Part 17

Again, the heating power of the Sun in each hemisphere is greatest at the period of the longest days, because of its greater altitude in the heavens; and least at the period of shortest days. Thus long days and a high sun operate together to produce the high temperature of summer; while long nights and a low sun cause the low temperature of winter.

The following table gives the length of the longest day, excluding the time of twilight, and of the shortest night, in the different latitudes, with the difference of duration in hours and minutes, thus exhibiting more clearly the above law.

TABLE OF UNEQUAL DAYS AND NIGHTS

=============+=============+================+============ =LATITUDE= |=Longest Day=|=Shortest Night=|=Difference= -------------+-------------+----------------+------------ Equator | 12.0 hours | 12.0 hours | 00.0 hours 10° | 12.7 „ | 11.3 „ | 1.4 „ 20° | 13.3 „ | 10.7 „ | 2.6 „ Tropics | 13.5 „ | 10.5 „ | 3.0 „ 30° | 14.0 „ | 10.0 „ | 4.0 „ 35° | 14.5 „ | 9.5 „ | 5.0 „ 40° | 15.0 „ | 9.0 „ | 6.0 „ 45° | 15.6 „ | 8.4 „ | 7.2 „ 50° | 16.3 „ | 7.7 „ | 8.6 „ 55° | 17.3 „ | 6.7 „ | 10.6 „ 60° | 18.7 „ | 5.3 „ | 13.4 „ Polar Circles| 24.0 „ | 0.0 „ | 24.0 „ 67-1/2° | 1 month | 0.0 „ | ... 69-1/2° | 2 months | 0.0 „ | ... 73.3° | 3 „ | 0.0 „ | ... 78.3° | 4 „ | 0.0 „ | ... 84° | 5 „ | 0.0 „ | ... North Pole | 6 „ | 0.0 „ | ... -------------+-------------+----------------+------------

LAW OF VARIATION OF DAY AND NIGHT

The inequality of day and night increases slowly in the tropical regions, but more and more rapidly towards the polar circles. Beyond these circles the Sun, in the hemisphere in which it is vertical, makes the entire circuit of the heavens, without sinking below the horizon, for a period varying from twenty-four hours to six months; while in the opposite hemisphere there is a corresponding period of continuous night.

RESULT OF THIS LAW IN DIFFERENT ZONES

In the tropical regions, where the days and nights vary little in length, the temperature is nearly uniform throughout the year; while the increasing inequality of day and night towards the Poles, causes an increasing difference between the summer and the winter temperature.

Again, the length of the day, in the summer of high latitudes, compensates for the diminished intensity of the Sun’s influence; so that the temperature, in the hottest part of the day, may equal, or even exceed, that within the tropics. A summer day in Labrador or Petrograd may be as warm as one under the Equator; but in the former latitudes there are only a few days of extreme heat in the year, while with increasing nearness to the Equator the number of warm days constantly increases.

HOW THE SEASONS VARY IN DIFFERENT LATITUDES

The high latitudes have short, hot summers, and long, severe winters. The transition seasons, spring and autumn, on account of the very rapid change in the length of the days, are short and scarcely perceptible.

In the middle latitudes the summer and winter are more nearly equal in length, with less difference in the extreme temperatures; and the transition seasons are distinctly marked. Farther towards the Equator the summer increases in length, and the winter diminishes, while the tropical latitudes have constant summer.

WINDS AND OTHER AIR CURRENTS

The winds appear to be caused by partial changes in the density of the atmosphere in a great measure arising from a diverse distribution of heat. When air is warmed it becomes less dense, or, in other words, it occupies a greater space. If an adjacent stratum of air be cooler, it will on coming in contact with the warmer air expand and pour into space occupied by the latter, thus forming a current. The greater the difference between the temperature of the one or other portion, the greater will be the force which the cold portion will rush into the space occupied by the warm portion, or, in other terms, the more violent will be the wind. In temperate climates the winds are variable; but in some parts of the world they blow with great regularity, and in others are subject to periodical changes.

WHAT CAUSES THE TRADE-WINDS

The most remarkable of the regular winds are the trade-winds. The atmosphere at the surface between the tropics is much warmer than in the higher latitudes; and since air expands when heated, the light warm air of intertropical regions perpetually rises, and its place is as perpetually supplied by the colder air from the north and the south. If it were not for the Earth’s rotation, these would be merely north and south winds; but like the equinoctial water-currents, these cool currents of air coming from regions which have not an equal velocity of rotation with the air at the equator, pause and hang back, and thus these aerial currents acquire a westerly direction, forming north-easterly constant winds in the northern hemisphere, and south-easterly in the southern hemisphere.

MONSOONS AND THEIR LOCATION

The monsoons or periodical winds of the Indian Ocean owe their origin to the same cause which gives rise to the trade-winds, though they acquire a different character in consequence of the proximity of the land. In the southern portions of the ocean which are remote from this cause of disturbance, the trade-wind blows with its wonted regularity; but in the seas occupying the region between the eastern coast of Africa on the one side, and the Malay peninsula and the island of Sumatra on the other, the course of the trade-wind is reversed for half the year. This change occurs from April to October; the sun at that period being vertical north of the equator, and the land in the adjacent regions acquiring in consequence a high temperature, and the air over the sea being cooler than that over the land, a south-west wind prevails. This wind, called the “south-west monsoon,” commences at about three degrees south of the equator, and passing over the ocean arrives charged with moisture, and accordingly usually deposits copious supplies of rain in India and some of the adjoining territories. In the remaining half of the year, or from October to April, the wind assumes the ordinary north-easterly direction of the trade-wind.

Sea-breezes, which occur in regions bordering on the ocean in hot climates, are produced by causes similar to those which give rise to the south-west monsoon, but on a more limited scale of action, and changing their direction daily.

THE WHIRL OF THE HURRICANE

Hurricanes are storms of wind which sweep or whirl round a regular course, and are at the same time carried onward along the surface of the Earth. In the northern hemisphere the whirling motion follows the course of east, north, west, and south to east again, and in the southern hemisphere it takes the opposite course. In the Atlantic Ocean, the principal region of hurricanes lies to the eastward of the West India Islands. They are also frequent in the Indian Ocean, at no great distance from the island of Madagascar. The “typhoons” of the China seas, and the “ox-eye” of the Cape of Good Hope, are also revolving storms.

TORNADOES AND OTHER CHARACTERISTIC STORMS

The tornadoes of the western coast of Africa, the pamperos of South America, and the northers of North America appear to be of a different character, and not to possess a revolving motion. The sirocco of Italy and Sicily, and the solano of Spain, as also the simoon of Arabia, and the harmattan of western Africa, are all winds which owe their origin to the heated surfaces of Africa and Arabia. The principal difference between these winds appears to be, that the sirocco and the solano acquire some moisture in their passage across the Mediterranean, and therefore do not possess that extreme degree of aridity which forms the distinguishing character of the simoon and the harmattan.

CLOUDS--THEIR FORM AND CLASSIFICATION

Clouds are continually varying in their form and appearance, but may be classed under the four principal heads of the cirrus, the cumulus, the stratus, and the nimbus.

The cirrus is a light, fleecy cloud resembling a lock of hair or a feather.

The cumulus or summer cloud is generally massive and of a round form; sometimes of small size, and sometimes covering nearly the whole sky, and occasionally appearing in the horizon like mountains capped with snow.

The stratus is a horizontal, misty cloud sometimes observed on fine summer evenings comparatively near the ground, and often crossing the middle regions of mountainous or hilly districts.

The nimbus or rain cloud has a uniform gray tint; it is fringed at the edges when these are displayed, but usually covers the whole sky. The region of clouds is a zone extending in the atmosphere from about one to four miles above the Earth. The most elevated clouds, which are light and fleecy, are those comprehended under the name of cirrus, and the lowest are those which are called stratus.

The cirro-cumulus, cirro-stratus, and cumulo-stratus are only modifications and combinations of the above-named principal classes.

FORMS OF ATMOSPHERIC VAPOR

Warm air is capable of holding suspended a larger quantity of moisture than cold air, and therefore the amount of vapor present in the atmosphere is subject to great variations.

WHAT CAUSES DEW

These facts also account for the formation of dew, which is caused by the reduction of the temperature and the deposition of the moisture which the warmer atmosphere of the day had held in suspension. Dews will hence be usually most abundant when cool nights succeed warm days, and on a clear night than when the skies are obscured by clouds, because a cloudless sky is usually much colder than a beclouded one. It is also essential for the copious formation of dew, that the ground or other substance on which it is deposited should be much cooler than the superincumbent air; for if the ground be warm it will impart its temperature to the air near its surface and dew will not be formed.

FORMATION OF MISTS AND FOGS

When the ground or water is warmer than the air, mists and fogs are frequently formed; and since water and marshy surfaces cool less rapidly than dry land, mists and fogs are of more common occurrence in low, damp situations than in dry, elevated districts. They are formed by the condensation of the vapor, or, in other terms, its transformation into the minute globules of water, which instead of descending to the earth in the form of dew, remain suspended above the land or the water.

RAIN, HAIL AND SNOW

Clouds are formed by the condensation of vapor at considerable but various elevations in the atmosphere. Vapor is always invisible, clouds, therefore, are not vapor but water, and consist of a fine watery powder, the size of each particle being exceedingly minute; and consequently they are so light that clouds formed of an accumulation of such particles are readily borne forward by the winds. Clouds are sometimes suddenly formed and as suddenly disappear, probably owing to sudden and partial changes of temperature. When a considerable difference of temperature prevails in the aerial currents which may come in contact with the local atmosphere, a further condensation takes place, and the particles of this fine watery powder unite into drops, and, becoming heavier, fall to the earth in the form of _rain_, _hail_ or _snow_.

SNOW AND SNOW-CRYSTALS

Vapor condensed in air having a temperature below thirty-two degrees Fahrenheit freezes, or passes to a crystalline form, producing snow. Snowflakes occur in a great variety of forms, which usually present the outline of either a regular hexagon or a six-pointed star.

Their size depends upon the temperature and the relative humidity of the air through which they fall, for, like raindrops, they increase by successive additions from the vapors with which they come in contact in descending. Thus in mild weather they are much larger than in very cold weather.

When the lower air is warm enough partially to melt the crystals, they form minute balls. When raindrops, formed in the upper air, fall through a cold current, they are often frozen, producing _sleet_ instead of snow.

WHERE PERMANENT SNOW EXISTS

Though the winter snows upon the plains, and the slopes of mountains of medium height, disappear during the warm season; yet, in all latitudes, the tops of high mountains are covered with a layer of permanent snow, which the summer heat of these great altitudes is not sufficient to melt.

The lower limit of perpetual snow, called the snow line, is found, within the tropics, about three miles above the level of the sea. In temperate latitudes it occurs at the height of a little less than two miles; and at the northern limit of the continents, it is about half a mile above the level of the sea, or, perhaps, even less than this.

On the Arctic Islands, vast fields of snow remain permanently, at a few hundred feet above the sea level.

The winter snows, falling into the icy waters of the polar oceans, are but partially dissolved; and, remaining upon the freezing surface, they help to form those vast ice floes which encumber the polar seas at all times.

The following table gives the observed height of the snow line in the different latitudes:--

HEIGHT OF THE SNOW LINE

==========+============================+======= =Lat. N.= | =New World= |=Feet= ----------+----------------------------+------- 75° |North Greenland | 2,300 54° |Unalaska | 3,500 48° |Mt. Baker, Oregon, about | 8,000 43° |Rocky Mountains |12,500 39° |Rocky Mountains |14,500 38° |Sierra Nevada |11,000 19° |Popocatepetl, Mexico |14,900 5° |Tolima, Columbia |15,300 Lat. S. 1°|Andes of Ecuador |15,800 17° |Andes of Bolivia, west side |18,500 17° |Andes of Bolivia, east side |15,700 33° |Andes of central Chili |14,700 42° |Andes of Patagonia | 6,000 54° |Andes of Straits of Magellan| 3,700 75° |Bear Island | 600 71° |Mageroe, Cape North | 2,300 67° |Sulitelma, Lapland | 3,800 61° |Scandinavian Alps | 5,300 50° |Altai Mountains | 7,000 46° |Alps, north side | 8,800 46° |Alps, south side | 9,200 43° |Caucasus |11,000 35° |Hindu Kush |13,000 31° |Himalaya, south side |16,200 31° |Himalaya, north side |17,400 12° |Abyssinian Mountains |14,000 Lat. S. 3°|Kilimanjaro |16,000 44° |New Zealand Alps | 7,500 ----------+----------------------------+-------

HOW SNOW AND ICE FORM GLACIERS AND ICEBERGS

Glaciers (from the French glace, ice) are vast streams of ice which descend from the lower edge of the perpetual snows, like long icicles from a snow-covered roof. They follow the windings of the Alpine valleys, and terminate abruptly in a massive wall of ice, from beneath which the waters of the melting glacier escape, through a large icy vault.

MOST FAMOUS GLACIER REGION

The mountain systems in the middle latitudes, with abundant snows and alternate warm and cold seasons, are most favorable to the formation of glaciers. The best known, and probably the most remarkable glaciers are those of the high Alps, in the heart of which are Mont Blanc, Monte Rosa, and the Bernese Alps. Late explorers have found large glaciers in the Caucasus and in the Himalayas, the last being of the grandest proportions. In the Scandinavia are many which descend, in the deep western fiords, nearly to the sea level.

In the New World glaciers are less frequent. On Mount Shasta and Mount Rainier fine examples are in evidence.

By far the most extensive glaciers however, are found on the snow-covered islands of the polar oceans.

Vast masses of ice, broken from the ends of these glaciers, form the enormous _icebergs_ (mountains of ice) which are so numerous in the polar seas, and are transported by the currents even to middle latitudes.

CLIMATE AND WEATHER

The term _climate_ is used to express the combination of temperature and moisture which prevails at any particular place, or, in more familiar terms, the prevailing _weather_.

The most prominent causes of diversity of climate are the heat of the sun, the respective position of land and water, and the elevation of land above the level of the sea. To these may be added, as producing considerable though less marked effects, the nature of the soil, the prevailing winds, the position of mountain ranges, and the currents of the ocean.

THE SUPREME INFLUENCE OF THE SUN

The sun is the grand agent in diffusing heat over the earth’s surface. While the sun is above the horizon of any place, that place is receiving heat; and when the sun is below the horizon, it is parting with it by the process called “radiation.” Whenever therefore the sun remains more than twelve hours out of the twenty-four above the horizon of any place, and consequently less than twelve hours below, the general temperature of that place will be above average; and when the reverse occurs, it will be below average. If the temperature depended solely on the heat of the sun, then indeed a tolerably accurate view of the respective climates of the zones of the globe might easily be assumed; but it is so greatly modified by other circumstances, that considerable differences prevail in countries situated in the same parallels of latitude.

HOW AFFECTED BY POSITION OF LAND AND WATER

The relative position of the land and water is an essential cause of this diversity. The waters of the ocean are of very equal temperature, and have a tendency to moderate both heat and cold, wherever their influence extends. Thus when a cold wind passes over the sea, it becomes warmed, while a hot wind becomes cooled; and thus islands generally experience milder winters and more temperate summers than continents. Such countries are said to possess an insular climate. But when any region experiences great severity of cold in winter and a high degree of heat in summer, it is said to possess an extreme or excessive climate. The most striking instances of an extreme climate are drawn from places like Yakutsk, situated in the depths of Siberia, where the difference between the average temperature of winter and summer amounts to the astonishing sum of 101 degrees Fahrenheit.

INFLUENCE OF ELEVATIONS

A gradual decrease in temperature takes place in the ascent from the sea to the line of perpetual snow. This line, which is called the snow-line, varies in different latitudes, and sometimes, owing to local causes, differs on the same latitude; as a general rule, however, a gradual decrease in elevation of the snow-line takes place as we recede from the equator north and south. The height of this line within the tropics varies from 16,000 to 17,000 feet above the level of the sea, and in the northern hemisphere meets the level at about the eightieth parallel.

MODIFICATIONS BY PREVAILING WINDS, MOUNTAINS AND OCEAN CURRENTS

Countries where the prevailing winds sweep across a wide expanse of ocean are not subject to extremes of heat and cold. Thus the climate of oceanic islands is always moderate, and the climates of all coasts are more equable than in the interior of continents.

Climate is also modified greatly by the position of mountain ranges, especially when ridges extend east and west, screening it from the north or leaving it exposed unsheltered in that direction.

Thus the Carpathians screen Hungary from the cold blasts of the north; while Poland, to the north of that range, and therefore unprotected from those piercing winds, suffers from a very cold and humid atmosphere.

The currents of the ocean are likewise potent agents in the formation of climates, and render places which would otherwise be uninhabitable, fit for man’s habitation. Thus the Polar currents coming to the equatorial regions cool, and the Gulf Stream making its way to Polar regions warms, otherwise extreme temperatures.

RAINLESS AND RAINY REGIONS OF THE EARTH

In some parts of the Earth extensive tracts exist where rain is never known to fall, and if at all only at intervals, and then in small quantities. The rainless districts of the New World include the flat territories of northern Chili and Peru, some parts of Mexico, and some parts of California. In the Old World an extensive rainless band extends from the western shores of Africa to the central regions of Asia, including the Great Sahara Desert, Egypt, part of Arabia, and the Desert of Gobi. Countries so circumstanced, unless like Egypt rendered fertile by the irrigation of a great river, constitute the most arid and desolate regions of the earth.

The quantity of rain which falls in any region depends greatly on local causes, such as the variations of the surface, the prevailing winds or the proximity of the ocean. Rain is usually more copiously deposited in mountains and well-wooded islands than in any other description of surface.

In tropical regions the rains follow the sun, i. e., when the sun is north of the equator, the rains prevail in the northern tropic, and when south of that line in the southern tropic. This forms the rainy and dry seasons to which countries so situated are subject. This does not, however, apply to the whole intertropical regions, for in a zone extending from the fifth to the tenth parallels on each side of the equator there are two rainy and two dry seasons.