A very simple model of atmospheric transmittance uses as its input regularly observed and archived measures of upper air humidity. The five model parameters were determined empirically to optimize the agreement with radiation measured at six sites in the northwestern contiguous U.S. in summer and winter 1990. The model uses the relative humidity at 500 hPa, the relative humidity at either 950 hPa or 800 hPa depending on the altitude of the station, and the precipitable water. Although the model does not use cloud observations explicitly, it is partitioned into a low-humidity stage and a high-humidity stage with markedly different dependences on relative humidity. In the low-humidity stage the transmittance is approximated as the product of factors for dry air and for water vapor. In the high-humidity stage the transmittance is the product of the transmittances of each of the two layers used; the product is a strongly decreasing function of relative humidity, as a representation of the effect of cloud. A split-sample test using measurements for spring and autumn indicated that the model parameters were not badly distorted by conditions unique to the summer and winter seasons. In terms of the average daily global shortwave radiation received at the surface, the rms errors are 31 W m(-2) in spring, 33 in summer, 30 in autumn, and 18 in winter. The corresponding relative errors are 0.25, 0.17, 0.20, and 0.31. Because of high-frequency temporal variation in the model residuals, the error declines to about half of the daily error for 7-day averages and to about a quarter of the daily error for 30-day averages.