One example of deposition is the process by which, in sub-freezing air, water vapour changes directly to ice without first becoming a liquid. This is how frost and hoar frost form on the ground or other surfaces. Another example is when frost forms on a leaf. For deposition to occur, thermal energy must be removed from a gas. When the air becomes cold enough, water vapour in the air surrounding the leaf loses enough thermal energy to change into a solid. Even though the air temperature may be below the dew point, the water vapour may not be able to condense spontaneously if there is no way to remove the latent heat. When the leaf is introduced, the supercooled water vapour immediately begins to condense, but by this point is already past the freezing point. This causes the water vapour to change directly into a solid.