Sunburning Solar Radiation in Central Indiana

Abstract

This work combines radiative transfer calculations for clear skies with measurements of ultraviolet sunlight obtained over a range of cloudy-sky conditions to define the sunburning solar irradiances experienced in central Indiana. Clear-sky ultraviolet irradiances undergo a large annual cycle primarily in response to the changing elevation of the sun above the horizon over a year, with a lesser influence from the periodic behavior in atmospheric ozone amounts. Erythema, defined as a reddening of the skin after exposure to solar radiation, can occur in less than one hr under clear, haze-free skies during a 5-hr interval centered on solar noon in October, during a 7-hr window in April, and a nearly 9-hr span in July. Scattering by clouds attenuates the ground-level irradiance to values that can vary erratically over time scales of minutes to hours
and from one day to the next. One-fourth of the measured ultraviolet irradiances are 52% or less of the clearsky value during June, July, and August. At the opposite extreme, one-fourth of the measured irradiances fall between 82% and 100% of the expected clear-sky result. In mid-July near solar noon, the time required for a minimal reddening of the skin varies from a minimum of 14 min under clear skies to 27 min or longer for the 25% of cases with the thickest cloud cover.