How does rime icing occur on an aircraft?

Rime icing occurs primarily through the process of small supercooled water droplets freezing instantly upon contact with the aircraft's surfaces. These small droplets, which are typically found in clouds, have a temperature below freezing but remain in a liquid state due to their small size and the energy they possess. When they collide with an object like an aircraft, they solidify rapidly upon impact, forming a rough, white, and opaque ice accumulation known as rime ice. This type of icing is common in conditions where clouds are present with low temperatures, specifically at altitudes where supercooled liquid water is prevalent. The characteristics of rime ice include its irregular shape and lower density compared to other forms of ice, contributing to the specific challenges it poses for aircraft performance and safety.

In this context, the other options don't align with the fundamental mechanism of rime icing. Large droplets do not generally create rime icing because they tend to break up upon impact, and conditions with low humidity or elevated surface temperatures do not facilitate the immediate freezing necessary for rime ice formation.