Solar radiation

the weather. All growth in plants depends upon the application of solar energy. Our atmosphere is the source of carbon, which is a principal plant con...
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In this article the importance of solar radiation for lip-kt, power, plant growth, and beauty i s indicated; and the principles of its measurements, and results as to quantity are explain,ed. T h e absorption by terrestrial gases and vapors, the importence of water and ozone absorptions for life, the variability of the s u n and five n m l y discowered regular periodicities therezn, and the distribution of radiation over the solar disk and in wawe-lengths i s discussed. N m observations at the Smithsonian Institution o n radiation and plant growth and experiments in solar cooking and solar power are included. Directly or indirectly our most important interests depend on the solar radiation. The sun rays keep the earth warm enough to sustain life. Variations of their intensity associated with summer and winter, and with night and day, produce climates. Slight variations of the original output of rays from the sun itself seem to he highly infiuential in altering the weather. All growth in plants depends upon the application of solar energy. Our atmosphere is the source of carbon, which is a principal plant constituent. The trifling percentage of carhonic-acid gas contained in air is the essential food of plants, hut it cannot nourish them without the help of radiation. The health of animals, including man, re~uiresradiation. The prevention of rickets by the curious direct and indirect infiuences of ultra-violet rays has formed a fascinating chapter in the story of recent investigations. Power is principally derived indirectly from solar radiation. Solar heat evaporates the oceans, drives the clouds inland, precipitates the rain and snow, and thus maintains the world's hydroelectric power sources. Enormous as these are, they are, nevertheless, trifling compared to the power derived from oil and coal. Oil, the less important of these two sources, comes mainly from animal l i e that was sustained ages ago by the vegetation fed by the ancient sun. Coal, on the other hand, is the end-product of decomposition of vegetation. The enormous deposits of coal, which are now the world's principal sources of power, represent but a trifling percentage of the solar energy lavished on the earth in former geologic ages. These are highly indirect applications of solar radiation for power supplies. I t is possible, however, as numerous inventors have shown, to produce heat for driving engines by the direct absorption of solar rays. If the devices now available for this purpose should he but a little more improved, the desert regions of the world could supply cheap solar power in quantities beyond the largest possibilities of future world demands.

* Presented before the Physical Science Section of the Eleventh Ohio State Educational Conference, April 9-11, 1931. **Secretary. 416

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A lesser, but still interesting aspect of solar radiation, lies in its use for domestic purposes. Food may be cooked, and water may be heated by simple and inexpensive contrivances for utilizing solar radiations. It may be that none of the services rendered to us by the sun is more important than its esthetic use. The colors of all flowers represent the fragments of the complete solar spectrum remaining after the pigments of the plant world have absorbed certain rays. Unabsorbed remainders are reflected and produce gorgeous mixtures of colors. Without the green of the grass and trees, the blue of the sky, the brilliant hues of flowers, and the more somber, but yet pleasing shades of the soil, and thousands of familiar objects on all sides, we should be sad indeed. Though thus obviously so important, it is little more than a century since measurements began to be made of the intensity of the solar radiation. Pioneers in this investigation were C. S. M. Pouillet, Sir John Herschel, and J. D. Forbes. They all devised instruments adapted to absorb the solar radiation as completely as possible and thus to convert it into heat. By appropriate devices for measuring the heat thus produced, they obtained measurements of the intensity of solar radiation. So intimate is the association between radiation and heat that many people have confused the two. Yet they are distinct and different. Radiation comprises a mixture of impulses which traverse a vacuum a t the enormous speed of 186,000 miles per second. These impulses are separable by prisms or gratings into innumerable regular periodicities. These periodicities produce diierent sensations of color in our eyes. Many of them, indeed, are invisible to us. Since radiation can traverse the vacuum and nevertheless seems to comprise transverse waves of exceeding shortness, philosophic minds have not been contented to contemplate so great a paradox as waves travelmg in nothingness. They have devised the idea of the luminiferous ether as a medium filling all space. At present, our ideas are in a state of flux regarding this abstruse subject, but a t least we can think of radiation as something which can exist where there is no matter in the ordinary sense, for i t comes to us across an immense void from the sun and the stars. Heat, on the contrary, is well recognized to be the motion of the molecules of material substances. The energy of radiation is competent to stir up this mode of motion called heat, when, for instance, radiation is absorbed by a black object. If the object is "absolutely black," absorption is complete and all the energy of the ray is transformed into heat. The measuring instruments of Pouillet, Herschel, Forbes, and others exposed surfaces blackened with lampblack. This substance is not quite "absolutely black," for it reflects away about 3 per cent. of ordinary sun rays. Kirchhoff, about seventy years ago, proved that a closed chamber, whether blackened or not, must be a perfect absorber. However small the percentage of rays absorbed a t one impact, there is no escape, and the rays

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must be reflected hither and thither until by innumerable impacts their intensity is reduced below any assignable minimum. This principle of the "absolutely black" chamber has been incorporated into the Smithsonian water-flow pyrheliometer. This instrument thus far is the world's standard for measuring solar radiation. Assuming, therefore, that we have accomplished the complete absorption of the solar ray, and its entire transformation into beat, and have devised means for the exact measurement of the heat thus produced, we then may express the intensity of solar radiation a t the earth's surface. We are accustomed to express it in terms of the amount of radiation absorbed upon a square centimeter of surface in a minute of time. We measure the heat produced in calories. In these terms we fmd the heat of solar radiation near noon on clear days to be approximately 1.4 calories per square centimeter per minute. To state the matter in more common terms, the solar radiation, if transformed completely into work, would produce roughly a horsepower per square yard whenever the sun is high in the sky. Thus even a t 93,000,000 miles, the sun rays are tremendously powerful. The sun itself sends them out in every direction continually. This output equals the heat of the burning of 400,000,000,000,000,000,000,000tons of anthracite coal per year. Measurements of solar radiation a t the earth's surface are subject to losses by absorption and scattering of the rays in our atmosphere. High up at an altitude of nearly 40 miles, there exists a small quantity of ozone which is that form of oxygen whose molecules contain 3 atoms instead of the usual 2. Ozone is a complete absorber of all rays in the extreme ultraviolet from wave-length 2900 A. U. onward for a considerable range. This is very fortunate. Otherwise our skin would be blistered and our eyes blinded, for these short-wave rays which are totally absorbed by ozone are highly destructive to animal tissues. On the other hand, it is not less fortunate that ozone allows some rays on the border of its absorption band to pass, for these rays between wave-lengths 2900 A. U. and 3100 A. U. are indispensable to prevent rickets. The total thickness of gas of the atmospheric ozone layer, if it could be brought down to sea level, would be less than one-eighth of an inch. It is astonishing and even terrifying to contemplate the narrow margin of safety on which our lives thus depend. Were this trifling quantity of atmospheric ozone removed, we should all perish. If it were ten times greater, we could not live. Rickets would prevail universally. Other little-considered atmospheric gases of great importance to our comfort are water vapor and carbonic acid, both minor constituents of the air, quantitatively speaking. Without the water vapor, our earth, like the moon, would cool far below freezing every night within a few moments after the sun sinks below the horizon. Water vapor hinders the incoming

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rays of the sun to the extent of 10 to 15 per cent., but i t hinders the escape of the long-wave rays emitted by the earth by over 70 per cent. Thus water vapor acts like the gardener's glass cover of his hotbed. It lets sun rays in profusely, but holds the earth rays from escaping, and so is an efficient regulator of climate. Carbonic-acid gas also acts in a somewhat similar way, but less efficiently than water vapor. The indispensable function of carbonic-acidgas in our atmosphere is to be the essential food for plants. Other atmospheric constituents which greatly modify the income and outgo of radiation for the earth's surface are clouds and dust. The permanent gases, oxygen and nitrogen, alter the incoming sun rays, i t is true, but weaken them only slightly, and hardly alter the outgoing earth rays a t all. The molecules of the permanent atmospheric gases, oxygen and nitrogen, scatter sun rays more and more powerfully toward the violet end of the spectrnm. But what is thus lost to the direct solar beam comes to us almost wholly in the beautiful blue rays of the sky. Dust also scatters sun rays, but without much selection of color. A dusty sky is therefore nearly white. Sunset and sunrise owe their beautiful colors to the scattering of sun rays by the atmosphere. When the sun is near the horizon, its rays shine very obliquely through the atmosphere and therefore by enormously long paths. The consequence is that the powerful tendency to scattering of the blue and violet rays so far depletes that part of the spectrum that the sky light which reaches us has a yellow or even a red tinge. S. P. Langley, third secretary of the Smithsonian Institution, was a profoundly interested student of solar radiation and atmospheric absorption. He wrote (1):

If the observation of the amount of heat the sun sends the earth is among the most important and difficult in astronomical physics, it may also be termed the fundamental problem of meteorology, nearly all whose phenomena would become predictable, if we knew both the original quantity and kind of this heat; how i t affects the constituents of the atmosphere on its passage earthward; how much of i t reaches the soil; how, through the aid of the atmosphere, i t maintains the surface temperature of this planet; and how, in diminished quantity and altered kind, i t is finally returned to outer space. Fifty years ago Langley invented the bolometer, an exquisitely delicate electrical thermometer sensitive to a millionth of a degree. He took it to Mount Whitney, California, in 1881, to measure the rays of the solar spectrum under the purest of skies. He also perfected and applied a method for determining the losses suffered by solar rays in traversing the turbid and absorbing ocean of atmosphere which always overlies even the choicest of observing stations. With some improvements, we still use the Langley bolometer and the Langley method.

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FIGURE 1.-VARIATION OF THE SUN'S RADIATION Comparative results of monthly mean ohsenrations 5000 miles apart a t Montezuma, Chile, and Table Mountain, California. The two stations agree on it within an average discrepancy of 0.2 per cent. For the past twelve years the Smithsonian Institution has maintained stations on high mountains in desert lands where daily measurements of the solar radiation are made. Our best station a t Mount Montezuma, Chile, nine thousand feet in altitude, lies in a desert where the rain seldom falls, and where neither animal nor vegetable life can exist. The observers must bring even water itself from the town twelve miles distant. The observations are carried on in such a way that the losses caused by the atmosphere are determined accurately. Thus we are able to measure the intensity of solar radiation as it would be found outside our atmosphere altogether, as if one were on the moon, for instance. We allow for the ellipticity of the earth's orbit, and thus reduce the results to a constant solar distance. Following long c:stom, we call the resulting value "the solar constant of radiation." I t is on the average 1.94 calories per square centimeter per minute.

I N SOLAR VARIATION FIGURE 2 -PERIODICITIRS Monthly mean "solar constant" values, 1918 to 1930, analyzed into periodicities of 65, 45, 26, 11, and 8 months shown in curves C, D, E, F. and G. Their sum in curve B dosely parallels the original curve A. The yeam 1931 and 1932 forecast, curve I. Periodicities of shorter length found in 1924, curve H.

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Yet the solar-radiation intensity is not perfectly constant. It varies through a range of twelve per cent. Figure 1 shows how two of our stations, one a t Montezuma, Chile, the other a t Table Mountain, California, agree witha 0.2 per cent. in tracing the variation of i t by their monthly mean results over the past five years. The total range of variation shown by these monthly means is 1.5 per cent. Figure 2 shows in curve A the monthly Montezuma values since 1918. The total range in this period is 2.5 per cent. Curves C, D, E, F, G are regular periodic curves of 68, 45, 25, 11, and 8 months whose sum, given in curve B, almost exactly repro-

FIGURE 3 . S O L A R VARIATION, 1924 TO 1930 Daily observations at the Smithsonian Station, Montezuma, Chile: , Circles satisfactory, cross?s nearly satisfactory, points unsatisfactory. Rmng and falling sequences of solar changes are indicated by full and dotted curves, respectively.

duces the variation shown in the original observations given by curve A. Other shorter periods may be found in solar variation, as indicated for the year 1924 in curve H. I have ventured to forecast in curve I the probable march of solar variation in the years 1931 and 1932. Short-interval changes are also found as shown in Figure 3. I have indicated by curved lines, full and dotted, respectively, over 100 cases each of rising and of falling sequences extending over several days each. The smallest ranges considered in these short-period changes are 0.45 per cent., and the largest found is 2.5 per cent. I have compared with these sequences

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the weather of Washington, Williston, and Yuma. The interesting result comes out that both temperatures and barometric pressures in weather show opposite courses depending on whether they accompany and follow rising or falling solar radiation. This is shown for Washington weather in Figure 4. Apparently major changes in weather are caused by small fluctuations in the solar radiation. If this is so, we ought to expect that the regular periodicities which are proved by Figure 2 to make up the principal solar changes since 1918 ought to be rdected in the weather. In Figure 5 one may see that this is indeed so. The prinapal changes in Washington temperatures since 1918 are represented as the sum of six regular periodicities, of which five are those

FIGURE 4.-WASHINGTON TEMPERATURES AND PRESSURES ASSOCIATEDWITH AND FOLLOWING SEQUENCES OP SOLAR CHANGESHOWN IN FIGURE 3 The results given here are average results of a period of seven years. Note the opposition between the full and dotted curves corresponding t o rising and falling solar radiation, respectively. found in the solar radiation. This gives us hope that weather may be susceptible to long-range forecasting. It would, indeed, be a great boon if the characteristics of coming seasons and years could thus be approximately known in advance. But much further study must be made before this hope can be thoroughly tested. We see from these exhibits that solar variation is of two types, the longrange and the short-range, respectively. Two kinds of causes probably are involved. The long periods of 68, 45, 25, 11, and 8 months are closely related to the well-known interval of 1 1 1 / 4 years in which the numbers of sun spots wax and wane. This suggests that these longer range periodicities are due to increasinq and decreasing agitation in the gaseous flnid which

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composes the sun. It is to be regarded like the stirring of a fire with a poker which brings up from below the hotter materials, and throws out temporarily a greater radiation in our rooms. The short-period changes of solar radiation, which run their courses in a few days, are probably caused in other ways. We may suppose that patches of increased or diminished radiating power form occasionally on the solar surface. An example of this, indeed, is often seen in the bright faculae which surround sun spots. On the other hand, there may be areas of diminished intensity above the sun spots due to the outrush of gases from within the sun and their consequent cooling by expansion as they reach

FIG-

5.-PERIODICITIES IN WASHINGTON TEMPERATURE ASSOCIATED WITH PERIODICITIES IN SOLAR R ~ D I A T I O NSHOWN IN FIGURE 2 Curves C, D, E, F, G, and H represent periodicitics of 68,45,25,18,11, and 8 months. Their sum in curve B roughly parallels the original curve A. These results indicate a real hope for long-range weather forecasting from solarradiation observations. THE

elevations of diminished pressure. If in either of these ways local irregularities occur in the sun's surface brightness, the complete solar rotation which takes place in about four weeks must present these fluctuating intensities toward the earth in their turns, and so produce short-interval variations of the solar constant of radiation. There is another variation of solar radiation, not periodic, but exceptionally interesting to theorists, as it throws light on the sun's inner nature. I refer to the difference of brightness between the edge and the center of the solar disk. Figure 6 illustrates this observation and shows how different is the phenomenon when viewed in differently colored rays. In the nltraviolet, the sun's center appears about three times as bright as its edge, while

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in the infra-red the edge is almost as bright as the center. A great deal of theoretical investigation has been based on exact measurements of these phenomena, which have been carried out in the years 1913 to 1920 by Smithsonian observers on Mount Wilson, California. This study is also associated with the determination of the distribution of brightness as be-

BETWEEN CENTERAND FIGURECONTRAST OP BRIGHTNESS EDGE OF THE SUN'S DISKAS SEEN IN DIFFERENT COLORS From Smithsonian observations.

tween different wave-lengths in the solar spectrum. Smithsonian observers have determined this as shown by Figure 7. The study of the dependence of plant growth on radiation has lately been taken up by the new Division of Radiation and Organisms a t the Smith-

AND INTENSITY IN THE SOLAR FIGURE~.-WAVE-LENGTH SPECTRUM Smithsonian observation.

sonian Institution of Washington. In order to have a better control of radiation intensity and hours of exposure than clouds and night would permit if solar rays alone were employed, we use mainly electrical lamps of special construction; the results will be applicable to the understanding of plant growth under natural conditions.

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The essence of the problem lies in this, that plants grow by taking in carbonic-acid gas from the air through millions of little mouths called stomata which dot the under su~facesof the leaves. But this feeding occurs only when certain rays found in sunlight and other sources shine upon the plants. The questions are: Which are the effective rays? What

differencesare created in plant growth when the rays are changed either in color, in intensity, or in time of exposure per day? What are the chemical processes which go on in the laboratories of the plant leaves by which cellulose, sugars, odoriferous materials, fruit and nut substances, poisons, and the host of organic chemicals which plants produce are built up?

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Finally, how does radiation cause plants to bend in those interesting ways illustrated by the sunflower and nasturtium, and by the twining stalks of the beans and peas? All of these questions are being studied a t the Smithsonian Institution. The investigation is still young, so that little as yet has been published. Figures 8 and 9, however, give some idea of the ingenious apparatus and interesting work which have been developed already under the direction of F. S. Brackett. I will but mention the phototropic experiments in which a little oat sprout is being used as an indicator. It is situated between two

lights of different colors, whose intensities may be graduated until the oat sprout grows vertically. Then, of course, the two lights are equal in their tendencies to produce bending. I t is found that green is a thousand times more active than yellow, and blue thirty times more active than green to produce bending. Red and infra-red are like darkness, having no bending influence a t all. Another very promising line of investigation is that illustrated in Figure 10. Pure organic chemicals such as benzene, chlorobenzene, and others of greater and greater complexity are introduced in a spectroscope between the source of light and the recording thermopile which automatically

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measures the spectrum energy. Absorption effects are produced on the infra-red rays. These effects are found to be characteristic for each chemical studied. In this way we are building up a system of organic chemical analysis, whereby chemical structure can be determined without making combustions. It is yet uncertain how powerful this method will eventually prove, but we hope it will throw much light on the abstruse reactions of plant growth under the influence of radiation. Figure 11 shows an experimental cooking plant which I erected a t the Smithsonian station on Mount Wilson. Sun rays falling upon the great concave cylindrical mirror, 7 feet by 12 feet in surface, and moved by

FIGURE 10.-ADSORPTION SPECTRA OF BENZENEDERIVATIVES Observed under the direction of Dr. F. S. Brackett. Smithsonian Institution. clockwork to foUow the sun, are reflected upon a blackened brass tube incased by a vacuum-glass jacket. Within the tube, which lies parallel to the earth's axis, is high-test engine cylinder oil. It grows hot, expanding, and rises up into a reservoir containing about 60 gallons of oil. Two ovens for cooking are inserted in this reservoir of hot oil. A return tube from its bottom completes the circulatory system, bringing cooler oil to be heated by the mirror. For weeks a t a time on Mount Wilson, the ovens remained hot enough to bake bread both day and night. All kinds of cooking except frying and broiling are readily done with this solar cooker. The kitchen where food is prepared remains cool, and the housewife has a delightful view

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of the mountains as she steps out to place i t in the ovens. The apparatus is too costly to be anything but a luxury. The most successful solar power installation thus far made was located near Cairo in Egypt, and was used to pump water from the Nile for irrigation. A description of it is given in the Smithsonian report for 1915.

The principle is the same as that just illustrated in the solar cooker, but vacuum jackets were not introduced to enhance the heating. Literature Cited (I)

LANGLEY, S. P.,"Report of the Mount Whitney Expedition." Prqfessional Papers of the Signal Service. XV, p. 11.