At what temperature is carburetor ice most likely to occur?

Carburetor ice is most likely to occur at temperatures below 70 degrees Fahrenheit due to the principles of ventilation cooling and the effects of fuel vaporization. When air passes through the carburetor, the pressure drop associated with the venturi effect causes the temperature of the air-fuel mixture to decrease. This temperature drop can lead to the condensation of moisture present in the air, which can then freeze, forming ice.

While carburetor icing can occur at temperatures below freezing (32 degrees Fahrenheit), it is important to note that it can also happen at higher temperatures, particularly when humidity levels are high. The critical factor is the combination of temperature and relative humidity, with conditions being ripe for icing as air temperatures drop significantly below 70 degrees Fahrenheit. Therefore, the correct answer reflects the upper limit where the risk of carburetor ice begins under humid conditions, making it especially relevant for pilots to monitor conditions when operating in warmer temperatures which may still support icing, rather than solely focusing on colder temperatures.