Higher humidity leads to lower atmospheric density.

• A. Density increases with higher humidity
• B. Humidity has no effect on density
• C. Density remains unchanged with humidity at constant temp and pressure
• D. Higher humidity decreases density

Answer: (D)

At a fixed pressure and temperature, air density depends on the mixture’s mean molar mass. Humidity adds water vapor to the air, and water vapor has a lower molecular weight (18 g/mol) than the main components of dry air (about 29 g/mol). Replacing some heavier dry-air molecules with lighter water vapor lowers the average molar mass of the air, and since density ρ = PM/(RT), the density decreases. So as humidity increases, density goes down.

That’s why the statement about higher humidity leading to lower density is the best choice. The other possibilities imply the density would rise or stay the same, which isn’t the case when the air’s composition shifts toward lighter water vapor at the same temperature and pressure.