Sublimation is the transition of a substance directly from the solid to the gas phase without passing through the intermediate liquid phase.
If the initial pressure of the ice at 0F is 0.0505 psia, heat transfer to the ice first results in an increase in temperature to 20F. At this point, however, the ice passes directly from the solid phase to the vapor phase in the process known as sublimation. Sublimation is a special term used for cases in which the transition between the solid phase and the vapor phase occurs directly, without passing through the liquid phase. Further heat transfer would result in superheating the vapor.