What is a necessary condition for structural icing to form during flight?

Structural icing occurs when supercooled liquid water droplets come into contact with the aircraft's surface, and for this interaction to happen, visible moisture is essential. Visible moisture can manifest in various forms, including clouds, fog, or rain; however, it specifically signifies the presence of water droplets in the atmosphere that can remain in liquid form even at temperatures below freezing.

Cold temperatures can contribute to conditions that allow icing, but without the presence of visible moisture, there would be no source of liquid water to freeze upon impact with the aircraft. Clear skies often indicate a lack of moisture in the air, thus reducing the likelihood of icing formation. While altitude can affect temperature and moisture conditions, it does not directly contribute to the necessary conditions for icing to occur in the absence of visible moisture. Hence, visible moisture is a critical component for structural icing to form during flight.