Cloud droplets must grow in volume by a factor of about a million before they become raindrops.

Collision-coalescence growth of cloud droplets.

Cold clouds often have supercooled water droplets.

At saturation (in=out) the vapor pressure over water is higher than over ice..

Due to the difference in saturation vapor pressure, ice crystals grow at the expense of liquid water droplets.

Ice crystals also grow by collecting liquid droplets (accretion) and other crystals (aggregation).
These images were taken from snowcrystals.com; they have additional pictures and good information.


Ice crystal shape depends on temperature.


Example of the stellar-plate type of crystal.


Example of the stellar-dendrite type of crystal.


Example of the plate-type of crystal with dendrites.


Example of a bullet rosette type of crystal.


Example of the capped-column (column & plates) type of crystal.


A lightly rimed sectored plate.


A heavily rimed dendrite.


Irving Langmuir and Vincent Schaefer at the cloud cold chamber.


Seeding a stratus cloud.


Bernie and Kurt Vonnegut.