The Atmosphere: Its Extent, Character, Use and Effect
The Earth is enveloped in its own atmosphere, which like a transparent covering surrounds it, and revolves with it. This atmosphere does not extend to more than forty or fifty miles above the earth’s surface, and is higher at the equator than at the poles.
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What the Atmosphere Is Composed Of
The atmosphere is an elastic fluid consisting of a mixture (not a compound) of oxygen and nitrogen, in the proportions of about twenty-one of the first to seventy-nine parts of the last named. It also contains a small quantity of carbonic acid gas, and a yet smaller quantity of ammonia; and water in the form of invisible vapor is always present in it, though the quantity is subject to great variations. All these substances move freely among each other, and are continually changing places: the oxygen being ever ready to perform the office assigned to it of sustaining life and combustion; the carbonic acid to promote the growth of vegetation; the nitrogen to perfect the fruits of the earth, and the vapor to descend to the thirsty ground, in the form of showers and dew.
The atmosphere is elastic, and therefore capable of expansion and compression; and is also a ponderable body. The consequence of these properties is, that it is much lighter and thinner in the upper regions than nearer the earth’s surface; for at the sea-level its whole weight presses on its lower strata and gives it greater density. Ascending from the earth’s surface it becomes gradually lighter and thinner, and at great elevations is so rarefied as to be unsusceptible of sustaining life.
How the Atmosphere Is Weighed and Measured
The weight of the atmosphere at the level of the sea is equal to about fourteen and one-half pounds on every square inch of surface. This weight is balanced by a column of mercury thirty inches in height; but at an elevation of 18,000 feet it would be balanced by a column of only fifteen inches in height, and at 36,000 by one only seven and one-half inches in height. It is on this principle that the mercurial barometer has been constructed; and since the mercury in the barometer stands at the same point at all places at the sea-level, and falls in a regular ratio on ascending therefrom, this instrument forms a most useful standard for measuring altitudes.
As we ascend from the sea the atmosphere becomes colder; but, as with the density, the temperature does not appear to pass through regular gradations of change. From experiment, however, it has been assumed that the atmosphere loses one degree of heat by Fahrenheit’s thermometer for every 350 feet of ascent; and hence even in the hotter regions very lofty mountains are covered with perpetual ice and snow.
Distribution of Temperature over the Earth
The amount of heat produced by the sun upon the Earth’s surface, is greatest near the Equator, and diminishes gradually towards the Poles. Three general causes, each referable to the spherical form of the Earth, combine to produce the gradual diminution of temperature from the Equator to the Poles.
1. The angle at which the Sun’s rays strike the surface. In the Equatorial regions they are perpendicular to the surface of the sphere, and there produce their maximum effect; but, on account of the curved outline of the globe, they fall more and more obliquely with increasing latitude, and the intensity of action diminishes proportionately. At the Poles their effect is practically nothing.
2. The area on which a given amount of heating power is expended, is least at the Equator, consequently the resulting heat is greatest. The area covered increases, and the effect diminishes, with the increasing obliquity of the Sun’s rays in higher latitudes, which, as we have seen above, results from the spherical form of the Earth.
3. The absorption of heat by the atmosphere, as the Sun’s rays pass through it, is least where they fall perpendicularly,—that is, in the Equatorial regions,—and increases, with their increasing obliquity, towards the Poles.
Effect of the Motions of the Earth
The Earth revolves constantly around the Sun, and at the same time rotates upon an axis inclined twenty-three and one-half degrees towards the plane of its orbit. In consequence of the inclination of the axis, the declination of the Sun, or its angular distance from the Equator, varies with the advance of the Earth in its orbit, causing periodical variations in the length of day and night, and, consequently, in temperature.
Vernal Equinox. On the twentieth of March, at mid-day, the Sun is vertical at the Equator. Rising directly in the east it ascends the heavens to the zenith, and, descending, sets directly in the west.
The illuminated hemisphere extends from pole to pole, and embraces half of every parallel of latitude; hence every point on the Earth’s surface is under the rays of the Sun during half of the diurnal rotation; the days and nights are equal all over the globe; and the heating power of the Sun is the same in both the northern and the southern hemisphere.
Summer Solstice. As the Earth advances in its orbit the vertical Sun declines northward; and on the twenty-first of June, at the Summer Solstice, it is over the northern Tropic, twenty-three and one-half degrees from the Equator.
The illuminated hemisphere, extending ninety degrees on each side of the parallel of the vertical Sun, reaches twenty-three and one-half degrees beyond the North Pole; but, at the south, it barely touches the Antarctic circle. It embraces more than half of each parallel north of the Equator, hence throughout the northern hemisphere the day is longer than the night, the difference in their duration increasing with the latitude; and all points within the Arctic circle are in the light during the entire rotation.
In the southern hemisphere, less than half of each parallel being illuminated, the night is longer than the day, and within the Antarctic circle there is constant night. The heating power of the Sun is now at the maximum in the northern hemisphere, while in the southern it is at the minimum.
Autumnal Equinox. On the twenty-second of September, the distribution of light and heat upon the two hemispheres is the same as at the Vernal, and at the Winter Solstice, on the twenty-second of December, it is the reverse of that at the Summer Solstice.
WHAT CAUSES THE SEASONS AND DAY AND NIGHT
FIGURE ILLUSTRATING THE CHANGE OF SEASONS THROUGHOUT THE YEAR
The change of seasons is caused by the revolution of the earth around the sun, and the inclinations of the planes of the equator and ecliptic. These causes also account for the difference in the length of the days and nights and the difference in the height of the midday sun. The exact duration of the seasons we get by observing the dates of equinoxes and solstices.
FIGURE SHOWING THE CAUSE OF DAY AND NIGHT
The revolution of the earth gives us the length of the year; its rotation on its axis, the length of the day and night, by causing the risings and settings and daily apparent motion of the sun and stars.
Effect of Unequal Days and Nights on Temperature
The inequality in the length of the days in different parts of the year, occasioned by the inclination of the Earth’s axis, is of itself sufficient to produce a marked variation in temperature.
During the day the Earth receives from the Sun more heat than it radiates into space; while during the night it radiates more than it receives. Hence a succession of long days and short nights results in an accumulation of heat, raising the average temperature and producing summer; while long nights and short days result in a temperature below the average, producing winter.
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 | „ |
| 671⁄2° | 1 | month | 0 | .0 | „ | ... | |||
| 691⁄2° | 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.
PICTORIAL CHART OF THE CLOUDS, SHOWING THEIR FORMS AND POSITION
Clouds
1. Cirrus (sir´rus).—Small curl-like clouds, usually high in the heavens. 2. Cirro-stratus (sir-ro-strā´tus).—Intermediate between the cirrus and stratus. 3. Cirro-cumulus (sir-ro-kū´mu-lŭs).—Resembling the scales of mackerel. 4. Alto-cumulus (al´tō-kū´mu-lus).—High cumulus clouds. 5. Alto-stratus (ăltō-strā´tūs).—High stratus clouds. 6. Strato-cumulus (strā´to-kū´mu-lŭs).—Forms of cumulus and stratus combined. 7. Nimbus (nim´būs).—A rain cloud. 8. Cumulus (kū´mū-lus).—A conical heap of clouds. 9. Cumulo-stratus (kū´mu-lo-stra´tŭs).—Intermediate between the cumulus and the stratus. 10. Stratus (strā´tŭs).—Arranged in a horizontal band or layer. 11. Fracto-stratus (frăk´tō-strā´tŭs).—Broken forms of stratus. 12. Fracto-cumulus (frăk´to-kū´mu-lus).—Broken forms of cumulus.
THE BEAUTIFUL CRYSTAL-FORMS OF SNOWFLAKES
1-3. Six-rayed stars. 4-13, 18-25. Combinations of six-rayed stars with decorated flat surfaces. 14, 16, 17. Combinations of stars and columns. 15. A true pyramid.
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.