Weather Conditions

Ceiling;

Visibility (i.e., vertical visibility, slant visibility and horizontal visibility); and/or,

Cloudiness (e.g., rain, fog or fog patches, haze, mist, smoke, snow, whiteout effect).

How do Visual Illusions Affect the Pilot’s Perception ?

Visual illusions result from the absence of or the alteration of visual references that modifies the pilot perception of his / her position relative to the runway threshold.
Visual illusions affect perception of heights, distances and/or intercept angles.
Visual illusions are most critical when transitioning from IMC and instrument references to VMC and visual references.
Visual illusions (such as the black-hole effect) affect the flight crew vertical and horizontal situational awareness, particularly during the base leg and when turning final (as applicable) and during the final approach.
Visual illusions usually induce crew inputs (corrections) that cause the aircraft to deviate from the original and intended vertical or lateral flight path.
Visual illusions can affect the decision about when and how fast to descend from  the MDA(H).
The following paragraph provides an expanded overview of all the factors and conditions creating visual illusions to discuss how each factor or condition may affect the pilot perception of:
The airport and runway environment;

The terrain separation; and,

The aircraft vertical or lateral deviation from the intended flight-path.

Usually, more than one factor is involved in a given approach, compounding  the individual effects.
Airport Environment

“Black hole” along the final approach flight path:

In case of approach over water or with an unlighted area on the approach path, the absence of visible ground features reduces the crew ability to perceive  the aircraft lateral and vertical position relative to the intended flight path.

Uphill or downhill terrain before the runway threshold:

An uphill slope in the approach zone or a drop-off of terrain at the approach end of the runway creates an illusion of being too high (i.e., impression of a steep glide path, as shown on Figure 1), thus:

Possibly inducing a correction (increasing the rate of descent) that places the aircraft below the intended glide path; or,

Preventing the flight crew from detecting a too shallow flight path.

Figure 1
Effect of Terrain Up-hill Slope on Flight Path Perception
 

Perceived Glide Path

Actual Glide Path

A downhill slope in the approach zone creates an illusion of being too low  (i.e., impression of a shallow glide path, as shown on Figure 2), thus:

Possibly inducing a correction placing the aircraft above the intended glide path ; or,

Preventing the flight crew from detecting a too steep flight path.
　
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