Dear Tom,
Snow can only form in clouds when the temperature is below freezing, but what is the best temperature for snow to form?
Mary Jacob, Chicago

Dear Mary,
Snow can form only when the following three conditions are met: The presence of water vapor in air at sub-freezing temperatures; a mechanism to chill the air below its saturation temperature; conditions that allow ice crystals to grow into snow flakes that actually fall to the ground.

Airborne ice crystals spring into existence when water vapor gas condenses out of the air at sub-freezing temperatures. The amount of water vapor that can be present in air increases dramatically with increasing temperatures (up to 32 degrees Fahrenheit, after which condensation occurs in the form of liquid water droplets). This means the greatest amount of snow occurs when the temperature is as high as it can be for condensation to produce ice crystals — 32 degrees. Above 32 degrees, water vapor condenses into liquid water droplets rather than snow flakes.

Condensation must occur at 32 degrees or lower to produce snow but occasionally the temperature at ground level is a little above freezing when snow is falling. This means the surface air (above 32 degrees) exists in a shallow enough layer for snow originating in below-freezing air aloft does not have time to melt before falling to the ground.