Clouds, fog, or dew will always form when
A) relative humidity reaches 100 percent.
B) water vapor condenses.
C) water vapor is present.
The answer to this question is B.
I know, it feels like a trick question, because in the lesson we teach that the ‘dew point’ is the temperature at which the air will have 100% humidity — it’s fully saturated with water vapor. And that when we reach the dew point, we’ll see clouds, fog or dew.
But Answer A only mentions the humidity, not the fact that the wator vapor needs to actually condense for clouds, fog or dew to ‘always’ form.
A humidity of 100% does not necessarily mean that rain is falling, or that there is dew or fog near the ground. It just means that the air is holding as much moisture as it can at a given temperature, in the form of water vapor. The water vapor doesn’t necessarily have to ‘condense’ when the humidity is 100%.
Answer A is partially right, but Answer B is a more correct answer. This is the kind of trickery to expect on your FAA Aeronautical Knowledge test!