system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does it snow and not hailing when its freezing outside?
gpt
Hail requires a pretty specific temperature in the atmosphere.

When it's cold, water begins to freeze.  This causes ice crystals to form.  These crystals stick together and form snowflakes.  Eventually, these become heavy enough that they fall from the sky.

Hail is a bit different.  For hail, a small raindrop begins to form.  This then begins to sink to a lower cloud level.  Here, the air is freezing, so the raindrop freezes.  But, because ice is less dense than water, the frozen drop can actually move back up to the warmer, wet layer of clouds.  Here, it collects more water, which makes it sink back down.  This water freezes, making it less dense so it floats back up.  The hailstone keeps going up and down between these layers of cloud, building up layers of ice like a gobstopper.  So, for hail to form, we need it to be just around freezing in the atmosphere.  Too cold, and that water layer doesn't exist.