Measurements of diffuse and direct irradiance at the surface of the earth under cloudless skies have been taken during the summer at a primarily-rural location in the Midwestern U.S.A. Based on observations taken under extremely clean skies, simple empirical expressions of the direct component I and the diffuse component D, as a function of solar zenith angle σ, provide a basis for ready comparison with more-turbid conditions. A particularly sensitive indicator of turbidity is the ratio D I , which is found to vary with wind direction at the measurement site in a manner such that considerably greater turbidity is found with southwest winds than with northwest winds. The data indicate that the gain in D associated with increased turbidity compensates for the attenuation of I by an amount that varies from 58% of the loss at θ = 65° to 70% at θ = 37.5°. If purely single-scattering processes and simple reflection from a surface with an albedo of 0.17 are assumed, it can be shown that the effect of adding suspended particles to the troposphere is to increase the effective albedo of the earth, with the magnitude of this increase being strongly dependent on the zenith angle.