Snow

Ice crystals grown from vapour, and what happens after they land.

Snow forms when water vapour deposits directly into ice around a nucleus, skipping the liquid phase. The crystal grows outward, and its shape depends on the temperature and humidity it grows in — plates, columns, needles and the familiar dendrites all come from different conditions.

Why six sides

Water molecules bond into a hexagonal lattice as they freeze. That lattice is the template, so growth is six-fold from the start. The branches match one another because all six faces of one crystal experience nearly identical conditions as it falls — not because of any organising force.

Snowpack

Fallen snow does not stay as it landed. Crystals lose their arms, settle, and bond at contact points in a process called sintering, so the pack densifies over days. Layers from separate storms remain distinguishable, and the weak layers between them are what matter for stability.

Avalanches

A slab avalanche happens when a cohesive layer sits on a weak one and the bond fails. Most occur on slopes between about 30 and 45 degrees — shallower and the snow does not slide, steeper and it sloughs continuously without accumulating. Recent heavy snowfall, rapid warming and wind-loaded lee slopes all raise the risk.