ATPL Meteorology Practice Test 2025 - Free Pilot Meteorology Practice Questions and Study Guide

Rime ice forms when supercooled water droplets freeze rapidly upon contact with a cold surface, such as an aircraft's leading edge. In this scenario, the supercooled liquid water droplets from the cloud are carried into the airflow and freeze almost instantly when they collide with the aircraft's surface, creating a rough and opaque layer of ice known as rime ice. This type of ice typically has a white or milky appearance due to air trapped within the ice crystals, which are small and granular in nature.

Clear ice, in contrast, forms when the water droplets freeze more slowly, allowing them to flow and create a smoother, transparent layer. Hoar frost forms under different conditions, often in clear, cold nights when water vapor sublimates directly onto surfaces, creating a delicate crystalline structure. Mixed ice refers to a combination of these types of ice and is generally used to describe scenarios where both clear and rime ice are present, but the question specifically highlights the mechanism of instant freezing at the leading edge, which is characteristic of rime ice formation.