What happens inside a cloud?
Micro-scale Processes - Ice Crystal Formation

The formation of ice crystals is essential for precipitation in middle and high latitudes, where clouds extend upward into regions where the air temperature is well below freezing ("cold clouds").

As with condensation, freezing requires a nucleus. In pure water droplets, molecules inside the droplet will join together in a rigid pattern to form an ice embryo when temperatures are just below freezing. However, the process of freezing releases heat. This and other thermal agitations break up the ice embryo, so the droplet needs to be below freezing in order for the entire droplet to freeze.

Small cloud droplets begin to freeze between -15 and -9 C (5 to 16 F), and large cloud droplets begin to freeze between -8 and -4 C (18 to 25 F). In tall clouds, there is a combination of ice and water between -40 and 0 C (-40 to
32 F). Above -40 C (also -40 F), the cloud is "glaciated", meaning it only contains ice.

Next page   -> Micro-scale processes - ice crystal formation continued

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