Snowflakes come from many snow crystals (or ice crystals). Most are not even one-half inch across. Larger snowflakes that are close to two inches form under certain conditions. This usually requires near freezing temperatures, light winds and unstable atmospheric conditions. Snow is white because it reflects most of the sunlight. Visible sunlight is white. Usually, natural materials absorb sunlight. That act gives them their color.
Snow is a type of granular material because it is formed from small ice particles. Snowflakes come in a variety of shapes and sizes. Unless packed by external pressure, snow is an open and soft structure. But how exactly does it form? Snow has to start somewhere right? It doesn’t just pop out of thin air.
It starts high up in the Earth’s atmosphere. It starts with water in what is known as a cold cloud. A cold cloud being a cloud that exists in air that is either at or below freezing. The water condenses into a tiny droplet. More and more water vapor will condense onto the droplet’s surface. The droplet grows as this is happening. Afterward, the cold air will freeze the water into a tiny ice crystal.
This tiny ice crystal grows in two ways. It will either grow by coalescence or by deposition. If the ice crystal collides and then sticks to the water droplets in the cloud, it is growing by coalescence. If water vapor molecules in the cloud are freezing directly on the ice crystal, it is growing by deposition. The ice crystal bonds with other ice crystals and will eventually form six evenly spaced branches.
The crystal will at some point fall from the sky. It picks up more water vapor as it falls. This causes it to continue growing. While falling, the crystal can come into contact with warmer air. This warmer air makes it melt a little. Because of this melting, crystals bond to form larger flakes. The ones that many people like to think of as the fluffy snowflake. A crystal melting too much and then refreezing before hitting Earth’s surface, then it is sleet and not snow.
A snowflake will continue to change shape even after reaching the ground. It melts and bonds with the other flakes. Of course this causes the snowflake to lose all of its intricate patterns. In cold temperatures, the snow may form a crusted top. In warm temperatures, the snow may start to melt. If it starts to melt, it will start to become slushy.
National Snow and Ice Date Center: http://nsidc.org/snow/faq.html