system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is snow formed?
gpt
way up in the clouds, it's very cold. Water vapour begins freezing piece by piece to tiny particles of dust (or something) called a "Nucleus." 

Once those particles get too big and heavy, they fall to the ground. If the temperature is high enough, that snow will warm and melt as it falls to become rain.

If it is winter, that melting doesn't happen.