Which type of environmental condition can cause moderate to severe icing during flight?

Moderate to severe icing in flight is most commonly associated with the presence of visible rain near freezing temperatures. When an aircraft encounters rain droplets in temperatures at or below freezing, these supercooled droplets can freeze upon contact with the aircraft's surfaces, leading to the formation of ice. This is especially hazardous because it can accumulate quickly and affect the aircraft's aerodynamics and safety.

The conditions related to high humidity can contribute to icing, but they are not as directly correlated as the presence of visible rain at freezing temperatures. High altitude winds themselves do not produce icing; rather, they are associated with turbulence and other atmospheric effects. Lastly, clear skies generally indicate low moisture content in the atmosphere, which significantly reduces the likelihood of icing occurring. Thus, the critical factor in the correct answer is the specific scenario of visible rain occurring at or below the freezing point, which is a primary cause of in-flight icing issues.