Snow forms when moisture freezes onto microscopic particles of pollen or dust at the atmospheric temperature of 32 degrees Fahrenheit or lower. More moisture freezes to the original ice crystal, forming snowflakes. If the temperature toward the ground is also below freezing, the snow falls.
As water droplets freeze and form crystals, they make up symmetrical six-sided patterns that vary according to the temperature. For instance, snowflakes forming at 23 degrees Fahrenheit tend to have longer, thinner appendages, while snowflakes at colder temperatures form in flat patterns. The biggest snowflakes normally measure at less than 1/2 inch across, but if snowflakes clump and freeze together, they can reach almost 2 inches in diameter.
Because moisture is a basic requirement to the formation of snow, snowfall is heavier in humid and moist areas. An area in Antarctica that gets very little moisture remains cold, but typically receives little or no snow. In order for snow to fall to the ground and not just melt in the air, the ground must also be at freezing temperature.
Snowflakes that fall through dry air result in powdery snow that doesn't stick together, which is ideal for skiing but not great for building snowmen or snowballs.