Читать книгу Great Disasters and Horrors in the World's History - Allen Howard Godbey - Страница 6

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“Earth has each year her resurrection hours

When the spring stirs within her, and the powers

Of life revive; the sleeping zephyrs rouse,

The blushing orchards clothe their naked boughs,

The swallow skims above the lakelet’s verge.

Swift summer speeds with fire in every vein,

And autumn’s glories crimson hill and plain.

Then warmth and life from Nature take their flight,

And winter robes her in a shroud of white,

While mournful Boreas chants her funeral dirge.”

O the seasons tread their ceaseless round in the temperate zones, and to a certain degree in the colder regions of the earth. But when we examine the change of seasons in the tropical world, we find a state of things so different that we are at once led to inquire the reason: and it will be found primarily in certain periodical winds.

When the sun is north of the equator: that is, while our northern summer is in progress, India enjoys a steady sea wind from the southwest, which brings a rainy season to the corresponding coasts of Hindostan and Farther India. When the sun returns to the south, the winds set in from the opposite direction, coming down across the great upland plateau of Central Asia, sometimes called, from its immense height and extent, the “Roof of the World.” These periodic winds are called monsoons: a corruption of the Arabic word Moussin, season. They are in reality a modification of the trade-winds.

A glance at a map will show that the northern half of the great Indian Ocean is enclosed by land masses as no other large body of water is. Consequently, while in the southern section the southeast trade is present, the northeast trade of the northern part is so modified by the surrounding land areas as to almost entirely lose its distinctive character. Hence, most tropical regions have, properly speaking, but two seasons: the rainy, and the dry. As the clouds swept in meet with an intensely heated region, the trade never chills them sufficiently to produce snow, except in extremely elevated regions.

This is the direct cause of the monsoons: During the northern summer, southern Asia, being under the rays of the vertical sun, becomes intensely heated; and the cooler and denser air of the adjacent ocean, and of southern Africa, flows towards it, producing the southwest monsoon, which lasts from April or May to September or October. The time of its beginning and its close varies in different latitudes, according to the time at which the sun is vertical in each.

During the southern summer, southern Africa being under the vertical sun and intensely heated, the cooler air of the surrounding seas, and of southern Asia, flows towards it. This produces the northeast monsoon, which lasts from October or November to April. This monsoon is, in fact, only the regular northeast trade-wind somewhat intensified.

A similar exchange takes place between Asia and Australia, but it is less marked, owing, perhaps, to the great islands lying between these continents.

The period of transition of the monsoons, in spring and autumn, is marked by sudden and violent gales, and terrific thunder storms. Destructive hurricanes, also, are of frequent occurrence. This corresponds with the period of equinoctial storms in higher latitudes.

There are narrow monsoon belts in the Atlantic, along the coast of Africa and of Brazil, also on the Pacific coasts of North and South America; but the phenomena they exhibit are of a much less striking character. On the African coast, in general, the winds blow from sea to land in summer, from land to sea in winter; on the Brazilian, the wind is from the northeast in summer, while in winter the southeast trade resumes its sway. The monsoons of the Pacific coast of America blow from the northwest and north during the southern summer; from the southwest and south during the northern. The regular trade-wind makes itself so strongly felt in northern Brazil, which is unusually level, that a boat can sail almost as rapidly up the swift current of the Amazon as it can row down: and Humboldt records that he found it of great strength at the foot of the eastern slope of the Andes.

Another modification of the northeast trade is found in the Etesian winds of Greece and the adjacent archipelago. This is a true intermittent trade, blowing only in the daytime, however, and lasting from July to September. The cool air of the peninsula rushes toward the extremely heated regions of the Mediterranean and north Africa.

Somewhat similar are the northers, or blizzards, of our Western States. By the laws already given, it is seen that northerly winds can prevail in any region only when some region further south is unusually heated. Now, the northern portion of America may be roughly compared to a trough. The cold polar current sets to the southward across the continent, and is turned to the east by the Rocky Mountain range, giving it a general southeast course. Hence, when the southern summer is in progress, our prevailing winds are from the northwest; and when the heated portion of the world is north of the equator, we have the return trade, giving us as our prevailing wind that from the southwest. When our return trade is unusually prolonged, we have a late fall; and if the southern summer is unusually warm, we have the polar current longer than usual, and a late spring in consequence. The polar current seldom makes its presence felt beyond the Texan plains; though occasionally it reaches the Mexican plateau, or sweeps across the Gulf to the Antilles.

A similar cold wind from Central France toward the Riviera is locally known as the Mistral. The cold winds from the south, which in crossing the plains of Patagonia, are turned eastward by the Andes, are called in Uruguay the pamperos, as their direction causes the popular belief that they originate in the pampas, or grassy plains. In Malta the cold wind becomes known as the gregale—in the Adriatic sea it is the tramontana; in Trieste and Dalmatia it is the bora. In New Zealand the corresponding cold blast comes from the south, and is known as the buster. When loaded with drifting snow, as in the blizzard of the United States, the cold wind of the Yenisei Valley, in Asia, is locally called the purga; in the steppes of Central Asia it is the bura.

Eastern Asia receives its prevailing cold current from the northwest; while western Asia and Europe receive their cold wave from the northeast, there being no range of mountains, as in America, to deflect the current, as the polar currents are disposed to follow the continents, having their origin in arctic lands; while for a similar reason the return trades reach their extremes on the ocean. Hence, lines drawn through the places which possess the same mean annual temperature reach a higher latitude at sea than on land.

These are the chief periodical winds of long periods. There is one other class to be noted: the diurnal land and sea breezes. These occur along all coasts, whether in the zone of trades or of variable winds; but the phenomenon is more strongly marked in the tropical regions, and in the summer of the temperate latitudes, because of the greater difference in the temperature of land and sea by day and by night.

During the hottest part of the day the air over the land frequently reaches a temperature of 100° Fahr., and even more, while that over the sea rarely rises above 80°. During the night the land radiates its heat with such rapidity that, towards morning, its atmosphere may be from 10° to 15° colder than that of the sea.

Soon after sunrise, the land being warmer than the sea, a sea breeze sets in, which increases in force until about three o’clock, when the difference of temperature is greatest. It then gradually diminishes until about sunset, when, the temperature of the land and sea having become equal, the atmosphere is at rest, the calm continuing for an hour or more.

Soon the land becomes cooler than the sea, and a gentle breeze from the former sets in. It increases in force as the night advances, becoming strongest a little before morning, when the temperature of the land is lowest; after which it rapidly dies away, and is succeeded by a calm, to be soon replaced by the sea breeze.

One other species of variable wind is to be noticed: the hot, dry, dust-laden blast from desert regions. Such occur more or less periodically, and are known by different names in different localities.

Tom Moore has told us that “love’s witchery” on the heart is

“Like the wind of the south o’er the summer lute blowing,

That hushed all its music, and withered its frame.”

The reference is to the simoom of Syria and Arabia. One who has not experienced this wind can have little idea of its oppressiveness. Apt to come at any hour during the hottest months of the year, with a temperature so great that a piece of silver exposed to it becomes hot enough to blister the flesh, and laden with the impalpable dust of the desert, vegetation is scorched and withered by it, and animals flee from it as from the pestilence. It may last but a short time: it may endure several days.

At the first indication of its approach, people flee to their houses; doors and windows are shut and every crevice that could allow any dust to enter is tightly stuffed: while the wind lasts no one ventures out. Such unfortunate animals as happen to be overtaken by it have literally to struggle for their lives. The wind is not steady, but comes in fitful gusts, sometimes differing as much as 20° in temperature. The streets are deserted; and were they otherwise, a person could hardly be seen at a few yards distance. Hours pass: that implacable enemy, the dust, sifts in at unknown chinks. By degrees it covers everything. Valuable lace and tapestry are nearly ruined. You put on a skull-cap; yet it penetrates your hair. It finds its way beneath the garments to the skin, producing distressing dryness and roughness. The lips parch and crack. The eyes are red and inflamed. You drink as if famished, and gasp for breath. You are excessively irritable; you reach the verge of complete nervous prostration. At length the ordeal is over. You creep into the street, to find your neighbors looking like corpses; some, it may be, actually dead from nervous exhaustion. Dead birds and animals lie on the earth. It is a case of the survival of the fittest. You pluck a leaf from a neighboring tree; it crumbles to dust in your grasp.

Such are the effects of an unusually protracted wind, even when most favorably situated to encounter it. But if a caravan be overtaken by such in the desert, happy are they who escape. The camels kneel and thrust their

THE SIMOOM.

noses into the sand, against each other, into a pack of goods—anywhere to avoid breathing that poisonous blast. The men throw themselves upon the ground behind the camels, and muffle their heads in their garments. The storm is at hand; perchance attended by whirling columns of sand. You raise your head: a thick, dun-colored cloud flies at you; a heat as of red-hot iron, it seems, holds you in its choking grasp. You find your way to your water bottle, and drink deeply. The lurid sun turns the sweeping columns of sand to pillars of fire. Superstitious fear seizes your Arab comrades. Gradually the storm passes on: the men pick themselves up and endeavor to shake the irritating dust and sand from out the folds of their clothing, and the party resumes its way, happy that they are not numbered among the dead whose bones are bleaching by the way. Tales are not wanting of great caravans completely overwhelmed by the sandstorms of the desert.

These storms are met with in their greatest severity in Egypt and Arabia. In Egypt, this wind is called the Khamsin, or fifty, referring to the period of fifty days—the latter part of April, May, and early June—when they may be expected. They never blow through the entire season: rarely so long as fifteen days at a time. In Arabia the simoom may travel from the center of the peninsula toward any point of the compass; the Khamsin of Egypt blows from the southwest. Winds of the same character cross the Mediterranean. In Spain the wind is known as the Solano, or Levanter, or Leveche: in Sicily and Italy it is the Sirocco. The distressing dryness is somewhat modified by the journey across the Mediterranean. The same wind in Syria is called Samiel; and a similar wind which blows from the Sahara southwest to the Guinea coast is called the Harmattan. In California a similar dry hot wind blows from the interior toward the coast, during the hot season, and is called the desert wind. Such occasional hot blasts are experienced in southeastern Dakota, coming from the “bad lands,” or sandy and rocky wastes along the upper Missouri river.

All these periodical or varying winds may be very properly, from their time and character, be called the season winds of the earth, as another means of distinction from the constant trades: as they in part bring changes of season, and in part are brought that way.

Into the question of climate and seasons one other element enters, of especial importance in regard to those disturbances of the regular winds, which we call storms. That factor is the quantity of moisture in the atmosphere, and the consequent rainfall or snowfall of a region. Without this element, the phenomenal disturbances known as tornadoes would hardly occur: or if they did, there would be greater difficulty in ascertaining their approach.

Water, in its vapor state, is but three-fifths the weight of the air, and in consequence rapidly rises. This evaporation, as it is called, goes on at all times: even when the water is frozen. A very thin sheet of ice, hung in the open air, will finally disappear, even though the temperature be always below freezing.

Now, all the phenomena of rain, snow, and hail, that are brought by different seasons, in different climes, depend upon a single simple law: that warm air can hold a much greater quantity of vapor than cold air. The amount of moisture that may be held in suspension at different temperatures is as follows:

Temperature of Air.	Weight of vapor in a cubic foot of saturated air.	Temperature of Air.	Weight of vapor in a cubic foot of saturated air.
20	deg.	Fahr.	1.30	grains	Troy.	70	deg.	Fahr.	8.00 grains	Troy.
32	“	“	2.13	“	“	80	“	“	10.95	“	“
50	“	“	4.09	“	“	90	“	“	14.81	“	“
62	“	“	6.15	“	“	100	“	“	19.79	“	“

This gives a second reason why storms of wind and rain closely follow extremely hot weather.

Now, as the vapor is so much lighter than the air, their mixture must also be lighter. So any unusual amount of moisture is at once detected by the barometer, an instrument for measuring the pressure of the atmosphere. If the air grow moister, and therefore lighter, the barometer falls; a storm is approaching.

Since cold air can retain but little moisture, if a warm moist current be chilled, it must lose a part of its vapor, which at once falls to the earth as rain. If the cold be somewhat greater, the moisture is crystallized into snow. Greely’s observations at Fort Conger show that, varied as are the forms of snow crystals, those that fall during any particular storm are invariably of the same types, even though they may be collected from localities widely removed from each other. All crystals of snow are hexagonal in plan, but there is much variety in detail. The laws that produce one variety at one time, and a second at another, are not yet known.

The subject of hail is a peculiarly perplexing one to the meteorologist. Hailstones are more or less spherical in form, and are made of alternate layers of soft opaque ice, and hard clear ice. It is evident that they must acquire this structure by being whirled about between clouds of different temperature and density. Some have supposed that they are formed in a whirlwind, whose axis is horizontal, but for the present we must be content with Lord Dundreary’s explanation, for “it ith one of thothe thingth which no fellah can underthtand.”

Raindrops from a great height are larger than those from below, for they increase as they pass through the vapor-masses. As the warmest currents are also the highest, it will at once be understood why warm and tropical rains fall in large drops, while drizzling rains, mists, and fogs are characteristic of cold regions and cold seasons.

The masses of more or less condensed vapor in the upper air currents are what are known as clouds. Their various forms and appearance are shown in the cut on the—— page.

The cirrus and cirro-cumulus clouds are the highest, are mostly in the altitudes of perpetual frost, and are supposed often to consist of minute ice crystals. In temperate latitudes they are usually formed in, and move with, the upper air current, or return-trade from the tropical regions.

The cumulus clouds are characteristic of the tropics, and of the summer days in middle latitudes, their height depending upon the relative humidity of the air. They are formed by local ascending currents, which carry a large amount of vapor into the cooler upper air. There the vapors are condensed, and are gradually heaped up into those heavy masses of sharply defined clouds, which look like vast snowy mountains. Their base is horizontal, and marks the height at which the dew point is reached and condensation begins.

The accumulation of vapors is often so great that these clouds form a column several thousand feet high. In this case the difference in the temperature and the electrical conditions of the upper and lower portions is such that electrical discharges take place, accompanied by condensation of a portion of the cloud, forming a thunderstorm.

Stratus clouds are most frequently seen in the morning or evening, and are always low. They are formed by the descent of the higher clouds and vapors of midday into the lower air as the temperature decreases. They are more frequent in winter and summer than in the intermediate seasons.

The nimbus cloud is more dense and heavy than the others, which may all be transformed into the nimbus by