What relative humidity percentage increases the likelihood of carburetor ice formation?

The likelihood of carburetor ice formation is significantly increased in conditions where relative humidity is at least 80 percent. At this humidity level, the amount of moisture in the air can lead to cooler temperatures within the carburetor as fuel vaporizes. This cooling effect can cause condensation of moisture, leading to ice formation in carburetor components, which can obstruct the airflow into the engine.

While lower humidity levels may also contribute to ice formation, it is at 80 percent and above that the risk becomes pronounced due to the saturation of the air with moisture, making conditions conducive for ice to form in the carburetor during the cooling process after fuel expansion.