High humidity significantly impacts night-sky visibility by increasing water vapor in the atmosphere, which scatters light and reduces the contrast between celestial objects and the background sky. This effect is similar to looking through a thin fog, making even bright stars appear fainter and obscuring fainter objects like galaxies and nebulae. High humidity also exacerbates light pollution, creating a brighter and more pervasive background glow, further diminishing the ability to observe faint celestial objects.