A temperature inversion occurs in the Salt Lake Valley when cold, dirty air gets trapped under warmer air. If there is no wind or a storm, the inversion can last for weeks. Snow doesn't melt, the air gets difficult to breathe, fog builds up and crystalizes on everything, and it stays bitter cold. I recall one winter when I was 16 when the inversion didn't lift for roughly a month; some days we couldn't even see across the street clearly.
Well, today in the Salt Lake Tribune, this article claims there is now a center that can predict these inversions a month in advance.
But most Salt Lake residents have always been able to predict temperature inversions. We call it "January."