Snow forms in the atmosphere when the proper temperature and moisture conditions are met. For snow to materialize, temperatures must be below freezing, or around 32 degrees Fahrenheit, and there must be a slight amount of moisture present in the air. In order for snow to have the best chance of reaching the ground, temperatures must be at or below freezing.
Once snow crystals develop in Earth's atmosphere, they expand by merging with surrounding water droplets. Snow is much less likely to occur in areas that are dry and cold, since the formation of snow depends not only on the right cold weather conditions, but also on the presence of moisture in the air. The majority of snowflakes are less than 1/2-inch across, but they can form as large as 2 inches under the right conditions.