Eighteen years of clear day solar radiation data accumulated at Phoenix, Ariz., are analyzed to yield monthly averages of the transmittance of the atmosphere for solar radiation. These figures serve as a basis against which theoretical calculations of the atmospheric transmittance are compared. Calculations based upon the complete set of attenuation relations proposed by Houghton average 2% lower than the measurements. When Houghton's water vapor absorption curve is replaced by the McDonald water vapor absorption relation, however, the yearly averages of the measurements and calculations are essentially identical. A procedure for approximating the effects of dust variability is introduced which further improves even these calculations in terms of monthly comparisons.