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Effect of humidity on air density

Remote Pilot (Part 107) practice question 8 of 25 on Weather / Effects on Performance. ACS element UA.III.B.K1a · difficulty: recall. Want to answer it in a timed quiz? Use the Remote Pilot (Part 107) practice mode.

Question bank updated 2026-09-22

QUESTIONHow does an increase in the water vapor content of the air affect air density and aircraft performance?

  1. AThe air becomes less dense, and performance decreases
  2. BThe air becomes more dense because water vapor is heavier than dry air
  3. CHumidity has no effect on density or performance
Show the answer and explanation

Correct answer: A. The air becomes less dense, and performance decreases

  • A — correct Water vapor is lighter than air, so moist air is less dense than dry air; the result is a higher density altitude and lower performance.
  • B — incorrect The study guide states the opposite: water vapor is lighter than air, so adding it makes the air less dense.
  • C — incorrect Humidity is not usually considered an essential factor in density altitude calculations, but the guide states that it does contribute to lower density and reduced performance.

BACKGROUND

The basic density altitude discussion assumes perfectly dry air, but real air always contains some water vapor. Water vapor is lighter than air, so moist air is lighter than dry air. As the water content of the air increases, the air becomes less dense, density altitude increases and performance decreases. Air is least dense when it holds the maximum amount of water vapor it can at a given temperature. Humidity alone is usually not treated as an essential factor in calculating density altitude, but it does contribute.

STUDY TIP

Remember the counter-intuitive point: humid air is lighter than dry air, so humidity hurts performance.

KEY POINTS

  • Water vapor is lighter than air, so moist air is less dense.
  • More humidity means higher density altitude and lower performance.
  • Humidity is a contributing factor, not the primary one, in density altitude.

SOURCES

  • remote_pilot_study_guide.txt ch3b "Effects of Humidity (Moisture) on Density"
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MORE ON WEATHER / EFFECTS ON PERFORMANCE

SkyDrill is not affiliated with the FAA. This is an original practice item based on 14 CFR part 107, 14 CFR part 89, and the FAA Remote Pilot – Small UAS Study Guide (FAA-G-8082-22). Regulations and handbooks are revised; confirm details against the current edition.