If the pressure altitude and temperature remain constant, but the air moisture content increases, what happens to the landing roll?

When the moisture content in the air increases while keeping pressure altitude and temperature constant, the overall density of the air decreases. This is because the molecular weight of water vapor is less than that of the dry air it displaces. As a result, the air becomes "lighter" or less dense.

In aviation, less dense air affects aircraft performance in various ways. A decrease in air density leads to reduced lift generation at a given speed and decreased engine performance, which translates to longer takeoff and landing distances. Since the aircraft will require a longer distance to come to a complete stop after landing due to the less dense air, this directly impacts the landing roll.

Thus, the correct understanding of aerodynamics indicates that an increase in the moisture content of the air, while maintaining constant pressure altitude and temperature, results in a longer landing roll because of the air's reduced density.