As far as I can tell, hoar frost is when water vapour in the air, what is a gaseous state, turns directly into a solid without passing through the liquid phase. This happens when the water vapour touches a cold surface. As this is the opposite of sublimation, which is when a solid is heated and turns straight into a gas without becoming a liquid, it is called desublimation. Rime ice is different because the water vapour turns into a supercooled liquid first before it then turns into a solid on hitting a cold surface. Rime is thus forms on the side of an object exposed to the wind. Now I need to find phase diagrams for H2O to show this.
