Snow is made of tiny ice crystals that form unique snowflake shapes. The word "snow" originated from the Old English word "sniwan" over 700 years ago. For snow to settle and accumulate on the ground, the temperature needs to be 3 degrees Celsius or lower. Avalanches occur when a heavy slab of accumulated snow slides down a sloped mountainside, often after a large snowstorm. Snow and hail are different, as snow is made of ice crystals while hail is frozen raindrops. As more snow falls and freezes on top of existing snow, it turns the snow into ice.