What visual illusion may cause a pilot to think their aircraft is at a higher altitude?

The visual illusion that can cause a pilot to perceive their aircraft as being at a higher altitude is associated with upslope terrain. When flying over land that gently rises, especially if the terrain is obscured by clouds or is otherwise difficult to discern, pilots may misinterpret their actual altitude. This is because the horizon appears to be farther away than it actually is, and the rising terrain creates a false sense of elevation. Consequently, pilots might level off or descend inappropriately, believing they are at a safe altitude, which could lead to dangerous situations such as flying too low.

Mountains in the distance can provide visual cues that help a pilot gauge their altitude but can sometimes create different types of misjudgment, primarily related to depth perception, rather than perceived altitude. Cloud cover below the aircraft usually serves to obscure the ground, making altitude more difficult to assess, but does not typically create the illusion of higher altitude as effectively as upslope terrain does. Fog, while it can limit visibility and create challenges in orientation, does not inherently change altitude perception but rather affects overall situational awareness.