Measurements of ultraviolet solar radiation in Washington, 1936 to 1942

Abstract

Continuing earlier measurements of ultraviolet solar radiation (RP877), a summary is given of similar measurements made during the years 1936 to 1942, in Washington, D. C. Marked variations in the spectral quality and total intensity of ul tr~v iolet solar radiation, attributable to variations in atmospheric turbidity, and, to a less extent, to well-known cyclonic and seasonal changes in ozone concentrations in the stratosphere, are recorded. On the clearest days the biologically effective ultraviolet of wavelengths shorter than 3132 A, incident directly from the sun and the surrounding sky at midday, r anges from about 75 microwatts per square centimeter (p,w/cm2) in midsummer to about 12 p.w/cm~ in midwinter.

A daily record of ultraviolet solar and sky radiation in Washington, 1941 to 1943

“…This great difference is especially noticeable in the months of comparable solar heights, March and September, April and August, February and October, and January and November. As noted in a previous paper [2], this is due to the presence of a less amount of ozone in the stratosphere (hence, a greater amount of incident ultraviolet solar radiation) in the autumn than ' in the spring.…”

“…In previous publications [2,14], it is shown that on the clearest days, at the noon hour, in Washington, the ultraviolet solar intensity of wavelengths 3132A and shorter at normal incidence ranges from about 75J.1.w/cm 2 in June to one-sixth this value in December. For comparison, in figure 7 are depicted the average intensities of solar and sky radiation incident on a horizontal plane on four very clear days in Washington.…”

“…1 As noted in a previous paper [2), the evaluation of the biologically effective ultraviolet solar radiation is still in its infancy, and the uncertainties involved are somewhat comparable with those experienced in the beginning of the evaluation of the total incident solar radiation of all wavelengths half a century ago. However, there is 1 Numbers in brackets indicate the literature references and notes at the end of this paper.…”

“…Thus far, however, this procedure has not been practicable for either type of instrument. Hence, in connection with the use of instruments for measuring solar and sky radiation incident on a horizontal plane, two methods of standardization [4] were undertaken: (a) by calibration against a standard of ultraviolet radiation [11], and (b) by calibration against an ultravioletintensity meter [3, 9] that measures radiation incident normal to the' photoelectric receiver [2] . The latter was calibrated against a standard of ultraviolet radiation [11] and gave ultraviolet solar-radiation intensities, of wavelengths shorter than 3132 A, in close agreement with.…”

“…It places the uncertainty of the calibration upon the pyrheliometer designed for measuring radiation at normal incidence, and, as above noted, in the case of the nonselective pyrheliometer there is half It century of research on the accuracy attainable. In the case of the spectrally selective radiometer (photocell), because of the above-noted uncertainty in the energy distribution in the biologically effective band of ultraviolet solar radiation, high accuracy in the absolute values is not yet attainable (p. 443 of [2]). …”