«ASTRONOMY: C. G. ABBOT 82 PROC. N. A. S. THE LARGER OPPORTUNITIES FOR RESEARCH ON THE RELATIONS OF SOLAR AND TERRESTRIAL RADIATION' By C. G. ABBOT ...»
ASTRONOMY: C. G. ABBOT
82 PROC. N. A. S.
THE LARGER OPPORTUNITIES FOR RESEARCH ON THE
RELATIONS OF SOLAR AND TERRESTRIAL RADIATION'
By C. G. ABBOT
SMITHSONIAN ASTROPHYSICAL OBSERVATORY
Communicated by the National Research Council, December 8, 1919
The earth is maintained at it§ present temperature by a balance be- tween the solar radiation received and the terrestrial radiation emitted.
The mean intensity of the solar radiation as it is at the earth's mean distance outside the atmosphere is believed to be well known. However, it appears to be variable from year to year and also irregular over short intervals of days, weeks or months. These variations, which have been discovered and to a certain extent followed for about fifteen years by the Smithsonian Astrophysical Observatory at its station on Mt. Wilson, have lately been compared with terrestrial temperatures and atmospheric pressures at different stations of the world by several meteorologists, notably by Clayton, and it appears that the effects of solar variations are important as governing changes of terrestrial temperature.
This indicates two highly desirable researches: First, to make daily observations of the sun competent to determine its variations. Second, to determine by further statistical inquiries the exact influence of the variations of the sun on the terrestrial phenomena.
In order to carry out the first research thus outlined, it requires not less than four special solar radiation observing stations in the four most cloudless regions of the earth. One such is now in existence at Calama, Chile, where it is being maintained by the private funds of the Smith- sonian Institution. If there was available the necessary income of thirty or forty thousand dollars a year, the Institution would establish the re- quired observing stations and would arrange for the daily wireless trans- mission of the results obtained on solar radiation so that they might be available for meteorologists in all parts of the world to use for forecasting purposes. The same could be done by international cooperation if due regard were given to making the measurements homogeneous. It is not.
probable that any steps of this kind will be taken within a year or two.
By that time the series of measurements being made at Calama, Chile, will enable meteorologists to determine to what extent the results will be valuable for forecasting purposes.
It would promote the proposed solar investigation if there could be devised autographic recording instruments for determining the exact value of the cloudiness at all times of the day when the sun is above the horizon. Hitherto measurements of cloudiness have been made at com- paratively few stations, and as they depend upon personal estimates of observers who are apt to overlook wisps of cirrus clouds especially dan- ge
or three times in the day, they are quite inadequate to enable the investigator interested in establishing solar radiation stations to select the regions of the earth best meeting his requirements of cloudlessness. It would, therefore, be a very valuable investigation if some suitable autographic method of measuring cloudiness could be devised simple enough to be employed at numerous stations reported to be cloudless.
The whole subject of the cloudiness of the earth is very important.
Recent measurements of Aldrich show that a cloud surface reflects about 78% of the solar radiation incident upon it. As the cloudiness of the earth is reputed to be about 50%, it is obvious what a large part clouds play in determining the temperature. They are effective not only in cutting off incoming radiation from the sun but also outgoing radiation from the earth.
If the simple autographic apparatus above mentioned were available, it should be employed at numerous stations in all parts of the world and if possible on the ocean.
The measurement of solar radiation at several stations on the earth's surface by means of the pyrheliometer has occupied observers in many countries. Several series of observations have been carried on for many years by the U. S. Weather Bureau, and are published in excellent form in the Monthly Weather Review. Many copies of the Angstrom pyrheliometer and of the Smithsonian silver disc pyrheliometer, whose constants are both so well known as to form together a homogeneous system of pyrheliometry, have been sent out to many quarters of the world.
Only a few series of regular observations are at the present time being conducted with them, and it is greatly to be hoped that a large number of such series may be conducted regularly in the future. The method of reducing the results and publishing them can hardly be improved over that which is followed by the United States Weather Bureau, and that may well be taken as a model.
The study of the outgoing radiation from the earth has lately been carried on by several observers, notably by Mr. Fowle, who has investigated the effects of water vapor in long atmospheric columns in diminishing the rays of long wave-length such as the earth sends out. His investigations extend only to a wave-length of 17 microns because no suitable optical media transparent to waves of longer wave-length are available. It appears that potassium iodide may be such a medium and it is hoped shortly to produce large crystals of the salt suitable for making prisms for the study of long wave-lengths of terrestrial radiation.
In the meantime, valuable information has been obtained on terrestrial and atmospheric long wave-length radiation by Dr. A. K. Angstrom by the use of the pyrgeometer or nocturnal radiation instrument devised by his father. An instrument applicable for the same purpose has also been, devised by Messrs. Abbot and Aldrich of the Smithsonian Institution and is called by them the pyranometer. Neither instrument is entirely 84 ASTRONOMY: C. G. ABBOT PROC. N. A. S.
satisfactory for very long waves since both depend upon a knowledge of the absorption of blackened surfaces to long wave-lengths, and this is a matter which still lies in doubt. For satisfactory progress in this line of the measurement of nocturnal radiation, a new instrument depending upon the principle of the hollow chamber or absolutely black body ought to be devised.
The study of the effect of ozone on the temperature of the earth is one which ought not to be neglected. Next to cloudiness and atmospheric humidity, ozone is thought to play the most important part in its effect on terrestrial radiation; first, because the quantity of ozone in the atmosphere is believed to be variable, and, second, because its effect upon terrestrial radiation occurs near the wave-length 10 microns where water vapor is almost completely transparent. This research should include the determination of the variations of the quantity of atmospheric ozone and the measurement of the effect of its changes on the ozone band near 10 microns in wave-length.
A fuller discussion of the several points which have been raised above will be found in the following papers.
INVESTIGATIONS IN SOLAR RADIATIONSurvey of the Present State of the Field We may inquire: First, what is the intensity of the radiation of the sun on which all the life of the earth depends; second, is this intensity constant or variable from day to day and from year to year, and what are the effects of variability, if any?
1. The Solar Constant.-The question of the absolute amount of the solar radiation interests us not only for our own sake but for the sake of the future generations. What a valuable thing it would be if we knew at this time the intensity of the sun's radiation as it was in the times of the Egyptians and the Babylonians, atd as it was in the intervening periods of Rome and the Middle Ages. It is plainly our duty to transmit to posterity accurate measure of the intensity of the sun's radiation as it is now, so that they will be in a better position in this branch of science than we find ourselves. We cannot expect to know the intensity of the sun's radiation as it would be in space at the earth's mean distance (that is, the so-called "solar constant of radiation") to that high degree of accuracy which we are accustomed to demand in many branches of physics.
If we determine that value to one per cent, it is all we may reasonably hope for, because the intervention of the earth's atmosphere, with its changing amounts of haze and cloudiness will always interpose to the investigator an insuperable obstacle to the highest degree of accuracy. It may be that in the future apparatus can be shot up by means of rocket devices to go outside of the atmosphere altogether, but probably the sources of error of automatic apparatus for such a research will be found so large 85 ASTRONOMY: C. G. ABBOT VoL. 6, 1920 that no greater degree of accuracy than to one per cent will be possible even with them.
The spectrobolometric investigations of this subject by the Astrophysical Observatory of the Smithsonian Institution which have been continued for the last seventeen years have yielded the general indication that the value of the solar constant of radiation is within one per cent of 1.93 calories per square centimeter per minute. This result is confirmed by experiments made with sounding balloons by the same observatory in cooperation with the U. S. Weather Bureau in 1914. Automatic recording pyrheliometers were sent to a height of about 25 kilometers.
It was found that the intensity of the radiation there, at a point where 24/25 of the atmosphere was beneath the observing instrument, fell within the range of solar constant values as obtained by spectrobolometric work at the surface of the earth. The actual value obtained by the balloon work was 1.84 calories per square centimeter per minute. Allowing for the obstruction of the sun rays by the still superincumbent atmosphere a reasonable extrapolation would give 1.88 calories. This differs from
1.93 calories less than the error of the observations and is well within the range of solar constant values obtained at different days from surface observatories.
Apparently the close agreement of these widely different independent methods of obtaining the intensity of solar radiation outside our atmosphere permits us to say authoritatively that the mean value of the solar constant is at any rate between 1.9 and 2.0 calories per square centimeter per minute and most probably not greatly different from- 1.93. A. few men cling to the view that much- higher values should be set for it-values between 3 and 4 calories per square centimeter per minute, but their arguments carry little weight, as it seems to me, and those best qualified to know are agreed that the values given above have a strong foundation.
The methods of observing the solar constant of radiation by means of the spectrobolometer have been carefully worked out and the whole subject has been published with satisfactory fullness and is in shape to be transmitted to the scientific men of the future in order to enable them to reproduce the measurements or to compare their own measurements with the results now obtained.
2. The Sun's Variability.-Passing now to the second branch of the subject, the investigations of the Smithsonian Astrophysical Observatory conducted at Washington, Mt. Wilson, Mt. Whitney, Bassour (Algeria), and now the investigations supported by the Smithsonian Institution from its private funds in North Carolina and Chile have all united in giving the impression that the solar radiation is not constant' but varies from day to day through a range of certainly five and possibly at times ten per cent. The conclusion that the sun is a variable star is confirmed in several ways but most notably by the results of measurements made by ASTRONOMY: C. G. ABBOT
-86 PROC. N. A. S.
the Smithsonian Astrophysical Observatory at Mt. Wilson, California, on the distribution of energy along the diameter of the solar image. These measurements indicate, as was well known before, that the edge of the sun's disc is less bright than the center, and that the contrast of brightness between the center and the edge varies according to the wave-length of light, being greater for short wave-lengths, less for long.
But the measurements of recent years have shown that not only is there a variation of contrast by wave-length, but also a variation of contrast with the time. The contrast in each wave-length is different for different days of observation and, on the average, for different years of observation. The changes of contrast have been compared with the changes of total radiation of the sun determined by the aid of the pyrheliometer and spectrobolometer, and it is found that there is a moderate degree of correlation between them. The correlation is of two kinds. For variations of long periods of years, high values of the solar constant are found associated with high values of contrast between the center and edge of the sun.
On the contrary, for the short period variations of the solar radiation, occupying a few days, weeks, or months, it is found that high values of the solar radiation are associated with diminished values of the solar contrast.
The cause of this two-fold variation is reasonably explained. When the sun grows hotter and thus increases its output of radiation along with increased solar activity, as indicated by sun spots, prominences, and othervisible solar phenomena, this would tend to cause a greater degree of contrast. For since if the solar temperature were zero there would be zero contrast, the higher the temperature the higher the contrast. But the sun is probably entirely gaseous, and certainly its outer layers are so, and these may become more turbid at times, just as the earth's atmosphere becomes more hazy at some times than at others. Accompanying increased turbidity of the solar atmosphere there would be found a diminished value of the solar constant of radiation. But since the path of the solar ray is oblique in the solar atmosphere near the edge of the sun, the path is longer there and the effects of the turbidity would be greater at the edges rather than at the center. Thus with the increase of turbidity the contrast of brightness would increase accompanying a diminished value of the solar constant of radiation. In this way it appears that the two-fold variations of the sun which have been found may be reasonably explained.