Supercooling is the process by which a liquid or gas is cooled past its freezing temperature without it becoming a solid. Under normal freezing conditions, such substances would freeze into crystals around a seed nucleus, a process known as heterogeneous nucleation. When a seed crystal or nucleus does not exist, the substance can remain a liquid until the point of homogeneous nucleation, which occurs at a much lower temperature.
Pure water normally freezes at 273.15 K (0 °C or 32 °F) but it can also be “supercooled” to -43.6 °F (-42 °C). Water must be distilled for supercooling to take place. Its transition temperature is much colder and harder to determine, but studies estimate it at about 165 K (−108 °C/−162.4 °F). Glassy water can be heated up to approximately 150 K (−123 °C/−189.4 °F).
Droplets of supercooled water often exist in stratiform and cumulus clouds. Aircraft flying through these clouds seed an abrupt crystallization of these droplets, which can result in the formation of ice on the aircraft’s wings or blockage of its probes, unless the aircraft are equipped with an appropriate de-icing system. Freezing rain is also caused by supercooled droplets.
The opposite process of supercooling (the melting of a solid above the freezing point) is way more difficult and a solid will almost always melt at the same temperature for a given pressure. This is the reason why the melting point is identified by using melting point technology. It is, however, possible, at a given pressure to superheat a liquid above its boiling point without it becoming gaseous.
As supercooling is usually confused with freezing point, it is necessary to know that there’s a difference between them both. Supercooling is the cooling of a liquid below its freezing point without it becoming solid and freezing point is when a solution can be cooled below the freezing point of the corresponding pure liquid due to the presence of the solute (eg: it occurs when sodiumchloride is added to pure water).