Water droplets which exist in liquid form at temperatures below 0°C.
"Supercooled large droplets (SLD) are defined as those with a diameter greater than 50 microns” - The World Meteorological Organization.
“Supercooled Large Drop (SLD). A supercooled droplet with a diameter greater than 50 micrometers (0.05 mm). SLD conditions include freezing drizzle drops and freezing raindrops.2 - FAA AC 91-74A, Pilot’s Guide to Flight in Icing Conditions
The freezing point of water is 0°C but it might be more accurate to say that the melting point of ice is 0°C. This is because, for a number of complex reasons, water exists in liquid form well below 0°C. Supercooled water exists because it lacks the ability to complete the nucleation process. Two of the factors influencing the freezing of supercooled droplets are the need for a freezing nuclei (usually ice crystals) and latent heat which is released when water freezes.
In "cold" clouds, where the temperature is below 0°C, ice crystals and supercooled droplets co-exist. In these 'mixed' clouds, the air is close to being saturated with respect to liquid water, but is super-saturated (an unstable phase) with respect to ice. Consequently, in mixed clouds, ice crystals grow from the vapour phase much more rapidly than do the nearby droplets. This is usually known as the Bergeron - Findeisen or "ice crystal" process.
- At temperatures between 0°C and -15°C most clouds are composed of supercooled water droplets.
- Between -15°C and -40°C most clouds contain a mixture of ice crystals and supercooled water droplets.
- Below -40°C almost all clouds consist entirely of ice crystals, with the notable exception of Cumulonimbus (Cb) clouds, in particular the “anvil clouds”.
Supercooled droplets are in an unstable state and usually start to freeze when brought into contact with ice crystals and particles with a similar structure to an ice particle (freezing nucleus). The ice crystals may form directly from water vapour in the cloud or fall into the cloud from above.
When water freezes, latent heat is released. This means that if a whole droplet was to freeze instantaneously, the latent heat liberated would, unless the initial temperature was below -80°C, raise the temperature of the droplet above 0°C which would be contradictory since ice cannot exist above 0°C. In fact only a small proportion of the drop freezes instantaneously, not more than enough to raise the temperature above 0°C. Further progressive freezing takes place as the droplet loses heat by evaporation and conduction.
When freezing of supercooled droplets takes place spontaneously, the larger water droplets tend to freeze more readily than smaller droplets. The freezing of droplets becomes more probable as the temperature decreases.
In aviation, small droplets will tend to freeze on the wing rather instantaneously, forming rough, white, opaque, ice deposits which are relatively easy to remove, called rime ice.
The latent heat released in the freezing process serves to warm the air immediately surrounding the droplet relative to the air surrounding the cloud, thereby promoting instability and upward development of the cloud.
Supercooled Large Droplets (SLD)
A term often used in discussions on in-flight airframe icing is the "supercooled large droplet". If an SLD is large enough, its mass will prevent the pressure wave traveling ahead of an airfoil from deflecting it. When this occurs, the droplet will encounter the airfoil surface, and because of its size only the part of the drop immediately hitting the airfoil may freeze. The rest of the droplet will be swept back by the airflow, and the droplet will freeze in the same manner as on initial contact, until all the droplet is frozen. This swept-back ice formation tends to leave a transparent, smooth, realtively difficult ice to remove, and is called clear ice.