Snow occurs when water droplets in clouds freeze, and these droplets then act as a nucleus onto which molecules of water vapor adhere, forming larger ice crystals. Once the growing snowflake is too heavy for the movement of air in the clouds to support, it falls to the ground. Snow can form in any clouds that are below freezing temperature but only does so under the right conditions.
Clouds below freezing temperature do not automatically produce snowflakes. This is because water droplets do not necessarily freeze even at temperatures below zero degrees Fahrenheit. Water droplets in clouds can be supercooled because their molecules must randomly encounter each other in the right way to begin the freezing process. In clouds warmer than minus 31 degrees Fahrenheit, this requires a tiny solid particle to act as a nucleus for crystallization. Particles of clay, dust or biological substances can fill this role, as well as human-introduced nuclei such as silver nitrate or dry ice.
Individual snowflakes vary in shape by the temperature at which they formed. They alternate between flat, plate-like shapes and longer, thin shapes as temperature decreases. The nuclei on which the snowflakes formed also influences their final shape. Most snowflakes are visibly irregular.