What condition must be present for carburetor ice to form on an airplane?

Carburetor ice forms when the temperature and pressure inside the carburetor are right. The air flowing through the carburetor cools the fuel, causing it to vaporize. This process absorbs heat from the air, which can cause the temperature inside the carburetor to drop below freezing. If the air is also humid, the water vapor in the air can freeze on the cold surfaces inside the carburetor, forming carburetor ice.