Definition
The formation of ice on an aircraft when it flies through a cloud containing supercooled water droplets. The droplets, though below freezing, remain liquid until they strike the airframe, at which point they freeze on contact and build up as ice on leading edges, propellers, antennas, and other exposed surfaces.
Plain English
Ice that forms on the aircraft because it is flying through a cloud full of tiny water drops that are colder than freezing but still liquid. When those drops hit the aircraft, they freeze onto it.
Context Anchor
Seen in weather briefings, icing forecasts, pilot reports, and in-flight decisions about whether to enter or avoid cloud layers near freezing temperatures.
Derivation
"Supercooled" is the key idea behind this term. Water in the open air can stay liquid well below 0°C if it has nothing to freeze onto. A cloud is full of such droplets. The aircraft becomes the surface they freeze onto -- hence ice forms in cloud, not in clear air.
Why Pilots Care
Ice on the wings and control surfaces reduces lift, increases drag and weight, and can quickly degrade handling qualities or lead to loss of control.
Grounding Statement
If you fly into a cloud and the outside air temperature is near or below freezing, expect ice to start forming on the aircraft.
Intuition Check
Do not assume a cloud must look stormy or contain visible ice for icing in cloud to occur. The key danger is tiny liquid water drops in the cloud that freeze only after they hit the airplane.
Example Sentence 1
The pilot requested a lower altitude after icing in cloud began forming on the wing leading edges.
Example Sentence 2
After entering the cloud layer the wings began to collect ice, confirming the presence of icing in cloud.