From Glossary of Meteorology
Because the displacement increases with distance, a horizontal surface, such as that of a body of water, appears to bend upward and one's perception is that of being inside a broad shallow bowl. Indeed, the upward bending surface results in an (optical) horizon that can be much farther from the observer than in the absence of a mirage. Looming is an example of a superior mirage. The opposite of looming is sinking. Looming occurs when the concave side of light rays from a distant object is down, and this in turn occurs when the refractive index of the atmosphere decreases with height. This is very common, but the displacement is usually sufficiently small as to be unremarkable. However, when there is a temperature inversion over the surface, the looming can be striking.