Why Does Sublimation Happen?

Some elements and substances under certain conditions have certain characteristics which results in them turning to the gaseous state from the frozen state directly. A classic example is dry ice or frozen CO2. Carbon Dioxide at room temperature and pressure prefers to directly convert to its gaseous state. It is different from evaporation which involves liquids turning to vapour; sublimation avoids the liquid state completely.

Another unique example is that of a comet. When the comet comes in proximity to the sun the heat from the sun allows the frozen gases located near its nucleus to sublimate. This is how the comet gets its tail.

Sometimes conditions can be induced to promote sublimation such as in a frost free freezer. The temperature being sub zero and the constant circulation of air creates an arid environment where the sublimation process is accelerated.