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

Carburetor ice forms in an airplane when the temperature and humidity conditions are conducive to ice formation, typically when the outside air temperature is between 20°F and 70°F (-7°C to 21°C) with high humidity. The cooling effect from fuel vaporization and the drop in pressure within the carburetor can lead to temperatures below freezing, allowing moisture in the air to freeze. This ice can obstruct the fuel-air mixture, leading to engine performance issues. Pilots should be aware of these conditions and take preventive measures when operating in such environments.