In order for snow to form, the atmospheric temperature must be 32 degrees Fahrenheit or below. If the temperature increases too much as the snow falls, it turns into sleet or rain. As long as the ground temperature is at 41 degrees Fahrenheit or below, the snow should eventually stick.
At 32 degrees Fahrenheit, water vapor in the atmosphere freezes, turning into tiny ice crystals. These collide to form snowflakes, which are too heavy to remain aloft. Most heavy snowfalls take place when there is modestly warm air, which holds more moisture, near the ground that melts the edges of the flakes, causing them to refreeze with others. Windy air or air below 19 degrees Fahrenheit produces smaller, dryer snowflakes that do not stick together as well.