What conditions lead to structural icing forming?

Structural icing commonly occurs when there are specific atmospheric conditions that promote the formation of ice on an aircraft or unmanned aircraft system (UAS). The correct answer encompasses the scenario where clouds or rain are present at temperatures at or below the freezing mark (0 degrees Celsius).

When an aircraft flies through cloud cover or rain in these temperature ranges, supercooled water droplets can exist. These droplets remain liquid despite being below freezing due to their physical properties. Upon contact with the aircraft surface, they freeze rapidly, leading to structural icing. This situation is particularly dangerous, as the ice accumulation can alter the aircraft's performance and handling characteristics.

Understanding the conditions that lead to icing is crucial for any pilot or UAS operator. While rain at temperatures above freezing, snowfall under specific conditions, or clear, cold skies do not generally result in structural icing, the combination of clouds or rain at freezing temperatures presents a high risk for ice formation, making option B the most relevant choice.