“Supercooled” refers to the temperature of the water droplet- it’s cooled below freezing yet remains a liquid. How is that possible you ask? I’m so glad you asked…it’s because the freezing of pure liquid water into ice does not happen until -40°F (Which actually is also -40°C by the way). You may be thinking…than why do we call 32°F the freezing point? Well we really shouldn’t call it that, but rather 32°F should be called the melting point of ice. Ice will always begin to melt (no matter how slowly) at 32°F or warmer. But pure liquid water will not always freeze at 32°F or colder.
Notice I’ve said “pure” liquid water. That’s the catch. At 32°F liquid water can freeze if there is an ice nuclei present. Basically, an ice nuclei is something the liquid water can freeze onto. Ice nuclei can be as tiny as a dust particle or ice crystal in the cloud and once liquid water touches that ice nuclei, the liquid water will freeze rapidly into ice crystals. Because the dust or soot have a similar crystal structure to ice, they act as ice nuclei. Sometimes an airplane will act as the ice nuclei and the supercooled water droplets suspended in a cloud will freeze onto the plane causing icing.
Supercooled water droplets are also what fall from the sky during an ice storm. In this case they typically form as snow high above the ground then fall into a layer of air above 32°F and melt. Then as they fall closer to the surface, the air gets colder than 32°F. At this point they become supercooled water drops and will freeze on contact with power lines, sidewalks and streets, basically any object that is colder than 32°F. All of this is due to the warm layer of air in the mid levels of the atmosphere that melted the snow.