Snow falls from clouds that contain enough cloud droplets and ice crystals for precipitation to form. In order for snow to reach the ground without melting, the wet bulb temperature of the air near the ground must be below freezing. The wet bulb temperature is the temperature measured by a thermometer that has a little wet sock covering the bulb before it is swung in a circle in the air. Thus, you can have snow fall reaching the ground even when the air temperature is slightly above freezing.
What is Snow?
