Definition
Liquid water droplets that remain in liquid form at temperatures below freezing (0°C / 32°F). They are unstable, and freeze on contact when they strike an aircraft surface, forming structural ice.
Plain English
Tiny drops of water in the clouds that are colder than freezing but somehow still liquid. The moment they hit something — like a wing — they instantly turn to ice.
Context Anchor
Seen in instrument flying and weather discussions about structural icing in clouds, freezing drizzle, and freezing rain.
Derivation
‘Super-cooled’ means cooled below the normal freezing point without actually freezing. The prefix ‘super-’ here comes from Latin meaning ‘beyond’ — so ‘beyond cooled,’ i.e., cooled past the point you'd expect freezing to occur. This helps explain why the droplets are unstable: they are past the point where they should be ice, and the smallest disturbance (like striking a wing) triggers freezing.
Why Pilots Care
Contact with an aircraft causes these droplets to freeze instantly, building ice that reduces lift and can lead to loss of control.
Grounding Statement
Picture flying through a cloud at -5°C. The cloud looks like any other cloud, but every droplet inside it is liquid water waiting for something to freeze onto. Your wing provides that something.
Intuition Check
Do not assume super-cooled droplets are already ice. They are still liquid water until they freeze, often when they hit the aircraft.
Example Sentence 1
The pilot reviewed the icing forecast and noted the cloud layer at 8,000 feet was likely full of super-cooled droplets.
Example Sentence 2
Flying through visible moisture at sub-freezing temperatures often means encountering super-cooled droplets that can rapidly coat the airframe.