When does the stall speed of an aircraft increase?

The stall speed of an aircraft increases when frost is present on its wings. Frost disrupts the smooth airflow over the wing surfaces, which adversely affects their aerodynamic performance. This disruption causes an increase in the angle of attack required to achieve the same amount of lift. As the stall speed is defined as the minimum speed at which an aircraft can maintain level flight without stalling, any condition that increases the angle of attack at which stall occurs will elevate the stall speed.

The presence of frost adds roughness to the wing surface, which effectively alters the wing's characteristics, necessitating a higher speed to maintain lift and avoid a stall. Clean wings, on the other hand, operate optimally to produce lift at lower stall speeds, thus supporting why the correct answer is linked to frost presence rather than factors like weight or aerodynamic efficiency.