What happens to jet engine thrust when humidity increases?

• A. Increases
• B. Decreases significantly
• C. Decreases slightly
• D. Remains unchanged

Answer: (C)

Humidity changes the density of the air. Jet engine thrust mainly comes from accelerating the air that enters the engine. If the air is more humid, its density is lower at the same temperature and pressure, so the engine draws in slightly less air mass for a given intake volume and engine speed. With less mass being accelerated, the momentum change—and thus thrust—drops a little. The effect is small, which is why the correct choice is that thrust decreases slightly. The other options would imply a larger change or no change, which isn’t supported by how humidity alters air density and mass flow.