What type of weather condition can cause moderate to severe icing during flight?

The presence of freezing temperatures combined with supercooled droplets is critical for the formation of moderate to severe in-flight icing. Supercooled droplets are liquid water droplets that exist at temperatures below the freezing point. When an aircraft flies through a cloud containing these droplets, they can impinge on the aircraft surfaces and freeze upon contact with the cold surface of the aircraft, leading to ice accumulation. This type of icing can be particularly hazardous as it can affect the aircraft's ability to fly safely, altering the aerodynamics and increasing weight.

In contrast, clear skies don't provide moisture necessary for icing, high humidity with warm air generally keeps water in its liquid state and avoids freezing conditions, and calm winds don't contribute to the circulation of air required to encounter supercooled droplets. Hence, the combination of freezing temperatures with supercooled droplets presents the most significant risk for icing conditions during flight.