in which the image
of distant objects is displaced downward.
Because the displacement increases with distance, a horizontal surface, such as that of a body of water, desert
, or road, appears to bend downward and one's perception is that of being on top of an inverted bowl or possibly on a planet with a very much smaller radius. Indeed, the downward bending surface results in an (optical) horizon
that can be very much closer to the observer than in the absence of a mirage. Sinking is an example of an inferior mirage
. The opposite of sinking is looming
. Sinking occurs when the concave side of light
rays from a distant object is up, and this in turn occurs when the refractive index
of the atmosphere
increases with height. This only happens near a surface when the heat flux
is upward and so the temperature
gradient decreases with height. This is common over a warm surface (such as might occur over sun-warmed ground or a lake at night). Sinking is often accompanied by a two-image inferior mirage. Compare stooping
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