Snow forms in clouds that are below freezing. Much of the rain that makes it to the surface started out as snow high aloft and then melted before reaching the ground. When saturated air rises it condenses. In other words the moisture turns from a gas and into a liquid. When saturated air cools the moisture condenses out similar to the way in which you will see moisture develop on the outside of a glass of ice-water. When it is below freezing the vapor turns into ice. Due to the nature of the water molecule the ice will develop into 6 sided crystals. If you look at a snowflake you will notice it is likely a bunch of 6 sided ice crystals joined together. When a snowflake gets big enough in a cloud then gravity will push it down toward the surface. The snowflake will continue to get bigger as it joins with other falling crystals. This is especially true if the snowflake is sticky. By sticky that means the snowflake has some liquid water coating on it. The biggest snowflakes tend to occur when the temperature is near freezing. When it is very cold (well below freezing) the snowflakes tend to be smaller and they do not stick together as well.

To get snow the temperatures need to be cold enough to allow the snowflake to fall to the surface without melting. There also needs to be clouds that build up enough to develop the snow. Many snow events occur due to either rising moisture on the cold side of a low pressure system, upslope rising air that produces snow, and snow to the north of a warm front. Snow is so beautiful to watch. I look forward to each time it snows.