What effect does an increase in humidity have on density altitude?

An increase in humidity affects density altitude by increasing it. Density altitude is a measure of how high the air is compared to the standard atmosphere, and it is influenced by various factors, including temperature, pressure, and humidity.

When humidity increases, the amount of water vapor in the air rises, displacing some of the heavier air molecules, like nitrogen and oxygen. Because water vapor is less dense than these air molecules, humid air is lighter than dry air at the same temperature and pressure. However, despite the air being lighter, the presence of more water vapor results in a decrease in overall air density, which makes the air less effective for generating lift and can reduce engine performance.

As humidity increases and the density of the air decreases, the density altitude will effectively rise. This means that the aircraft operates as if it is at a higher elevation than it actually is, which can adversely affect performance during takeoff and landing.

Therefore, the correct understanding of how humidity impacts density altitude is that increased humidity leads to an increase in density altitude.