This study focuses on the sensitivity of significant severe weather climatology to proximity criteria. Six independent definitions of proximity are used. These criteria are then used to develop a climatology of several sounding derived parameters for significant wind, hail, and tornado cases. Geographical and significant severe type comparisons are made. One of the major findings is that little variance occurs in distributions of the parameters studied over the range of proximity criteria considered, namely, from 40 km and 30 min to 185 km and 3 h. Therefore, criteria on the upper end of this range can be confidently applied to significant severe storm climatologies in order to maximize sample size. Substantial differences between the climatological significant severe thunderstorm environment in the High Plains and that of other regions of the country are noted. However, significant tornado cases in all the regions studied are found to be associated with higher values of wind shear between the surface and 1 km, and lower mean layer LCL heights. The climatology compiled in this study describes mean significant severe weather environments for eight regions of the United States.