Snow is a meteorological event that most often occurs in cold environments such as the Earth’s North and South Poles. It is frozen precipitation that falls from the sky like rain, but instead of being made of water droplets, it is made up of ice crystals. Snow appears white when it compiles on the ground because the ice crystals reflect sunlight equally in all directions. The type of snowflake that we are most familiar with – the six-sided symmetrical flake – only forms under very specific temperature and humidity conditions. When these conditions aren’t met, other types of flakes can form, including irregular ones or those with hollow centers.
The process by which snowflakes form begins when water vapor in the atmosphere condenses onto a particle of dust or pollen. This creates a tiny droplet of water around the particle, which then freezes into an ice crystal. As more water vapor condenses onto this central nucleus, the ice crystal grows larger and complex shapes begin to emerge. The final shape of a snowflake depends on several factors, including temperature and humidity. At warmer temperatures (near freezing), snowflakes tend to be smaller and less symmetrical than at colder temperatures (well below freezing). And at very high humidities, snowflakes can even stick together to form clumps called graupel.
Once formed, snowflakes begin to fall through the atmosphere towards Earth’s surface. During their descent they might pass through areas with different temperatures and humidities, which can cause them to change shape slightly. They also might collide with other flakes or objects, which can break them apart or cause them to stick together (this is how snowball fights get started!). Eventually they will reach a layer of air near Earth’s surface where the temperature is below freezing point and they will melt into tiny drops of water again. If enough flakes accumulate in one place – on trees, rooftops, cars or even people! – we call it a “snowfall”.