What causes abrupt crystallization and ice formation when an aircraft flies through cumulus clouds?

Supercooled water is the correct answer because it refers to water droplets that remain in a liquid state even at temperatures below freezing. Within cumulus clouds, temperatures can drop significantly, allowing these droplets to exist in a supercooled state. When an aircraft enters a region of supercooled water, the temperature difference between the aircraft's surfaces and the droplets causes the droplets to freeze upon contact. This rapid crystallization forms ice, leading to abrupt ice formation on the aircraft.

Understanding this concept is crucial for pilots and those working with aircraft, as flying through cumulus clouds containing supercooled water can pose serious safety risks if ice accumulates on the wings, tail, or other critical surfaces of the aircraft.