FAA Part 107 Remote Pilot (UAG)Loading and PerformanceMedium
A client asks how humidity affects a fixed-wing sUAS compared to an otherwise identical hot, dry day. Which statement correctly describes the effect of high humidity?
- AHumid air is denser than dry air, improving lift
- BWater vapor displaces heavier air molecules, making humid air less dense and reducing performance
- CHumidity has no measurable effect on air density
- DHumid air always increases engine or motor power output
Show answer & explanationAnswer & explanation
Correct answer: B. Water vapor displaces heavier air molecules, making humid air less dense and reducing performance
Water vapor molecules are lighter than the nitrogen and oxygen molecules they displace, so humid air is actually less dense than dry air at the same temperature and pressure, reducing lift and power performance.
Why the other options are wrong
- A. Humid air is less dense than dry air, not more.
- C. Humidity measurably reduces air density and is part of density altitude calculations.
- D. Humid air generally reduces, not increases, engine/motor performance.
Humidity's Effect on Air Density
Water vapor is lighter than dry air's nitrogen and oxygen molecules, so humid air is less dense, reducing lift and engine/motor performance.
- Humidity is a factor in density altitude
- Humid air is less dense than dry air
- Reduces lift and power at the same temperature
Memory trick: Water vapor is a lightweight guest crowding out the heavy molecules.