A critical study of the values published during the past 20 years leads to profound respect for the skill, energy, and devotion to science evidenced in connection with their determination. The mean of these values is generally accepted as correct within the limits of the accuracy of the observations. This paper has to do principally with the two following sources of error, and their probable effect upon the day-to-day variations in the solar constant values:

  • (1) Pyrheliometer readings. Numerous comparisons between different instruments indicate a probable error in the readings of one, provided both are equally good, of about ±0.30 per cent. For Montezuma, Chili, Dr. Abbot computes the error to be ±0.20 per cent, and since a solar constant value depends upon the readings of two instruments, the error would be only 
    formula
    This enters directly into the determination of the solar constant values.
  • (2) The difficulties of extrapolating pyrheliometer readings to the outer limit of the atmosphere, which is accomplished through determinations of atmospheric transmissibility for monochromatic radiation, (a), by the bolometer, or (b) by the pyranometer. An examination of individual determinations made at Montezuma and Harqua Hala on days having not less than 6 determinations at the two stations, gives probable errors of from ±0.2,3 per cent to ±0.74 per cent, and the day-to-day variability seems to be a function of the number of determinations upon which the daily values are based.

Since the probable error, or the probable variability, in the day-to-day values of the solar constant derived from determinations at these two stations is less than ±0.5 per cent, it is seen that solar variability, if it exists, is of the same order of magnitude as the probable error of the determinations. Therefore, this fact must be taken into account in attempting to correlate solar constant values with weather and other phenomena.

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