Snow is a form of ice crystals that undergo changes on the Earth's surface. It starts when ice crystals form in the atmosphere, increase in size, precipitate and accumulate on surfaces, metamorphose in place, and ultimately melt, slide or sublimate away. Snowflakes nucleate around particles in the atmosphere by attracting supercooled water droplets, which freeze in hexagonal-shaped crystals. Snowflakes take on a variety of shapes, basic among these are platelets, needles, columns and rime. Accumulated snow metamorphoses over time, by sintering, sublimation and freeze-thaw.