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

Abstract

A descript ion is given of a photoelectric ultraviolet-intensity meter and automatic integrating and recordin g apparatus for measuring the biologically effective component of ultraviolet radiation, of wavelengths 3132 A and shorter, from the sun and the entire sky, incident on a horizontal plane, under various meteorological conditions. Methods of standardization, in absolute value, are described (see RP1542 for supplementary data). A continuous graphical r ecord of the in tegrated daily total amount of biologically effective ultraviolet solar and sky radiation observed during a period of 3 years (1941 to 1943) in Washington, D. C. is given . The monthly totals of biologically effective ultraviolet, in absolute value (milliwatt minutes per square centimeter) mw min/cm2 are also given graphically. On the clearest days the biologically effective component of ultraviolet radiation of wavelengths 3132 A and shorter, incident from the sun and the whole sky at midday, ranges from about 180 microwatts per square centimeter, (p.w/cm 2 ) in midsummer to about 30 /Lw/cm 2 in midwinter. A series of erythema tests is given, correlating the physical (radiometric) measurements with the physiological reaction of the untanned skin, which information is of interest in helotherapy and bioclimatology.

Bioclimatic Measurements of UV–Solar and Sky Radiation in Washington, D. C., 1941–1944

“…For a complete description of the photoelectric cell, methods of standardization and measurement, and a summary of observations extending over a period of 3 years, reference is made to previous papers [1,2,4].…”

“…The top part of an improved model, in which the photosensitive receiver, R, is inclosed in a large glass bulb (about 8 cm in diameter) to eliminate the effect of electric charges [2] is shown in FIGURE 1.…”

“…By analogy, and to keep the information out of the realm of mystery it is a simple matter to report the photoelectrical^ integrated ultraviolet intensities in "milliwatt minutes" (or "hours"), i.e., numbers that are not cumbersome to use. Moreover, since the erythematogenic efficiency of ultraviolet solar radiation varies with the solar height and depends upon skin pigmentation (and numerous physiologic factors); also because the information is useful for other purposes, the hereinreported data are the physiologically "unweighted" measurements of the integrated ultraviolet solar and sky radiation comprised in the spectral band of wave lengths including 3132A and shorter, which is specific in producing erythema, in preventing and curing rickets, and in producing vitamin D. The energy required to produce an erythema [2] is a measure of the dosage that can be given at one time without causing a burn. Obviously as indicated in the previous paper [2,reference No.…”

“…These photoelectric cells differed in intrinsic sensitivity, in spectral range of response, and in 'cosine'') response with angle of incidence of ultraviolet radiation. Part of the difference in response with angle of incidence upon the receiver was due to variation in transmission, caused by inhomogeneity and lack of uniformity in thickness of the glass dome of the photoelectric cell, which affects the results depending upon the distribution of clouds [2]. Nevertheless, in spite of these defects an intercomparison of daily and hourly integrated amounts of ultraviolet radiation received by the various photoelectric cells show irregular variations in the relative amounts recorded, just as though there was an unrecognized photochemical production, and variation in composition of gases in the stratosphere, causing irregularities in the relative amounts of selective absorption of ultraviolet solar radiation at 2900A to 3100A, which would have a different effect upon photoelectric cells differing in range of spectral response.…”

“…This combination of apparatus has been in uninterrupted operation since February 1, 1941 [2]. Aside from the collaboration of R. Stair [2,4] in making a primary calibration of several of these instruments, because of the military exigency throughout the past 4 years it fell to the lot of the writer to service the various instruments studied and to maintain the primary calibration.…”