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.
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.