Definition
Liquid water droplets that remain in liquid form at temperatures below 0°C (32°F). When these droplets strike a surface such as an aircraft, they freeze on contact, producing structural icing.
Plain English
Tiny drops of water that are still liquid even though they are colder than freezing. As soon as they hit something solid, they turn to ice almost instantly.
Context Anchor
Seen in cloud, weather, and icing discussions, especially when an aircraft is flying through visible moisture in below-freezing temperatures.
Derivation
‘Supercooled’ means cooled below the normal freezing point without actually freezing. Pure water in still air can stay liquid well below 0°C because freezing needs something to start the ice forming — like a surface or a particle. An aircraft flying through such droplets provides exactly that trigger.
Why Pilots Care
When an aircraft flies through them, the droplets freeze on contact, rapidly forming ice that reduces lift and increases drag.
Grounding Statement
Picture a cloud at -5°C full of liquid droplets just waiting for something to bump into. Your wing flies through, and each droplet freezes the moment it touches the leading edge.
Intuition Check
Do not assume water below 32 °F is always ice. Supercooled water droplets are below freezing but still liquid until something causes them to freeze, such as contact with the aircraft.
Example Sentence 1
The pilot avoided the cloud layer because temperatures aloft and visible moisture suggested supercooled water droplets and a high risk of airframe icing.
Example Sentence 2
Forecasts warned of supercooled water droplets along the route, so the crew activated the de-icing boots before entering the clouds.