From AMS Glossary
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.