In what weather conditions is de-icing typically required?

De-icing is typically required during or after snowfall, freezing rain, or freezing fog because these weather conditions create the potential for ice accumulation on aircraft surfaces. Ice can significantly affect the performance, control, and safety of an aircraft during takeoff and landing. When snow falls or precipitation freezes, it can form a layer of snow or ice that needs to be removed before an aircraft can operate safely. The presence of freezing fog can also lead to the formation of ice due to the high humidity and low temperatures, exacerbating the need for de-icing procedures.

In contrast, heavy rains do not generally require de-icing since the rain tends to wash away any ice or snow that might be present. Clear sunny days typically do not produce conditions conducive to ice formation, making de-icing unnecessary. Thunderstorms may involve heavy precipitation and wind but do not directly correlate with the need for de-icing unless they lead to a change in weather that results in freezing conditions.