A tabulation of percent frequencies of inversion and/or isothermal conditions, based below 500 feet above station elevation, for Weather Bureau radiosonde stations in the contiguous United States for four observation times, provides a sampling of daytime and nighttime stability conditions for all time zones. These data are analyzed with respect to nighttime surface wind speed and cloud cover. Radiosonde data are compared to vertical temperature gradient data obtained from meteorologically instrumented towers and valley-ridge stations. Finally, an attempt is made to delineate geographically and climatologically, the percent frequency of low-level stability for the entire country.