Popular Science Monthly/Volume 80/June 1912/Tropical Sunlight




JUNE, 1912




THE subject of the influence of sunlight in the tropics has been the subject of extended discussion for many years, and the general opinion seems to be that the intensity of insolation is the most important factor influencing the physical welfare of the white inhabitants in those parts of the world lying within the regions which are generally considered as having a tropical climate. In considering the question of what may be regarded as a tropical climate, we are too apt to be influenced by preconceived opinions as to what the dominating factors are, and we are prone to lose sight of the fact that there is as much difference between tropical climates as between those in the temperate zones. Persons living in the tropics are almost certain, during their early years of residence, through ignorance or otherwise, radically to change their mode of living and subject themselves to hygienic conditions which they would consider inadmissible in their former homes. The races of people native to the tropical zones have no knowledge of bacteriology and pathology which would enable them to understand the measures to be taken to avoid infectious and other diseases, whereas they live in regions where the absence of a pronounced winter is favorable to the rich development of microscopical life. As a consequence, many of the ill effects which are attributed to sunlight may in reality be due to entirely different causes. Again, races native in the tropics, as a rule, do not have access to the complete food supplies of persons in temperate zones, although this statement may sound paradoxical, and this fact in turn may morphologically modify the peoples to a greater extent than other influences.

It can readily be understood that a place recognized to be within the tropics, may, by reason of its proximity to the sea, its altitude, relation to mountain chains, and other natural surroundings, have a climate so modified that the actual sunlight may have less influence than in localities which may be situated upon the borders or even within the temperate zones. Another factor influencing local conditions may be the color of the soil and the resulting modification of the intensity of the heat rays coming from it, because the radiation from the soil is of importance.

In other words, pronounced differences may be found between the climates of two places in the tropics which may geographically be close together, as may readily be seen by comparing the meteorological data from Alexandria, Cairo and Aswan, in Egypt. Prevailing winds may so modify the climate of a region that during the nights the temperatures may closely correspond to those found in places more favorably placed. Indeed, in comparing two regions we may find the anomaly that the tropical situation, for long periods, may have a more temperate climate than is found at the place nominally lying outside of the zone in question.

The general trend of the discussion in the literature has been that the rays of the sun lying in the region of the spectrum comprising the violet and ultraviolet are of the greatest importance in determining the influence of insolation upon human beings, and these rays have a special, though undeserved, designation as actinic rays. Studies of the influence of these rays have not been lacking; it has clearly been shown that they are destructive to great classes of microorganisms, and methods have also been devised to measure the relative proportion of these rays as compared with other parts of the spectrum. However, no thoroughly systematic work in this line has been carried out, and comparisons in the tropics are decidedly lacking.

Therefore it seems evident that the entire question of the influence of insolation upon the inhabitants of the tropics exists as a legitimate field for experimental study and that comparative data from different regions may be obtained so as to solve many of the questions in an impartial manner. The Bureau of Science at Manila is very fortunately situated in regard to this work. Its equipment is ample and the composite nature of the staff makes it possible to carry it on in a number of lines simultaneously. In addition, the newly organized University of the Philippines offers opportunity to call upon the faculty of that institution for cooperation. The opportunity being at hand, it seemed advisable to begin cooperative work on the subject of tropical sunlight. and, therefore, about three years ago, several members of the staff from the two institutions undertook different lines of investigations with the object ultimately of coordinating the results into a monograph on the subject.

The first problem was to obtain data regarding the relative influence of the rays of shorter wave-length on different days and in different latitudes; and to obtain reliable figures, it was necessary to determine upon the photocatalytic reaction to be used. The decomposition of oxalic acid in the presence of uranyl salts, which is brought about almost entirely by the rays in the spectrum of the sun extending from 550 to 291 , was decided upon. Although this method is not free from grave objections, which have been pointed out by me in another article,[2] nevertheless extended experimentation brought the decision that it was best adapted to the ends sought. Having decided upon the method and thoroughly learned the factors which influence the reaction, cooperation was requested in a number of laboratories in various parts of the world. The returns have not all been received, but so far extensive comparative measurements have been conducted in Manila (latitude 14° 36′), Kuala Lumpur in the Straits Settlements (latitude 3° 10′), Honolulu (latitude 21° 10′), Washington (38° 59′), Khartoum, Egypt (latitude 14° 36′), and a few data have been obtained from Tucson, Arizona (latitude 32° 12′). As yet, the returns from a number of other places have not been received. The results are surprising. In each one of the localities mentioned above, days of maximum insolation were observed which were practically identical, and in a number the averages were very close together. So, for example, at Manila the average percentage of oxalic acid decomposed for one hour during one year was 13.45; at Kuala Lumpur, 11 degrees farther south, 15.29; at Honolulu, 7 degrees farther north, 13.81, and at Khartoum in the Sudan, 17.6. The great differences do not lie in the averages obtained, but in the minimum observed on cloudy days. So, for example, both Kuala Lumpur and Khartoum show surprisingly uniform degrees of insolation, whereas in Manila and Honolulu the proportion of cloudy days is so great as to bring the average down materially. In Manila the minimum observed was 1.15; in Honolulu, 3.48; whereas at Kuala Lumpur the same figure was 9, and at Khartoum 14.7. Therefore, the difference of climate of these places is not due to geographical location, but is purely a meteorological phenomena. Washington, with a winter climate and presumably much greater atmospheric disturbances, on the entire average gave about 33 per cent, less decomposition than Manila, yet the maxima are practically the same and at times Washington shows an astonishingly high average, and there is but little difference, between the summer and winter months in the latter place. As mentioned above, we have but few data from Tucson and, therefore, at present we can scarcely make a comparison, but doubtless when a longer series of observation is at hand we shall discover that there are many days in Tucson showing a maximum as high or even higher than Manila, and an average of about the same. The fact is also observed that two days apparently equally clear will show marked differences between each other during corresponding hours, so that the proportion of the rays reaching the earth and lying between 550 and 291 varies from day to day. A comparison of the total effect of these rays with the measurements obtained by the black bulb thermometer also demonstrates that the two are not functions of each other. Of course it is clear that there must be a certain relationship because, obviously, on clear days both black bulb readings and photocatalytic measurements will be high, but they need not necessarily be high in the same proportion. The sun's rays which lie in the portion of the spectrum under discussion, on reaching the atmosphere, suffer molecular scattering, refraction and other changes which modify the proportion of direct sunlight that reaches the earth's surface, and, of course, these changes vary with the condition of the atmosphere. Nevertheless, it is very interesting to observe that even in the tropics the shortest wave-lengths appearing in the spectrum of the sunlight are very close to 291 and that no shorter rays reach the surface of the earth. The same observation was made in northern latitudes, so that it can confidently be stated that the range of the spectrum everywhere is the same, the difference, if any, being in the intensity of the light.

The average of the measurements made at Baguio, which lies a little north of Manila at an altitude of 1,432 meters, shows that the photocatalytic action of the sun in that locality is much the same as it is in the lowlands. The maximum is slightly higher, being practically identical with that of Honolulu, and the average is 1.75 per cent, more than in Manila, and 1.09 less than at Kuala Lumpur, and 0.39 more than Honolulu, so that the ascent of 1,432 meters has produced the same effect upon the photocatalysis as would a transfer from Manila to Honolulu.

The above is a very brief summary of the results so far obtained in a study of photocatalytic reaction brought about between the rays lying between 550 and 291 , and in view of the results the more extensive remainder of the spectrum, which extends upward from the point mentioned into the red and infra-red and which would include the heat rays, must be considered. It is self-evident that comparative measurements in this field involve much greater difficulties, as, at present, no photocatalytic reaction is available. The best means at hand is by a comparison of a series of readings with the Angström pyrheliometer of the total solar radiation per square centimeter of surface normal to the ray of incidence.

Unfortunately, measurements in the tropics with this instrument are lacking and in Manila we have not yet been able to carry them out, because the instruments which we have ordered have suffered great delays in delivery. A large number of available data have been gathered by Dr. Herbert H. Kimball, of the Mount Weather Observatory, and comparisons of the maximum intensity of solar radiation extending from Cape Horn, on the south, to Treurenburg, on the north, show that the variations are not great and those which appear, in the belief of Kimball, are due to instrumental errors rather than to atmospheric conditions. Angström gave some measurements from Teneriffe (20° 30′ north) and obtained practically the same figures as those quoted by Kimball. Dr. Rudolph Schneider in February, in Vienna, found a maximum higher than that given for Cape Horn and practically the same as that at Katherinenburg, and the observations made by him for the time close to the noon hour show a close resemblance to those in Washington. Harvey N. Davis, of Providence, Rhode Island, obtained practically the same results. Kimball, in discussing the annual march of radiation, as compiled by him, states that "a rather surprising uniformity throughout the year (is shown) in the maximum intensity of radiation, the December minimum being only 8 per cent, less than the April maximum." Even if we take the annual total we find that Warsaw actually has 85 per cent, of the radiation received at Washington, although it is 20 degrees farther north.

Because data with the Angström pyrheliometer in the tropics were lacking, we had recourse to animal experiments in Manila. In this connection it should be remembered that although air temperatures in some regions may be low and in others high, the effect of the sunlight on solid objects, as in the case of the black-bulb thermometer, may be very great and bear no relation to the shade temperature; so, for example, Davos, Switzerland, shows an average maximum black-bulb thermometer reading for three years of 53°.8 with a highest absolute maximum of 67° in 1910, whereas the maximum in Manila in one year was 56° and at Helwan, Egypt, 70°.8 during a period of three years. Alexandria during the same period gave 57°, Aswan Reservoir in June, 1910, showed a maximum of 81°, and Ley, Thibet, with an altitude of 3,517 meters, a maximum black-bulb thermometer reading of 101°.7 with a shade temperature of 23°.9. These figures refer to maxima only and do not take into consideration averages or the shade temperatures which might be high or low. but it is evident that the occurrence of days of extreme insolation is not so much a matter of latitude as of situation and it is evident that even in the tropics we might come to averages decidedly lower than in certain more northern climates. It is obvious that in any one of the places mentioned and which lies outside the tropics, a living body might encounter days in which it would be heated by solar radiation to a much greater extent than in the tropics, and the only question would be whether the possibility of cooling, such as low air temperature, low humidity, winds or other means would compensate to avoid the effects of such insolation.

Our studies were undertaken with animals having fairly well developed means of heat regulation. The most interesting results were obtained with monkeys and human beings. To obtain comparable data, means had to be devised to give accurate and rapid measurements of the temperatures of the skin and of the inner parts of the body, and eventually a very satisfactory apparatus was completed by Dr. Hans Aron, of the department of physiology of the College of Medicine and Surgery, which gave the temperatures by means of specially prepared thermocouples, the changes being read by a tangent galvanometer. Monkeys are naturally at home in the tropics, and we should suppose that they would best be able to withstand the effect of sunlight. They have a system of sweat glands, but this is not so highly organized as it is in man, so that their physical heat regulation is brought about not only by evaporation of sweat, but also to a very great extent by water evaporated from the lungs and mouth through increased respiration. If a monkey is exposed to the sunlight in Manila, his subcutaneous and rectal temperatures rise rapidly, the former more rapidly than the latter, and the animal will die within 1 hour and 20 minutes to 1 hour and 50 minutes, the temperatures gradually reaching maxima. Entirely different results are obtained if the animals are shaded, even by such a small area as is produced by an umbrella or a piece of board, all other conditions being similar, except that the direct rays are excluded. Under these circumstances the skin and rectal temperatures never exceed 40° and the animals remain healthy. Similar results are obtained if the animals are exposed to full insolation, but care is taken to conduct away the excessive heat increment by means of a brisk current of air from a fan. Under these circumstances the subcutaneous and rectal temperatures remain the same as when the animal is shaded. In this form of experiment the monkey is exposed to all the rays of the sun, including those of lesser refrangibility, heat waves alone being conducted away. If untoward effects are to be attributed to the absorption of the ultraviolet rays, then surely the animal is in the same condition to absorb the latter as he is when no blast of air is present, and their effect should be apparent. On absorption, a large proportion of these rays is presumably converted to heat and conducted away as such, so that it can be assumed that the effects which we observe on exposing these animals to the sun is one of heat, and these conclusions are borne out at autopsy where post-mortem examinations give protocols clearly pointing to heat stroke. Monkeys enclosed in tight boxes, with only the heads exposed, and placed in the full sun, suffer no inconvenience, although the hair temperature on the scalp may reach 47°. Of course, it must be understood that the monkey's skin is protected by fur and is not sensitive to the irritating effects of the ultraviolet rays as would be the case with the skin of a Caucasian, who, if exposed to the sun, would be sunburnt, whether in a strong blast of air or not. This latter effect is due to the ultraviolet portion of the spectrum, and, as these rays have but little power of penetration, the skin can in time amply protect itself by pigmentation. But even though protected, as is the monkey's skin, and hence not subject to sunburn, the heat effect still remains and brings about the results of excessive heat exposure.

Experiments on man have shown that the ultraviolet rays are easily guarded against, by shade or even by a white cotton shirt, the heat rays not. In man we have a subject with highly developed sweat glands, so that the means of heat regulation by evaporation are much more complete than in dogs, rabbits or monkeys.

Skin temperatures of men in this climate in the shade under normal conditions, as measured by our apparatus, vary within the extreme limits of 31° to 34°. On exposure to the sun, these temperatures rise rapidly on the sunny side, but as soon as the human subject begins to sweat, even slightly, the temperature begins to fall, and with muscular exertion may be as low as 31-33°. If the subject is at rest, the skin temperatures do not fall as rapidly, but after one hour they may be the same as at the beginning of the experiment or even more than a degree lower. In going over our long series of figures we found that fifty minutes of exposure caused no practical rise over the temperatures after the first ten minutes in white men and in Malays.

The comparison showed but little difference between the white and the Malay, the difference, if any, being in favor of the latter, but a Negro, in a series of observations, exposed to the sun at the same time as a blond European and a brown Igorot, showed a higher skin temperature by 1°.45 than the Caucasian. At the end of the experiment the final temperatures were decidedly against the Negro, slightly against the Tagalog, and in favor of the white skin. Therefore, so far as they have gone, our experiments seem to show, as regards rise in temperature upon exposure to the sun, that the white and brown skins are about equal, with a slight factor in favor of the white, but that in the case of the very dark-skinned Negro the temperature, on exposure, reaches a decidedly higher point than it does with either of the others. The dark skin of the Negro obviously will absorb heat more readily than the lighter one of the European, and also will radiate more readily, the heat taken up on the sunny side being rapidly lost on the shady one; but this balance evidently results in a greater rise of temperature for the Negro than for the white man. With the white skin we have the phenomenon of sunburn, with its resultant irritation of the nerve-endings and hyperemia of the peripheral tissues, and this will cause a rise which, apparently, just about offsets the rise in the pigmented brown skin due to the sunlight. To determine definitely the decided difference brought about by the color of the subjects, it was decided to use experimental animals which would show great contrasts; and white, gray and black rabbits gave the data sought. I will select one experiment. When exposed side by side to the sun, the black rabbit reached a maximum subcutaneous temperature of 47°.8 in thirty-one minutes and then died; the gray rabbit a final temperature of 44°.9 in one hour and twenty-six minutes and then died; the white rabbit a final temperature of 45°.7 and when put in the shade it recovered, although much exhausted. None of the animals suffer from sunburn as does the white man, and it is evident that the darker the coat, the greater the heat absorption and the more apparent do the effects of insolation become.

These experiments bring us to the conclusion that, all other things being equal, the Negro will suffer more from the heat effects of the sun than the lighter-skinned races, and all of the work tends to show that the rays of greater refrangibility in the violet and ultraviolet portions of the spectrum are not the important factors, except in so far as they cause sunburn and subsequent excessive pigmentation. However, protection from these rays is easily accomplished and has been accomplished so long as man has worn clothes. These experiments also show that the whiter the clothing the better it is adapted for protection against sunlight and that even in the tropics, if care is taken to seek the shade, no untoward effects can be observed. Indeed, Major W. P. Chamberlain, United States Army Medical Corps, who investigated the systolic blood pressure of a large number of residents in the Philippines, concluded that there is no progressive tendency for the pressure to increase or to decrease with a continued tropical residence covering periods of over three years, beyond which length of time his observations do not extend.

From all of our present studies it would seem legitimate to draw the conclusion that a climate such as we have in the Philippines, where we are surrounded by the sea which modifies the extreme of temperatures and where we have such a large proportion of cloud, is not by any means deleterious to the white man if he takes ordinary precautions which are not as elaborate as those he would take in a northern climate to keep out the cold. In the Philippines the nights are rarely too hot for comfort; they may even be quite cool.

The actual number of hours of insolation per year on the earth's surface, were the sky always clear, is greatest at the equator and diminishes toward the poles, the ratio between 0° and 45° being 1.83 to 1.34, although in the longer days in the temperate zone the sunshine reaching the earth when the sun is near sunrise or sunset is only a small proportion of that at midday. As a result we have in the tropics greater absorption and radiation from the earth's surface as a result of direct exposure to the sunlight to augment the influence of the sun's rays, so that, as it has been shown that the heat factor is the chief one to consider, this increment due to radiation from the earth will be of decided influence. This will naturally vary with different regions according to the hours and intensity of insolation and the color of the surface exposed, being least with green surfaces of vegetation and greatest with rocks, or red, clay soil, such as is common in India under the name of laterite.

Another factor needs to be considered, and that is the evenness of the tropical climate, which is devoid of severe contrasts, such as are given by the winters in northern climates, yet Chamberlain's results seem to indicate that this has but little effect.

I have endeavored in this short article to give a very brief résumé of the most important points which, up to the present, have been brought out. Any one can see that the subject under investigation is so complex and that it is influenced by so many factors that general conclusions at the present time are premature, excepting in so far as they are borne out by experimental evidence. Obviously relative humidity is of great influence on evaporation and varies with geographical locality, the season of the year, and other causes. Experiments carried on in Manila also seem to show that the Malay and the Negro possess relatively more sweat-glands than the European. The formation of ions in the air, the proportion of such ions, if any, due to the effect of the sunlight, and the total ionization brought about by radioactivity may be of influence in controlling the electro and other meteorological phenomena, and we have also begun work in this direction, but as yet are not in a position to publish the results.

Although the spectrum of the sun, as shown by the spectrograph, does not extend beyond 291 , still it may be possible that we receive rays the nature of which we have not yet determined and which, with our present physical technique, we can not determine. These may also be factors in the phenomena of insolation.
Photograph by Bailey Willis.

Mount Siyeh, Scene up Canyon Creek, near Altyn, Montana. Glacier National Park.

  1. The manuscript of this article was received by the editor two weeks after the cabled announcement of the lamented death of Dr. Freer. Dr. Freer had attained high distinction as a chemist and since taking charge of the scientific work of the government in the Philippines in 1901, had contributed greatly to the organization and advancement of the scientific work under our government.
  2. Phil. Journ. Sci., Sec. B. (1912), 7, 1.