Many of us saw the first freeze of the season. If not that, then it was certainly the coldest morning of the season so far. DFW Airport, the official reporting station, managed to stay a few degrees above 32°. The winds were calm and the air was dry, ideal conditions for radiational cooling…where the heat in the ground radiates back into the upper atmosphere, cooling things off here on the surface. Lower elevations get colder generally more than locations in slightly higher elevations in these conditions. This is why Decatur, which is at a higher elevation than nearby Bridgeport, stayed above freezing last night…but Bridgeport sank to 25°. The main reason the Metroplex stayed a bit warmed overnight is what’s called the urban heat island effect. Essentially there is more concrete in the heart of the city that hold more heat during the day and retain more heat during the overnight.