Air quality in southwestern Pennsylvania, as in most other areas across the globe, is very much influenced by surface-based temperature inversions. In fact, this region of the U.S., which includes Pittsburgh, has been known for historic air pollution episodes initiated by such inversions. To discover the severity and trend of substantial ground-level inversions, a 30-year climatology (1991-2020) for the Pittsburgh area was prepared that examined morning (7 am EST; 12 UTC) and evening (7 pm EST; 00 UTC) daily radiosonde soundings for surface-based inversions of at least 1oC in strength for morning inversions and 0.4oC for evening inversions. The investigation revealed that morning inversions occurred on 47 percent of all days with valid readings. The strength of the morning inversions was 3.9oC on average. The depth of the inversion layer measured an average height of 246 m above the ground. Morning inversions tended to dissipate by 10 am EST. The frequency of evening inversion occurrence during this period averaged 20 percent. The mean strength of the evening inversions was 1.1oC. The mean height of the inversion layer was 101 m above the ground. Overall, a generally declining frequency of surface-based inversions was discovered. The trend in inversion strengths, heights, and estimated break times over this duration were varied. Morning (7 am EST) inversion strengths generally declined while morning heights and break times were rather steady. Evening (7 pm EST) inversion strengths and heights increased overall. Comparison of individual decades revealed that on average morning surface-based inversion frequency and strength have declined over the three recent decades, while inversion heights increased in the middle decade and estimated break times remained steady. For evening inversions, frequency declines but strength increases in the final decade and height increases in the second and third decades. A look at monthly means shows morning surface-based inversions are most common April through November and evening inversions most frequent in October through December. There is a morning-evening overlap of some of the months with the most frequent substantial inversions during the Fall time of the year, coinciding with the time when the worst air pollution events occur in Allegheny County. And, while morning inversion strength tends to be greater in the latter four months of the year with depths their highest November through February, evening inversion strengths and heights generally peak in the winter and mid-year. Morning surface-based inversions break earlier in the day during the warm months of the year when the sunrise is earlier in the Northern Hemisphere.
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