At what temperature and humidity can carburetor ice form?

Carburetor ice can form under a range of conditions, typically in conditions where there are low temperatures and relatively high humidity. The key factor in carburetor ice formation is the cooling effect created when fuel is vaporized in the carburetor, which can lead to temperatures dropping below freezing, especially when humidity levels are high.

In the context of the choices provided, the correct scenario occurs when the outside air temperature is 50 degrees Fahrenheit and the humidity is 90%. This situation creates ideal conditions for the formation of carburetor ice because the high relative humidity contributes to the potential for ice to form at temperatures above freezing.

At 50 degrees Fahrenheit with 90% humidity, the combination allows for significant cooling and condensation to occur in the carburetor, making it susceptible to icing. Thus, this answer accurately reflects the critical environmental factors that contribute to carburetor ice formation in aviation.