When does Frost form?

Frost typically forms when the temperature of an aircraft's surface, as well as the dew point, falls below freezing and there is high humidity in the environment. This is due to the condensation process, where moisture in the air condenses onto cooler surfaces, and when that moisture is at or below freezing temperatures, frost develops.

High humidity indicates that there is a significant amount of moisture available in the air, which can condense and freeze on surfaces once the temperatures drop accordingly. In contrast, low humidity would not provide enough moisture for frost to form, even if temperatures are at or below freezing. Similarly, temperatures rising above freezing would prevent frost formation, as any existing frost would typically melt. Lastly, while having the dew point and air temperature the same can indicate saturation, it does not directly cause frost unless both are below freezing, which is not the only requirement for frost to form. Hence, the occurrence of frost is closely tied to both surface temperature, dew point, and humidity levels, confirming the correctness of the selected answer.