Erica's Answer:
That means the temperature has to be below the freezing mark not just at the surface, but also all the way up to the cloud where the snow is falling. If snow falls into a pocket of air where the temperature is above freezing, the snow will melt. Now, sometimes snow falls when the temperature at the surface is above freezing, for instance, 35 degrees. But the snow will melt when it hits the ground if the air temperature is above the freezing point.- Erica
