Why Does Rain Begin as Snow?

Step 1 of 5

The top of a rain cloud is freezing, even in summer.

Air cools steadily with height, and a few kilometres up it is twenty degrees below zero regardless of the season. A tall summer cloud has its head in that air, so its upper half is full of ice while its base is warm enough for shirtsleeves.

Step 2 of 5

Water can stay liquid well below freezing.

A cloud droplet has nothing to start freezing on, and freezing has to start somewhere, so it can sit at minus ten and stay stubbornly liquid. Give it a speck of dust of the right shape and it turns to ice on the spot.

Step 3 of 5

An ice crystal steals water from the droplets around it.

Water escapes more easily from a liquid surface than from an ice one, so in a cloud holding both, the droplets quietly evaporate and their vapour lands on the crystal. The crystal grows on exactly what its neighbours are losing.

Step 4 of 5

That is far faster than droplets merging.

Growing by bumping into other droplets is slow and often never gets anywhere, which is why plenty of clouds sit all day and do nothing. Growing on stolen vapour takes a crystal to falling size in minutes, and it is how most rain on Earth is actually made.

Step 5 of 5

Then it melts on the way down and you never know.

Below the cloud the air is above freezing, and a falling crystal melts long before it reaches you. Only when that warm layer is thin or missing does the ice survive the trip and land as something you can see it was.

Tropical clouds are warm all the way to the top and have no ice stage at all, so they must do it the slow way by collision — which is part of why tropical rain arrives in such different bursts.