From AMS Glossary
This is not a difficult mirage to find, especially along the sun-warmed walls of buildings. In many cases, it appears as nothing but an inferior mirage turned on its side. However, there are often interesting subtleties. Easiest to find, perhaps, are the high-order multiple images that result from inhomogeneities along the wall. These can arise both from the wall having a slightly wavy surface and from the periodic variations in the internal structure of the wall that alter the thermal conductivity and so produce periodic temperature variations. Curiously, unlike the inferior mirage, the lateral mirage seems to be capable of producing three images even in the absence of inhomogeneities. The temperature profiles normal to horizontal and vertical surfaces are slightly different. In the case of the inferior mirage, gravity acts normal to the surface, while in the case of the lateral mirage, gravity is parallel to the surface. This produces a flow up the wall that results in a temperature profile capable of giving the three-image mirage. Lateral temperature gradients in the free atmosphere, away from vertical surfaces, are not sufficient to produce lateral mirages; the rare reports of such sightings undoubtedly arose from misinterpretations of observations.