When are clouds formed by ice crystals instead of drops of liquid water?
Clouds are formed by ice crystals instead of drops of liquid water when water vapor is raised above or transported to the freezing layer. This is usually done by warm air advection, or the horizontal transport of warm air. When warm air collides with a cold air mass, the moisture freezes to form ice crystals. Of course, there has to be a sufficient amount of ice nuclei for high level clouds, such as cirrus, to actually form.
