Popular Science Monthly/Volume 60/April 1902/The Formation and Motions of Clouds
|THE FORMATION AND MOTIONS OF CLOUDS.|
By Professor FRANK H. BIGELOW,
U. S. WEATHER BUREAU.
THE most beautiful objects in the sky are clouds, and their daily procession from west to east in northern latitudes forms a moving tableau of living pictures for those who have eyes to see. The glories of the sunrise and sunset, decking the fading stars of the morning and the waxing lights of the evening with the pure colors of the spectrum, elevate the heart of man to a loftier adoration for the marvels of nature, than any works of art prepared by his own hand. The exquisite tints of the twilight in the northern, and the even richer tones of the tropic zones, are painted on the memories of those who have crossed the equator. I have seen on the Island of Ascension a set of spectrum bows spread over the evening clouds, as if several rainbows had conspired to illuminate the heavens at the same moment. There are possibly two or three other objects in the sky that rival or even excel these cloud pictures in delicacy of light and shading. They are the aurora with its quivering beams dancing in the cool atmosphere of the polar night, and the star clusters of the nebulae in the milky way near the Southern Cross, viewed through the great refractors of the south.
Such effects are produced by the prismatic action of the small spheres of condensed aqueous vapor that make up the cloud. The rays from the sun, when it is near the horizon, pass through these crystal spherules at such angles that the emergent light is spread out in numerous spectra. The white light coming from space is singly and doubly reflected and refracted within the surface of each aqueous globule, so as to become separated into the bands of color which correspond to the wave-lengths of the solar radiations. The rainbow is a typical illustration of this process, but an illuminated cloudlet represents a million minute bows intersecting each other in all possible directions. The colored clouds of the morning and evening are bright because the light from the sun passes at proper angles to the cloud, and from the cloud to the observer, through a given thickness of the vapor, till the refracted rays are brought down to the earth. The midday clouds are white and glistening, because the sun's rays pass through them as scattered instead of as refracted light, dancing from drop to drop in zigzag courses till the last reflection brings it down to the eye of the observer.
The formation of the vapor drops that make the cloud was long a puzzle to science, but modern research has at last succeeded in solving this mystery. It has been found by experiments that if pure dry air, and pure vapor of water, be mixed in a clean vessel, and then cooled down below the temperature of saturation, the drops of mist are not generally able to form. Purity means that all the particles of dust which float in the air have been perfectly filtered out, and that all traces of electricity have been removed from the air and the vapor before mixing them. It was further discovered that if fine dust powder is injected into the pure mixture, without changing the temperature or the pressure, the drops of water developed at once; also that if minute charges of electricity, carried on particles of matter which may be as small as one-thousandth part of the mass of an atom of hydrogen, are introduced, the drops are able to condense. It is inferred that nuclei of some kind, dust particles or electric particles, called ions or electrons, are required for the formation of water drops suspended in dry air, one nucleus for each drop. Hence, it is possible by counting the number of minute drops that form in a cubic inch, to estimate the number of motes of dust in the air, and even the number of ions charged with electricity in a given volume. The number of the ions contained in the air may be enormous, ranging from 20 per cubic centimeter to many millions. We perceive further that these minute drops coalesce to form rain, which falls from the clouds to the ground, and that they carry down the dust previously blown up by the winds and so purify the atmosphere from all sorts of small floating particles. They also bring electric charges to the earth, and this has something to do with producing the atmospheric electrical potential which always exists. These ions are a natural portion of the atmosphere itself, being continuously produced in it, even when no special cause seems to be present, and they have much to do with explaining some of the strange characteristics of atmospheric electricity which have so long baffled all efforts to comprehend. Investigators are now paying the closest attention to these ions from every point of view.
After clouds have been formed by the condensation of the aqueous vapor, which has been lifted by evaporation from the surface of water at the earth into the air, they take on very different shapes according to circumstances. They may be broadly divided into two classes, the cumulus type and the stratus type. Cumulus clouds are the fleecy, woolpack clouds usually seen on a warm summer afternoon; the stratus are the horizontal veils or sheets that cover the sky more or less completely and which may develop into a general rain. The cumulus of the lower strata, one mile high above the ground, may grow upwards so as to form large domes, and in hot weather they become cumulo-nimbus when the heads are very lofty, some having been observed to reach six or eight miles above the ground. See Chart 3. The alto-cumulus clouds are smaller, more huddled together like the backs of a flock of sheep, and they are from two to three miles high; the cirro-cumulus, from four to six miles high, appear to be still smaller, and they often arrange themselves in ranks or battalions and form the beautiful mackerel sky. All these clouds are formed by the rising of currents of air in a vertical direction, the flat bases showing the level where the temperature begins to be cool enough to cause the vapor to condense, while the sides and tops outline the relative amounts of pressure, temperature and vapor which are just sufficient for the saturation to begin. From a study of these elements we may compute the vertical gradients for each 100 meters, or for each 100 feet, of elevation above the surface of the ground, and these quantities are of great value to meteorologists. The stratus or veil clouds are formed in a very different way. Instead of producing the cooling necessary to condense the moisture by raising it to higher elevations, it may also be caused by the flowing of horizontal currents of air in contact with each other, as when a warm current passes over a cold one. When air from the south flows northward and air from the north moves southward into the same region, these currents generally overflow one another in two strata instead of mixing, since masses of air at different temperatures are quite reluctant to lose their individuality. This stratification of the air in horizontal sheets, flowing from the tropics and from
the polar zones, is always taking place in the atmosphere, and the stratus clouds are generally produced somewhere along these surfaces of contact. The cumulus clouds therefore indicate, as shown in Chart 1, that the air is rising vertically in certain layers, while drifting eastward, and the stratus that it is moving horizontally with a different velocity in adjacent strata. The lowest stratus clouds are elevated fog; the strato-cumulus is about two miles high; the altostratus averages four miles and the cirro-stratus six miles above the earth.
There is furthermore a stratification of the vapor contents of the atmosphere within every high cumulus cloud, which is interesting. Take a lofty cumulo-nimbus cloud such as rises in the summer afternoon before a thunderstorm, from whose base rain may perhaps be falling, while the top is even higher than Mount Blanc piled upon the summit of the Himalayas. See Chart 2. There are sections which should be passed through the cloud, so as to divide it into three parts, which in fact differ from one another physically though they look alike to the observer. The lowest plane separates the saturated from the unsaturated vapor and marks the flat base of the cloud; the second is at the top of the saturated part and at the beginning of the freezing stage; the third is at the top of the freezing and at the bottom of the frozen stage. The freezing stratum is thin and is the place where the
saturated vapor is passing into water at freezing temperature before it can crystallize into ice. A cloud has, therefore, these three portions, the lower consisting of vapor, the middle of water and the top of ice or snow. They appear to be alike because the light from the sun is reflected from drop to drop and from flake to flake in its passage through the cloud. The diagram gives the pressures, temperatures and vapor tensions at the ground, and at the several stages, while the height is indicated in miles, meters and feet. This illustration is taken from one of the loftiest clouds ever observed, and it was computed that the temperature fell from 26.4° Centigrade at the ground to -59° Centigrade or -74° Fahrenheit at the height of 8.8 miles. In summer time at the top of a high cloud the same temperature prevails that may be found at the ground in the polar regions during the coldest days of winter. The obvious correction for the suffering of humanity due to the oppressive hot waves of summer is to bring down this upper air, or else arrange to visit it in floating houses.
The reader may have noticed that hail falls usually in the summer instead of in the winter, and have wondered what is the reason for it. The answer is simple. If we arrange a diagram so that freezing temperature is on the left and 100 degrees Fahrenheit on the right, and plot the limits of the rain, hail and snow regions in altitude, the result is shown in a set of curves which indicate the height at which it is possible for them to form respectively. In the summer when the
weather is warm there is more moisture in the air, it rises higher, till in extreme cases it may reach about seven or eight miles. The stages each become wider, though the hail is always confined to a narrow wedge shaped region whose highest place is about four miles. Now in thunderstorms when there is powerful congestion and stratification of air currents in a vertical direction, the conditions are favorable for the forming of snow balls first by congealing the flakes in the lower parts of the upper stage; these fall slowly through the freezing stage and are coated with a layer of ice; they drop through the rain stage and collect a thicker covering of ice till they arrive at the ground as hailstones. Such strata may be intermixed and arranged in alternate layers so that a nucleus of snow may fall through more than one pair of snow and freezing regions in succession, and thus cover itself with several layers of snow and ice, like an onion. Such stratified hailstones are often found when they are cut open. This theory seems to account for all the facts that have been noted in a simple and natural manner.
The most important use of the cloud observations is not the study of their constitution, but of their motions relatively to the surface of the earth. A cloud is a meteorological meteor, and moves in the stratum of air at approximately the same velocity as the atmosphere itself, so that a measurement of its direction and velocity gives that of the air current, just as a chip floating on a stream shows how fast the water is running. Repeated measures of this kind, when classified, teach us that the atmosphere flows with certain typical movements, and that by them the laws controlling its average circulation can be determined.
Now it is a fact that because meteorologists could observe the motions of the wind readily at the surface of the earth, but not in the upper strata of the air, they have relied too much upon conjecture in constructing the theories of the constitution of storms. The mathematical analysis has had therefore only an imperfect basis upon which to rest, and consequently it has made slow progress towards a complete solution of the problem. The international observations on clouds, during the year May, 1896-July, 1897, had for their immediate object the accurate determination of the motions of the upper air, with a view of testing the existing theories, and constructing new ones wherever necessary. This period of scientific observation is similar to the Tycho Brahe and Kepler stage of astronomy, when observations of the motions of the planets were accumulated for the use of the coming Newton. To lay out the paths of motion in the atmosphere is just as important a work as that of finding the orbits of planets. There is more difficulty in doing it accurately because the motions of the air are much less steady and symmetrical than those of single masses like planets, comets and meteors, but it can be accomplished by patience and well-directed work. Unfortunately for lack of this sort of data much of our common meteorology is incorrect, and must be laid aside as of only an historical value. The subject is itself very complex, and it is unsuitable for a popular exposition, but an idea can be given of its scope and tendency by reference to the accompanying charts.
The Chart 4, marked 'Storm in the Lake Region' 'Winter Cyclone or Low Area,' is a composite map of the motions of the air around a winter storm central near Lake Superior. The upper or cirrus cloud movements are shown by the dotted arrows, the lower or cumulus, by those drawn with a broken line, and the surface wind by those with one unbroken line. The region affected by this storm extends from the Gulf of Mexico to the Lakes, and from the Atlantic Ocean to the Rocky Mountains. It is noted that the arrows drawn with an unbroken line and those drawn with a broken line generally blow nearly side by side, but that they differ from the dotted arrows in many cases, especially near the center of the storm and eastward to the ocean. This shows that the real storm circulation is confined to the lower strata as if it were quite independent of them. There is seen to be a slight deflection of the dotted arrows southward around the low; a corresponding chart of a high area would show a similar deflection to the northward of the center. This feature is brought out in Chart 5. Direction and velocity of the motions of the air in the cirrus, cumulus and on the surface. The arrows are not very smoothly laid down with reference to one another, but this is due to the fact that there were not enough observations taken in order to smooth out the local irregularities. Still, it is easy to trace a sort of wave motion, a crest over the high and a hollow under the low. As we come down from the cirrus, six miles high, towards the ground the sinuous motion becomes more pronounced, till in the strato-cumulus level the rotary component is distinctly seen, and in the cumulus it predominates. The length of the arrows shows the relative velocity, which is seventy miles per hour in the cirrus, on the average, forty-five miles in the alto-and strato-cumulus, fifteen in the cumulus and six on the ground. Compare Chart 1. The total motion of the upper air is made up of two parts, a rapid and quite steady eastward drift, and a local or secondary circular movement; these two combine and make up the observed circulation that actually exists. They are due to different causes, the first to the fact that the tropics are warmer than the polar regions, and that the earth is rotating on its axis; and the second to the fact that these two different temperatures seek to combine into an equilibrium by means of an interchange of heat through the air currents, especially in the strata from one to three miles above the ground. These counter currents flow against each other, and at or along their junction they produce rotations and whirl up cyclones or local storms. This is the law in the temperate latitudes from 30° to 50°; another law prevails in the tropics and possibly in the polar zones, though these belts have not yet been suitably studied. These counter currents can be readily seen on Chart 4 in the arrows drawn with a broken line, where a great stream flows from the northwest and recurves along the Mississippi Valley, while a second stream arising in the Gulf of Mexico flows northward, and recurves to the westward over the Lake Region. Along the edges of these currents is the place for rainfall, for the formation of cold waves, the production of tornadoes and thunderstorms. One can readily judge from relative lengths of the arrows on Chart 5 to what a disadvantage the forecaster is put in attempting to anticipate the probable storm action by using only the small arrows on the surface. He ought to have access to those of the higher strata as well, and this is the reason for the special efforts which have been made by the Weather Bureau through the cloud observations and the kite ascensions to in some measure overcome this defect. It is hoped that by means of nephoscopes and the necessary study of the facts already obtained to be able to make at least three simultaneous daily weather maps, one on the sea level, one on the 3,500-foot plane, and one on the 10,000-foot plane. This will give us three sections through each storm instead of one as at present. The forthcoming report of the Weather Bureau on the Barometry of the United States and Canada contains a discussion of these data and the necessary reduction tables for all the stations of the service. The sea level reduction tables were put in operation on January 1, 1903, and the tables for the other planes are nearly ready for use. There is reason for expecting that a good measure of success will attend these efforts to obtain practical results of considerable advantage in the art of forecasting.