inferior mirage
If only a single image of distant objects is seen, then the term sinking is often applied: A horizontal surface appears to curve downwards with increasing distance and terminate in a relatively nearby optical horizon. The inferior mirage is most striking when it exhibits two images; the second, lower image is always inverted and of reduced magnification. Sometimes textbooks suggest that there is but a single image: the lower, inverted one. The upper erect image is claimed to be the object. However, both are images, and have positions and magnifications that differ from that of the object. Also, the lower inverted image is sometimes misinterpreted as having resulted from a reflection and when this is seen over land, it leads to the assumption that there must be water in the distance causing the reflection. This is the origin of the long association of the mirage and illusory water, and this leads to the assumption that water is present on a dry surface. The mirage owes its name (from se mirer, to look in a mirror) to this impression of arising from a reflection, having been named by French mariners for images seen at sea. For vertical objects seen beyond the optical horizon, typically the lower portion of the object cannot be seen, and an upper portion of the object is seen twice: erect, and inverted. The farther away the object, the more of the lower portion of it will have vanished so that, for example, the upper decks of a distant ship might appear erect and inverted and apparently floating above and disconnected from the optical horizon while the lower decks will not be seen at all. Sometimes a scene such as this is misinterpreted as resulting from a superior mirage by a person who thinks the ship's images have been lifted up from the horizon. Actually, in this case, everything is displaced, but the horizon has merely been displaced more. Inferior mirages occur over a surface when the temperature decreases with height. The formation of a two-image inferior mirage also requires that the temperature gradient decrease with height. These conditions are met when the surface is relatively warm, resulting in an upward heat flux, such as over sun-warmed ground or a lake at night.
See sinking, stooping;
compare superior mirage, towering, looming.
