rapidly grew less in quantity and larger in size, some stones measuring three and a half inches in diameter, and one was found weighing one-fourth of a pound. This last precipitation continued for about thirty minutes, after which a cloud in the shape of a waterspout was seen forming in the southwest, and moving rapidly forward to the northeast. The cloud from which the funnel depended, seen at a distance of eight miles, appeared to be in terrible commotion; in fact, while the hail was falling, a sort of tumbling in the clouds was noticed as they came up from the northwest and southwest, and about where they appeared to meet was the point from which the funnel was seen to descend. There was but one funnel at first, which was soon accompanied by several smaller ones, dangling down from the overhanging clouds like whiplashes, and for some minutes they were appearing and disappearing like fairies at a play. Finally one of them seemed to expand and extend downward more steadily than the others, resulting at length in what appeared to be their complete absorption. This funnel-shaped cloud now moved onward, growing in power and size, whirling rapidly from right to left, rising and descending, and swaying from side to side. When within a distance of three or four miles, its terrible roar could be heard, striking terror into the hearts of the bravest.” The eye-witness judged that the funnel itself would reach a height of about five hundred feet from the ground. As the storm crossed a river, a cone-shaped mass came up from the earth to meet it, carrying mud, débris, and a large volume of water. The cloud then passed the observer’s house very near to 4 P. M. The progressive velocity at the time was considered to be about thirty miles per hour, although at Delphos, three and a half miles distant, it had slackened down to near twenty miles. A few minutes previous to and during the passage of the funnel, the air was very oppressive; but ten minutes after the wind was so cold from the northwest that it became necessary to wear an overcoat when outside.

The actual diameter of this storm appears to have been only forty-three yards. On the right of the track, destructive winds extended to a further distance of from one to two miles, sensibly deflected winds for another mile and a half, beyond which only the usual wind of the day was experienced. On the left or northern side of the tornado path, the damage did not extend quite so far, for the width of the belt of destructive winds was not more than twenty-eight yards across and that of sensibly deflected winds one mile and a quarter.

As a specimen of the damage done a large two-horse sulky plow, weighing about seven hundred pounds, was carried a distance of twenty yards, breaking off one of the iron wheels attached to an iron axle one and three-quarter inches in diameter. A woman was carried to the northwest two hundred yards, lodged against a barbed-wire fence, and instantly killed. Her clothing was entirely stripped from her body, which was found covered with black mud, and her hair matted with it. A cat was found half a mile to the northwest of the house, in which she had been seen just before the storm, with every bone broken. Chickens were stripped of their feathers, and one was found three miles to the northwest.

A few miles further on, another eye-witness says, “the dark, inky, funnel-shaped cloud rapidly descended to the earth, which reaching, it destroyed everything within its grasp. Everything was taken up and carried round and round in the mighty whirl of the terrible monster. The surrounding clouds seemed to roll and tumble toward the vortex.

“The funnel, now extending from the earth upward to a great height, was black as ink, excepting the cloud near the top, which resembled smoke of a light color. Immediately after passing the town, there came a wave of hot air, like the wind blowing from a burning building. It lasted but a short time. Following this peculiar feature, there came a stiff gale from the northwest, cold and bleak, so much so that during the night frost occurred, and water in some low places was frozen.”

END OF VOLUME TWO

FOOTNOTES:

[1] The lakes of Sweden, which cover one-twelfth of the surface of the country, exercise an important influence on climate according as they are frozen or open.

[2] Another variety or species of seal inhabits Lake Baikal.

[3] Count von Helmersen, however, has stated his belief that for this extreme northern prolongation of the Aralo-Caspian Sea there is no evidence. The shells, on the presence of which over the Tundras the opinion was chiefly based, are, according to him, all fresh-water species, and there are no marine shells of living species to be met with in the plains at the foot of the Ural Mountains.

[4] Archbishop of Spalato and Primate of Dalmatia.

TRANSCRIBER’S NOTE

Obvious typographical errors and punctuation errors have been
corrected after careful comparison with other occurrences within
the text and consultation of external sources.

Some hyphens in words have been silently removed, some added,
when a predominant preference was found in the original book.

Except for those changes noted below, all misspellings in the text,
and inconsistent or archaic usage, have been retained.

Pg 470: ‘chiefly Brachipods of’ replaced by ‘chiefly Brachiopods of’.
Pg 472: ‘these same familes’ replaced by ‘these same families’.
Pg 483: ‘constituing links’ replaced by ‘constituting links’.
Pg 563: ‘Camaroons Mountains’ replaced by ‘Cameroon Mountains’.
Pg 563: ‘with Teneriffe in’ replaced by ‘with Tenerife in’.
Pg 569: ‘existing, denundation’ replaced by ‘existing, denudation’.
Pg 650: ‘their relativ size’ replaced by ‘their relative size’.
Pg 718: ‘incalulable ages’ replaced by ‘incalculable ages’.
Pg 722: ‘greatly diminshed’ replaced by ‘greatly diminished’.