Page:Popular Science Monthly Volume 80.djvu/529

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TROPICAL SUNLIGHT

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