All snow starts as tiny droplets of supercooled water, suspended high in the atmosphere. That’s the only way it can freeze into delicate crystalline snowflakes!
Those water droplets will stay liquid as long as they’re suspended in the air without touching anything. However, the moment they’re jostled by anything, even a tiny speck of dust, the water molecules will instantly crystallize around it. More and more water molecules will join in until the newly formed snowflake’s heavy enough to fall.
Once the snowflake enters a layer of air that’s above freezing, it starts to melt. It doesn’t do it immediately. First, it’s got to heat up to melting temperature, which takes a lot of heat energy by itself. Water has one of the highest heat capacities known to science. Even after it’s at melting temperature, it’s still got to absorb a lot of heat energy to get it past the transition point.
All of that takes time. During that time, the snow is steadily falling down. Now it’s a race against time to see if the snowflake will melt first or make it all the way down to the ground with its structure still intact. If it’s dry enough, some of the snowflake may go directly from solid to water vapour. This process is called sublimation.
Of course, it’s not nearly that simple. Humidity plays a major role as well.
As frozen water melts or sublimates and water vapour evaporates, it also cools down its surroundings. This is why you put ice in a drink to keep it cold, or hang a wet towel around your neck to keep cool when it’s hot.
So, like water droplets skittering in a hot pan, little bits of the snowflake are sacrificed to keep the rest of the snowflake cold. It’s now a matter of whether the snowflake can lose enough heat through melting, evaporation, and direct sublimation to keep it frozen, while at the same time it’s gaining heat from the warmer air.
That’s where the humidity factor kicks in. If it’s very damp, the snowflake can’t shed heat fast enough, and it melts into sleet or rain. But if it’s very dry, it survives all the way down.
That’s when snow’s going to fall, even when the air temperature’s above freezing. If there’s enough snowflakes coming down, they’ll even cool the ground right down and start accumulating, at least until they stop falling and the air temperature finally melts them. The snowball fights will end, and the knee-high snowmen will melt. But they had a great run while it lasted!