“I thought cold air was supposed to sink. Why was there frost on my roof, but not on the ground?” a listener commented. There could be several explanations for that condition. One is that the ground is much warmer than the air. The radiant heat given off by the ground is keeping the lowest areas of the atmosphere warmer than the air at rooftop levels. This would trap a warm layer near the ground and prevent frost from forming. Another is that at the rooftop level, radiational cooling is occurring at a faster rate than at the ground, thus permitting the temperature to reach dew point more quickly – forming frost if the temperatures are cold enough.
Mary Knapp, Weather Data Library