Definition

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

Description

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.