Which condition is necessary for structural icing to occur?

Structural icing occurs when certain atmospheric conditions align, primarily involving temperatures and moisture. For icing to happen, the air temperature must be below freezing, which allows supercooled water droplets in the air (often found in clouds or precipitation) to freeze upon contact with aircraft surfaces.

Visible moisture is crucial here; it refers to conditions such as rain, snow, or clouds that contain water droplets. When these droplets strike a surface that is at or below freezing temperature, they freeze instantly, forming ice. This phenomenon can significantly impact aircraft performance and safety, making it critical for pilots and operators to understand and anticipate.

The other choices present conditions that do not support structural icing. High temperatures with low moisture do not create enough conducive conditions for ice formation. High humidity and low pressure could correlate with various weather phenomena, but they don’t directly imply the necessary temperature and moisture conditions for icing. Warm air, although it often contains clouds, generally does not lead to icing because the temperatures are typically above freezing, making it unlikely for ice to form.