mirage

Illusion caused by refraction of light in various atmospheric conditions. Mirages include the appearance of pools of water in the desert or on hot roads, or of places beyond the horizon apparently floating above it.

When there are hot layers of air near the ground, light rays from the sky near the horizon bend as they pass through them, so that they appear to come from the horizon. Because the light is from a blue sky, the horizon appears blue and watery. If, during the night, cold air collects near the ground, light can be bent in the opposite direction, so that objects below the horizon appear to float above it. In the same way, objects such as trees or rocks near the horizon can appear enlarged.