In which environment is aircraft structural ice most likely to accumulate quickly?

Aircraft structural ice is most likely to accumulate quickly in a freezing rain environment. When an aircraft flies through freezing rain, supercooled liquid droplets are present in the atmosphere. These droplets are in liquid form and can remain in a liquid state even when the air temperature is below freezing. When these droplets come into contact with the aircraft's surfaces, they freeze immediately, leading to rapid accumulation of ice. This can create significant hazards, including increased weight, altered aerodynamic properties, and potential for control difficulties.

The other environments listed may be cold, but they do not offer the same rapid ice accumulation potential as freezing rain. For instance, while freezing drizzle could lead to ice accumulation, it typically involves smaller droplets and a lower intensity than freezing rain. Cumulus clouds at below freezing temperatures can create ice as well, but again not as effectively as freezing rain. In clear skies with low temperatures, ice accumulation is generally minimal because there are fewer moisture-laden clouds to provide the necessary supercooled droplets that pose the greatest risk for ice build-up.