The Atlantic Monthly, Volume 06, No. 33, July, 1860

Without light, however, there is no color. Agriculturists and chemists understand this. Plants without light retain their oxygen, which bleaches them.

The theory of color has never been fully agreed upon. Some writers maintain that the character of its hues depends on the number of undulations of a ray. Goethe's theory is substantially, that colors are produced by the thinning or thickening and obstructing of light. Brewster contends that there are but three primary colors,–red, yellow, and blue. Wollaston finds four,–red, yellowish green, blue, and violet. But this, as well as the consideration of the solar spectrum of Newton, is more the specialty of Optics. The atmospheric relations of color are more apposite to our purpose.

The color of the clouds, which may be occasionally affected by electricity, is owing to the state of the atmosphere and its reflecting and refracting properties.

The color of snow is white because it is composed of an infinite variety of crystals, which reflect all the colors of light, absorbing none, and these, uniting before they reach the eye, appear white, which is the combination of all the colors.

Wind, the atmosphere in action, though not picturesque, is always wonderful, often terrible and sublime. The origin of wind, its direction and its force, its influence on the health of man, his business, his dwelling-place, and the climate where he perpetuates his race, have attracted the profound attention of the greatest philosophers.

To the rarefaction of the air at the equator, and the daily revolution of the earth, is attributed the origin of the Trade-Winds, which blow from the east or a little to the north of east, north of the equator, and east or south of east after we are south of the equator. The hot current of ascending air is replaced by cold winds from the poles.

But why are we not constantly subject to the action of north winds, which we rarely are? Because of the diurnal motion of the earth, which at the equator equals one thousand miles an hour, the polar winds in coming down to the equator do not have any such velocity, because there is a less comparative diurnal speed in the higher latitudes. The air at the poles revolves upon itself without moving forward;–at the equator, the velocity, as we have mentioned, is enormous. If, then, says Professor Schleiden, we imagine the air from the pole to be carried to the equator, some time must elapse before it will acquire the same velocity of motion from west to east which is always found there. Therefore it would remain behind, the earth gliding, as it were, from beneath it; or, in other words, it would have the appearance of an east wind. Lieutenant Maury adopts the same explanation. It is, indeed, that of Halley, slightly modified.

The warm air, ascending from the equatorial regions, rushes to the poles to be cooled in turn, sliding over the heavy strata of cold air below.

The northern trade-wind prevails in the Pacific between 2° and 25° of N. Latitude; the southern trade, between 10° and 21° of S. Latitude. In the Atlantic the trades are generally limited by the 8th and 28th degrees of N. Latitude. The region of calms lies between these trades, and beyond them are what are styled the Variables. In the former the seaman finds baffling winds, rain, and storms. Occasionally, from causes not yet fully explained, north and south periodical winds break in upon them, such as the Northers which rage in the Gulf of Mexico.

There are many curious facts connected with the Trades, and with the Monsoons, or trade-winds turned back by continental heat in the East Indies, the Typhoons, the Siroccos, the Harmattans, land and sea breezes and hurricanes, the Samiel or Poison Wind, and the Etesian. The Cyclones, or rotary hurricanes, offer a most inviting field for observation and study, and are an important branch of our subject. But we are obliged to omit the consideration of these topics, to be taken up, possibly, at some other opportunity. The theory of the Cyclones may be justly considered as original with our countryman, Mr. Redfield. Colonel Reid, Mr. Piddington, and other learned Englishmen have adopted it; and so much has been settled through the labors of these eminent men, that intelligent seamen need fear these storms no longer. By the aid of maps and sailing-directions they may either escape them altogether, or boldly take advantage of their outward sweep, and shorten their passages.

We have yet to ascertain the causes of the many local winds prevailing both on the ocean and the land, and which do not appear to be influenced by any such general principle as the Trades or the Monsoons.

The force of air in motion gives us the gentle breeze, the gale, or the whirlwind. At one hundred miles an hour it prostrates forests. In the West Indies, thirty-two pound cannon have been torn by it from their beds, and carried some distance through the air. Tables of the velocity of winds are familiar to our readers.

Let us next advert to the connection of the atmosphere with Vapor and Evaporation. The vapor rising from the earth and the sea by evaporation, promoted by dry air, by wind, by diminished pressure, or by heat, is borne along in vesicles so rare as to float on the bosom of the winds, sometimes a grateful shade of clouds, at other times condensed and gravitating in showers of rain. Thus it enriches the soil, or cools the air, or reflects back to the earth its radiated heat. At times the clouds, freighted with moisture, present the most gorgeous hues, and we have over us a pavilion more magnificent than any ever constructed by the hand of man. These clouds are not merely the distilleries of rain, but the reservoirs of snow and hail, and they are the agents of electric and magnetic storms.

Notwithstanding their variety, clouds are easily classified, and are now by universal consent distinguished as follows.

In the higher regions of the air we look for the Cirri, the Curl Clouds. They are light, lie in long ranges, apparently in the direction of the magnetic pole, and are generally curled up at one extremity. They are sometimes called Mackerel Clouds. They are composed of thin white filaments, disposed like woolly hair, feather crests, or slender net-work. They generally indicate a change of weather, and a disturbance of the electric condition of the atmosphere. When they descend into the lower regions of the air, they arrange themselves in horizontal sheets and lose much of their original type. The Germans call them Windsbäume, or wind-trees.

The Cumulus is another form of cloud, which floats along in fleecy masses, in the days of summer, but dissolves at night. Sometimes it resembles a great stack or pile of snow, sometimes it has a silvery or a golden edge, as if we saw a little of the lining. Sometimes they lie motionless in the distance, and are mistaken by mariners for land. They rest upon a large base, and are borne along by surface-winds. Their greatest height is not more than two miles. They carry large quantities of moisture with them, and, when preceding rain, fall rapidly into other shapes.

The Stratus, or Fall Cloud, is horizontal in its figure, lies near the earth, and its length is usually greater than its breadth. It floats in long bands with rounded or sharpened points, and is seen rising from rivers or lakes, at first as a fog. In the morning it indicates fine weather. The Fall Cloud never discharges rain.

This comes only from the Nimbus, which is quite unlike the others. It puts on a dark gray color, has irregular transparent edges, and increases rapidly so as to obscure the sky. It appears to absorb the other clouds, to be a union of their differently electrified particles, which are attracted to each other, form drops of water, and descend as rain.

Of the first three forms we have three modifications or varieties. The Cirro- Cumulus is a congeries of roundish little clouds in close horizontal position, varying in size and roundness, and often, to use the words of the poet Bloomfield, appearing as

"The beauteous semblance of a flock at rest."

The Cirro-Stratus is more compact than the Cirrus,–the strata being inclined or horizontal. It is sometimes seen cutting the moon's disc with a sharp line. The Cumulo-Stratus, or Twain Cloud, is denser than the Cumulus, and more ragged in its outlines. It overhangs its base in folds, and often bears perched on its summit some other form of cloud, which inosculates itself with it. Sometimes a Cirro-Stratus cloud comes along and fastens itself to it parasitically. It is one of our most picturesque forms of clouds.

Within the last two years we have twice observed in the city of New York, during the summer afternoons, large masses of clouds coming over from the southwest, and hanging rather low, which could not be well placed in any of the classes already described, or recognized as such by meteorologists. They consisted of a great number of hemispherical forms of large diameter, hanging vertically from a Stratus cloud or plane above them, and to which they appeared attached. They were regular in shape, and very distinct; they barely touched each other, and were of a gray color. They might be compared to a hay-field turned upside down, with innumerable hay-cocks hanging below it. Unfortunately, the circumstances under which the spectacle was observed did not; admit of any resort to the barometer, thermometer, or anemometer. Should further observations verify these remarks, it might perhaps be proper to style this variety the Hemispherical.

Dew is another atmospheric product. It is the condensation of the warmer vapor of the atmosphere, in calm and serene nights, and in the absence of clouds, by the cold surface of bodies on which it rests. In some countries it is copious enough to supply the want of rain. The earth radiates its own acquired heat, grows colder than the atmosphere, and so condenses it.

What is thermometrically called the dew-point is that degree at which the moisture present in the atmosphere, on being subjected to a decrease of temperature, begins to be precipitated or condensed. It is the same as the point of saturation. Daniell calls it "the constituent temperature of atmospheric vapor." It is our criterion for ascertaining how much moisture there is in the air, and at what degree of heat or cold it would be precipitated. When the air is saturated, a dry bulb and a wet bulb will read alike.

The dew-point has been a puzzle to most persons. Very few treatises explain it satisfactorily. The definition just given, though explicit, is not quite enough. For it will be perceived that an ordinary subtraction of the degrees of temperature on a wet thermometer, which had cooled down by evaporation, from the actual temperature indicated by a dry thermometer, will not give us the dew- point.

For example,–if a free or dry thermometer indicates 63°, and the one with the wet bulb has by evaporation cooled down to 54°, the difference would be 9°. The dew-point would not be 54°, but that degree to which the mercury would fall in the free thermometer, for the atmosphere to become saturated with the quantity of moisture then actually existing in it. It would be 46.8°.

This dew-point, which figures so largely in all well-kept meteorological reports, is the key to many important conditions of the atmosphere, affecting health, vegetation, and climate.

It is found that the air at different degrees of heat has different degrees of elasticity, different degrees of tension, and different degrees of capacity to hold vapor. Dalton, by a series of experiments with barometer-tubes, into which he introduced air and vapor at certain temperatures, found what its force was upon the mercurial column from degree to degree. He also experimentally determined the ratio of the weight of moisture and of air, the former being five-eights of the latter,–in other words, how many grains of moisture additional could be held by the air, advancing from degree to degree of temperature. This being ascertained, a table of factors was constructed, in other words, a set of figures contrived, which should, by a multiplication of the subtracted difference between the range of the dry bulb and the wet bulb of the thermometers, furnish the amount of deduction from the former which would indicate the dew-point, or the point to which the mercury in the dry thermometer must fall to show how much more moisture the air could hold without its condensation. These tables of factors have been constructed at the Greenwich Observatory, and are generally used.

The Hygrometer, invented by Mr. Daniell, gives the dew-point by inspection.

It is an error to suppose that dew falls like rain from the air; it forms on the body which is cooled down below the temperature of the air. It differs in quantity with the radiating or cooling surface; that which has absorbed and retained the most heat during the day radiates the most at night and furnishes the most cold in return.

Hoar-frost, such as we find on our window-panes, or on the grass, is the moisture of the warm air cooled down and frozen, and is produced when the cold at the surface is below the freezing-point. What we in common parlance call the action of frost, and which in this climate is well known to be very powerful, is not particularly injurious to organized bodies.

Mists are the vapor near the ground rendered visible by the temperature of the air falling below that of the vapor. When we see our breath in a cold morning, we see a mist. Where the surface is comparatively warm and damp, and the air is cooler, we have mists, which, if dense, are called fogs. These are found plentifully on the banks of Newfoundland; and with icebergs on the one hand and the Gulf Stream on the other, we must always expect to have them.

The distribution of rain, which is one of the offices of the clouds, is another of the more important features of Meteorology. The amount of water taken up by evaporation into the atmosphere is almost incredible. It is calculated by Lieutenant Maury that there is annually taken up in the torrid zone a belt of water three thousand miles in breadth and sixteen feet deep. Rain occurs regularly and irregularly in different parts of the earth. In some places it may be calculated upon to a day; in others it is quite unknown. Latitude and longitude may indicate the points of distribution, but the causes are dependent on temperature, winds, locality, and, what may seem a strange assertion, upon the conduct of man himself. The greatest quantity falls near the equator, diminishing towards the poles. Much more falls on islands and coasts than in the interior of continents,–more in the region of the variables and less in that of the trades. There are, however, tropical countries of great extent where rain is scarcely ever seen.

The influence of man upon rain is seen in the progress of civilization, the destruction of forests, and the drying-up of meres, swamps, and water- courses.

Forests undoubtedly affect the distribution of rain, and the supplies of streams and springs. Their cooling influence precipitates the vapor passing over them, and the ground beneath them not getting heated does not readily evaporate moisture. Lands, on the contrary, which are cleared of forests become sooner heated, give off larger quantities of rarefied air, and the passing clouds are borne away to localities of greater atmospheric density.

The Canary Islands, when first discovered, were thickly clothed with forests. Since these have been destroyed, the climate has been dry. In Fuerteventura the inhabitants are sometimes obliged to flee to other islands to avoid perishing from thirst. Similar instances occur in the Cape Verdes. Parts of Egypt, Syria, and Persia, that once were wooded, are now arid and sterile deserts.

In the temperate zones these results are not so immediately apparent. It is now much in doubt whether the climate of our country has changed its character within the last two hundred years. Jefferson and Dr. Rush both contended that it had. Our oldest inhabitants assert that in their day our winters began nearly two months earlier than they do now.

The general laws laid down in relation to rain are these:–

1. It decreases in quantity as we approach the poles.

2. It decreases as we pass from maritime to inland countries.

3. It decreases in the temperate zones on eastern coasts as compared with western coasts, but within the tropics it is the reverse.

4. More rain falls in mountainous than in level countries.

5. Most rain falls within the tropics.

The rainless regions, not deserts, are parts of Guatemala, the table-land of Mexico, the Peruvian coast, parts of Morocco, Egypt, Arabia, Persia, etc.

The electric character of the air is another subject of interest, and a leading one in Meteorology. What can be more magnificent, what more awful, than those storms of lightning and thunder which are witnessed sometimes even in our own latitudes?

Faraday, who as a chemist and philosophical writer is of the highest authority, professes to have demonstrated that one single gram of water contains as much electricity as can be accumulated in eight hundred thousand Leyden jars, each requiring to charge it thirty turns of the large machine at the Royal Institution.

It is not intended that this astounding statement should be received without some grains of allowance; but a very elegant and scientific writer, who adopts it without hesitation, adds, "We can from this crystal sphere [of water] evoke heat, light, electricity in enormous quantities, and beyond these we can see powers or forces for which, in the poverty of our ideas and our words, we have not names."

Flashes of electricity have been detected, during warm, close weather, issuing from some species of plants. The Tuberose and African Marigold have been seen to emit these mimic lightnings. (Goethe is the authority for this.) To atmospheric electricity we doubtless owe the coruscations of the Aurora, one of the most beautiful of our meteors.

The usual forms of lightning are the zigzag or forked sharply defined,–the sheet-lightning, illuminating a whole cloud, which it seems to open,–heat- lightning, not emanating from any cloud, but apparently diffused through the air and without report. There are also fireballs which shoot across the sky, leaving a train often visible for seconds and minutes. These last, when they project any masses to the earth, are termed aërolites.

Atmospheric electricity has much to do with the distribution of rain, the precipitation of vapor, the condition of our nervous system, and, according to Humboldt, with the circulation of the organic juices. Atmospheric electricity has heretofore been a great obstacle to the success of the Magnetic Telegraph, and curiously disturbs its operation; but there has recently been invented an instrument called a Mutator, which is connected with the wires, and carries off all the disturbing influences of the atmosphere without interfering with the working current. On the other hand, artificially created electricity has led to important advances in many of the arts and sciences.

Ice is water frozen under a very curious and peculiar law. Hail is the congelation of drops of rain in irregular forms, always sudden,–by some attributed to electricity and currents of air violently rarefied by it, and by others to rain-drops falling through a cold stratum of air and suddenly congealed. Snow, the ermine of the earth, is the crystallized moisture of the air, and is in subjection to unchanging laws.

Water contracts as it grows colder, until it falls in temperature to 42°. It then expands till it reaches 32°, when it becomes solid, though its density is actually diminished, and its specific gravity is reduced to .929, while that of unfrozen water is 1.000. Of course it is much lighter, and it floats. This admirable arrangement prevents our rivers being frozen up and our lakes becoming solid. Ice thickens because it is porous, and allows the heat of the water to pass up and the cold to descend; but this is happily a slow process, as ice is a bad conductor. Salt water freezes at the temperature of 7°, 25° below freezing- point. There are many things to be said about ice, whether as glaciers, or Arctic bergs, or, as it is found sometimes, contrary to its general law, at the bottom of rivers and ponds, its geological movements in the transportation of boulders, and as an article of luxury;–but we are compelled to leave them for the present.

Snow, which, in its crystallization, surpasses the most perfect gems, is invariably found arranged in determinate angles, to wit, 60°, and its double, 120°, and formed of six-sided prisms. More than one hundred kinds have been described by Dr. Scoresby and others, and all these are combinations of the six- sided prism. The uses of snow, from its non-conducting qualities, whether as appreciated by the Esquimaux as a material for huts, or by the agriculturists of our own climate as sheltering the seed, are too well known to require any particular remarks. Strange as it may appear, the proximate cause of the formation of snow is not yet fully agreed upon by the learned.

The connection between Sound and the atmosphere is an important one. The air is a conductor of sound, and in some conditions one of the best. A bell rung in an exhausted receiver gives no sound. In the Arctic regions ordinary conversations have been distinctly heard for the distance of a mile and a half.

All that we have thus far said in this article bears directly, in some form or other, on another of the great features of Meteorology, one of its great objects, and an unceasing topic,–namely, Climate.

The term Climate, in its general sense, indicates the changes and condition of the atmosphere, such as we have been considering. It has something to do with all of them; it is not entirely controlled by any. Thus, places having the same mean annual temperature often differ materially in climate. In some (we quote Mrs. Somerville) the winters are mild and the summers cool, whereas in others the extremes of heat and cold prevail.

Climates are not found coincident with lines of latitude; they are quite as often found parallel to lines of longitude. If you connect the extreme points of the mean annual temperatures by a line passing round the earth, you have a zone, but never a true circle. The curves are longitudinal.

Climate is dependent on temperature, winds, the elevation of land, soil, ranges of mountains, and proximity of bodies of water; and it is also the expression, if we may so term it, of the changes in the atmosphere sensibly affecting our organs. Humboldt refers it to humidity, temperature, changes in barometric pressure, calmness or agitation of the air, amount of electric force, and transparency of the sky.

When mountains range themselves in lines of latitude across a continent, they are barriers to civilization, to the mingling of races, and the union of states. Thus, the Pyrenees have always kept France and Spain apart, the Alps and the Apennines have secluded Switzerland from its neighbors. In our own country, Providence has placed our great mountains on a northern and southern axis; the slopes, the direction, the prevailing winds, the facilities for transportation and travel favor no one of our northern, southern, and western States more than another.

Climate affects vegetation and the distribution of animal life, and thus greatly modifies commerce.

Whatever of importance is accomplished in those countries where climate has overpowered a race is best and principally done by the men of the temperate zones, who carry with them perseverance, courage, and ability, and maintain their ascendency, true to their type, while they have their life to live.

But with our own eyes we may perceive how much climate affects agriculture. The humidity or dryness of soils, their natural or acquired heat or cold, the prevailing winds, the quantity of rain, the snows, the dews, all affect the planter of the seed and the tiller of the ground; they increase or diminish the aggregate of the products of countries, the value of their imports and exports,– -in short, their material power, their resources, their influence, their very existence.

The climate of our own country is exceedingly variable. The transitions from heat to cold are very sudden, the range of the mercury is very great. In the North, we have almost the Arctic winters; in the South, almost the peculiarities of the tropics. Of the State of Pennsylvania it has been said, that in this respect it is a compound of all the countries in the world. Mr. Jefferson and Dr. Rush, as before observed, insisted that our climate has changed; and Williams, the historian of Vermont, contends that New England has deteriorated in its seasons, temperature, harvests, and health, since its early settlement. Our winds blow from every point of the compass, but a due north wind is very rare. Our great western lakes have a large influence on our climate. Some learned men have asserted, that, if they were land, their area being about ninety-four thousand square miles, the region would be so cold as to be scarcely inhabitable.