In most cases, snow crystals form on microscopic particles, called ice nuclei, which are present in clouds with below-freezing temperatures. Snow crystals may develop when water vapor is deposited directly on ice nuclei. They may also form when the ice nuclei cause tiny drops of supercooled water to freeze. Supercooled water is water that has remained unfrozen at below-freezing temperatures. However, at temperatures below -40 degrees F (-40 degrees C), supercooled water freezes without the presence of ice nuclei.
