Snowflakes are born inside clouds at high elevations, where temperatures plunge to well below the freezing point of water. Silicate materials – clay minerals and micas – can act as the core of a fledgling snow crystal.
Water molecules join together in a rigid pattern – a tiny ice structure that’s the heart of a snow crystal. The crystal grows by continuing to collect water vapor, and by the attachment of water droplets to its surface.
As it grows, it gets heavier. It starts falling through the cloud. On its way down, it encounters millions more drops of water. The ice crystal gets bigger as the droplets in the cloud give their water to it. All over the cloud, the number of ice crystals multiplies – and soon the whole cloud might be ice. If the flakes of ice get heavy enough, they might fall out of the cloud, drift down to Earth, and blanket the ground as snow.