With no wind the water on my hot tub cover froze. The temp. was above 40 deg F.. It was a mild day about 65 deg. A cold front came through and got windy that day but this AM wind was still. The water in the tub is about 101 deg.

The temp must have hit 32 degrees and the water there was just a tiny thin amount so froze quickly. Temps vary all over the place, town to town, high, low, over water, over pavement, where a thermometer is (which might not be accurate), and at the end of summer, the ground is still warm and might be warmer than the air on any given morning.