Differences in the temporal behavior of the ultraviolet irradiance at 205 nm and the 10.7-cm radio flux, the ultraviolet irradiance at 121.6 nm, model calculations of the 205-nm irradiance derived from Ca II K plage emission, and the sunspot-blocking function are examined during a 5-year period near the maximum of solar cycle 21. Because of solar rotation the dominant variance in each of these time series occurs at 27 days, but real temporal differences arise because the five solar time series are each formed at different heights within the solar atmosphere and are associated with a variety of solar active region phenomena having different spatial and temporal characteristics on the solar disc. These differences may be important if the ground-based solar activity time series are used instead of the measured UV irradiances in correlation studies of solar variability with atmospheric parameters such as ozone densities and temperature. Recognizing the presence of autocorrelation in the UV irradiance time series is also important in solar terrestrial correlation studies, since it complicates the use of classical statistical techniques for estimating the significance of the results. ¿American Geophysical Union 1987