Snow forms in clouds when the temperature is within the range -20°C to —40°C (-4°F to —40°F). Ice crystals in the clouds begin to melt and join together with super-cooled water droplets. They then freeze together and form into snowflakes which, provided the air temperature is low enough, fall from the clouds. The process of forming snowflakes is called accretion.
Whether winter storms produce snow relies heavily on temperature, but not necessarily the temperature we feel here on the ground. Snow forms when the atmospheric temperature is at or below freezing (0 degrees Celsius or 32 degrees Fahrenheit) and there is a minimum amount of moisture in the air. If the ground temperature is at or below freezing, the snow will reach the ground. However, the snow can still reach the ground when the ground temperature is above freezing if the conditions are just right. In this case, snowflakes will begin to melt as they reach this higher temperature layer; the melting creates evaporative cooling which cools the air immediately around the snowflake. This cooling retards melting. As a general rule, though, snow will not form if the ground temperature is at least 5 degrees Celsius (41 degrees Fahrenheit).
While it can be too warm to snow, it cannot be too cold to snow. Snow can occur even at incredibly low temperatures as long as there is some source of moisture and some way to lift or cool the air. It is true, however, that most heavy snowfalls occur when there is relatively warm air near the ground—typically -9 degrees Celsius (15 degrees Fahrenheit) or warmer—since warmer air can hold more water vapor.
Because snow formation requires moisture, very cold but very dry areas may rarely receive snow. Antarctica’s Dry Valleys, for instance, form the largest ice-free portion of the continent. The Dry Valleys are quite cold but have very low humidity, and strong winds help wick any remaining moisture from the air. As a result, this extremely cold region receives little snow.
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