Frequency, Duration, Commencement Time and Intensity of Temperature Inversions at St. Paul-Minneapolis

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Abstract
This study is based upon temperatures for the period June 1961 through July 1968, obtained from thermistors installed on a television tower at heights of 70, 170 and 500 ft. The tower was located in the heart of the metropolitan area but well outside of the main business district of either city. Only inversions ≥2 hr in duration were counted.On an annual basis only 2.5 inversions per 100 days occurred in the lower level (70–170ft) compared to 26.3 inversions per 100 days in the upper level (170–500ft). The total frequency, including the two levels and the deeper inversions that extended over both levels, equalled 45.5 per 100 days.The average duration of inversions within both levels was 6.8 hr, the 8.2-hr average in October being the longest of any month. Midnight was the single most common hour for inversion formation. Inversion intensity (defined as the temperature difference between levels) varied directly with duration of the inversions. The average intensity of all inversions within both levels was 3.6F.An inversion rating index was developed that takes into account frequency, duration and intensity of inversions so that either time periods or sites may be quantitatively compared. The index indicated that the major inversion months were October, August and September, the minor months being April, March and May.A decrease in inversion intensity and frequency over the record period was assumed to indicate an increasingly urban influence on the temperature of the region.

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