What environmental condition must exist for airframe icing to occur?

Airframe icing occurs when there is a combination of temperature at or below freezing and visible moisture in the environment. This combination is crucial because icing requires moisture in the form of water droplets, which can be present in clouds, precipitation, or high humidity conditions. When the temperature is at or below freezing, these water droplets can freeze upon contact with the aircraft's surfaces, leading to the accumulation of ice.

In conditions where moisture is absent, even if the temperature is below freezing, ice formation is unlikely to happen because there are no water droplets to freeze. Therefore, identifying both cold temperatures and the presence of moisture forms the foundational criteria for understanding how environmental conditions contribute to airframe icing.