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

The formation of ice during flight through cumulus clouds can be attributed to supercooled water. Supercooled water exists at temperatures below freezing but remains in liquid form. When an aircraft enters a region containing these supercooled droplets, the rapid cooling of the aircraft surface can cause the droplets to freeze almost instantaneously upon contact. This results in an abrupt crystallization process and the formation of ice.

Supercooled water is particularly dangerous in aviation because it can lead to ice accumulation on wings and control surfaces, which can negatively impact the aircraft's performance and handling characteristics. The presence of this phenomenon in cumulus clouds is tied to the cloud's structure and the temperature gradients present in the atmosphere.

In contrast, melted droplets, frozen precipitation, and high humidity do not directly contribute to the same kind of rapid ice formation. Melted droplets typically wouldn’t freeze abruptly upon contact, frozen precipitation refers to ice that has already formed and does not involve supercooling, and while high humidity is relevant to cloud formation, it is not the primary cause of ice crystallization when flying through supercooled environments.