Mean radiosonde observations and normal maximum surface temperatures are used with the assumption of a dry adiabatic lapse rate to estimate monthly mean maximum mixing depths (MMD's) for 45 stations in the contiguous United States. From these data isopleth analyses of mean MMD's are presented for each mouth. Deviation from the assumption of a dry adiabatic lapse rate is discussed. Sixteen of the available 45 radiosonde stations are selected as generally representative of the United States and are subjected to further considerations. These considerations are (1) the relationship between monthly mean MMD's computed from mean observations and from individual observations, (2) comparison of monthly mean MMD's based on observations for 1 year with those based on observations for 10 years, and (3) the standard deviations of daily MMD's.