Ilmapakastus

Ilmapakastus, also known as air freezing or atmospheric freezing, refers to the natural phenomenon where atmospheric water vapor directly transitions into ice crystals without first becoming liquid water. This process is a form of deposition, a phase transition where a substance changes from a gas to a solid without passing through the liquid phase. It is distinct from freezing, which involves the transition from liquid to solid.

This phenomenon is most commonly observed in the formation of frost on cold surfaces. When the temperature of a surface is below the frost point and the air is saturated with water vapor, the vapor can deposit as ice crystals directly onto the surface. This is why frost can form even when the temperature is below freezing, but before it has rained or snowed.

Ilmapakastus plays a role in the formation of certain types of clouds and precipitation, particularly in very cold atmospheric conditions. For instance, it contributes to the growth of ice crystals in cirrus clouds and the formation of diamond dust, which are tiny ice crystals that fall from a clear sky in extremely cold weather. The specific conditions of temperature and humidity are crucial for ilmapakastus to occur.