How does increased humidity affect density altitude?

Increased humidity increases density altitude because it contributes to a lower air density at a given altitude. To understand this, it's important to consider how humidity impacts the overall density of the air. When humidity increases, water vapor displaces some of the oxygen and nitrogen molecules in the air since water vapor has a lower molecular weight than these gases. As a result, the overall mass of air becomes lighter for a given volume, effectively reducing air density.

Density altitude is a measure that combines actual altitude and atmospheric conditions to determine aircraft performance. Higher density altitude indicates that the air is less dense, leading to reduced engine performance, decreased lift, and longer takeoff distances for aircraft. Therefore, when humidity rises, density altitude also increases, reflecting poorer aerodynamic conditions and reduced aircraft performance.

By recognizing that increased humidity results in lower air density, it's clear why the correct answer is that humidity increases density altitude.