It's sunny in winter

The conditions for frosting on the ground are: clear night, no wind or light breeze, frost is easy to appear when the ground temperature is lower than 0℃. Its appearance does not depend on temperature. Usually, the outdoor temperature we refer to refers to the air temperature in the louver at a height of 1.5 meters from the ground, which is obviously different from the ground temperature (also known as the surface temperature). On a clear night in winter, the ground constantly emits long-wave radiation into the atmosphere, and the direct radiation of the sun is zero. However, because the sky is clear, the atmospheric inverse radiation is weak, and the effective radiation on the ground is positive, although the temperature is higher than 0℃, the ground temperature is lower than 0℃, so frost appears.