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Climatology of Diffusion Potential Classes for Minneapolis–St. Paul

Abstract This climatological study reports on the potential for atmospheric diffusion at Minneapolis–St. Paul, Minnesota, cities located in the heart of the North American continent. As such, the results can be considered typical of an urban setting within a continental climate. Data were obtained from a nearly continuous 8-yr record of vertical temperature and wind measurements made on a 152.4-m tower. Temperature lapse rates between 21.3 and 152.4 m were grouped into three stability categories: 1) isothermal–inversion, 2) subadiabatic, and 3) superadiabatic. A subdivision of each was based upon wind speeds of less than 4 m s−1 and greater than or equal to 4 m s−1, resulting in six classes that were examined according to wind direction, time of day, time of year and, most importantly, the associated synoptic conditions. The isothermal–inversion condition was limited to nighttime periods, especially when high pressure centers were dominant and winds were less than 4 m s−1. The highest frequency of occurre...