How does snow form?

For snow to form you need two things. An atmospheric temperature below freezing and sufficient moisture in the air.

The process begins with the formation of ice crystals.

When the air temperature is below freezing, water vapour that would normally condense into cloud droplets will change directly into ice without ever becoming a liquid, forming around tiny particles in the air, such as a grain of sand or soot.

As these ice crystals form, they grow as more water vapour freezes onto their surface and they collide with other crystals as they move within the cloud.

As they grow the crystals become snowflakes, which are heavy enough to fall from the cloud

If the snowflakes pass through warmer air as they fall, they will begin to melt, turning to either sleet or rain.

Snowflakes that fall through moist air that is slightly above 0°C clump together easily forming the “classic” large fluffy flakes.

Snowflakes that fall through really cold and dry air become powdery and don’t stick together as easily creating the perfect ‘powder’ snow for skiing.

Once on the ground, snow will remain if the air and ground temperatures are cold enough to keep it from melting.