In what condition does structural icing develop most often?

Structural icing typically develops when there is visible moisture present at or below freezing temperatures (0°C). This phenomenon occurs because the moisture, in the form of water droplets, can freeze upon contact with chilled surfaces, such as an aircraft's wings or other structures. As the temperature is at or below 0°C, these water droplets do not have enough energy to remain in a liquid state and quickly freeze, forming ice.

In contrast, clear skies and low humidity do not encourage the presence of moisture, which is necessary for icing to form. Rain above 0°C can be problematic for other types of weather phenomena but does not directly contribute to structural icing, as the conditions for icing require freezing temperatures. High-pressure systems are typically associated with stable and dry weather conditions, thus reducing the likelihood of visible moisture. Therefore, the presence of visible moisture at or below 0°C is the key factor that fosters the occurrence of structural icing.