What is necessary for the formation of structural ice?

The formation of structural ice occurs when certain conditions are met, most notably the presence of visible moisture combined with sub-freezing temperatures. This is because structural ice forms as water vapor condenses and freezes upon contact with a surface that is below 0 degrees Celsius (32 degrees Fahrenheit). In essence, visible moisture can include water droplets in the air, fog, or precipitation, which, when the temperature is sufficiently low, will freeze and adhere to surfaces, creating layers of ice.

In contrast, high humidity alone doesn't guarantee ice formation without the critical prerequisite of sub-freezing temperatures. Clear skies and low winds typically suggest a lack of moisture, which is necessary for ice formation, and heavy rainfall with warm temperatures would lead to liquid water rather than ice. Therefore, the specific combination of visible moisture with sub-freezing conditions is essential for the creation of structural ice.