Ice crystals that fall from the sky as precipitation are called snowflakes. Snowflakes take several forms, depending on how the ice crystals fall and the temperatures of both the air and ground.
Snow forms when droplets of liquid condensation in clouds freeze to create a crystalline structure. Snow forms best when air is able to push upward, carrying moist, often warmer, air into colder parts of the atmosphere to begin condensation. Lake-effect snowfall is common around larger bodies of water; the warmer air of the lake evaporates and is borne high into the atmosphere, where it condenses and freezes to form ice crystals. Snowflakes are simple ice crystals, however, snowflakes may pass through additional clouds on the way to the ground. When this occurs, condensation from the clouds freezes to the existing snowflakes to form round, crumbly pellets, or graupel.
Hoarfrost forms when a surface is colder than the surrounding air. In this situation, water vapor from the air freezes directly, forming a solid without condensing first. Hoarfrost sometimes forms on the surface of existing snowfall.
Earth is not the only planet in the solar system to experience snow. Research indicates that Venus has snow-capped mountains as well, but the temperature of Venus is far too hot for snow to form as frozen water. Venus's snow is actually metallic; it forms from a mist of lead sulfide and bismuth sulfide.