If the ground temperature is lower than 32 degrees Fahrenheit when snow flakes leave the clouds, it reaches the ground as snow. When the temperature on the ground is above 32 degrees Fahrenheit, the snow flakes melt and form rain.
Most precipitation begins in the form of snow flakes in the atmosphere. Whether they stay as snow or turn to rain depends upon the ground and air temperature they pass through. On occasion, a warm layer of air that the snow must pass through is so shallow that the snow reaches the ground before it melts, even when surface temperatures are above freezing.