Mirage comes from the French word mirer, to look at; or se mirer, to be reflected. The phenomena includes the appearance of lakes in arid deserts and hot asphalt roads, the images of ships and icebergs, frequently seen as if inverted and suspended in the atmosphere in the Polar Regions, the Polar Morgana, and “looming” as witnessed in mists or fogs.
Mirage optical illusions are caused by variations in the refractive index of the earth’s atmosphere. Our atmosphere is a mixed bag of temperature and density variations; each with its own layer. Each layer has its own refractive index, which is the amount that light is distorted as it streams through from above. The amount of distortion is normally very small but it’s enough to make stars twinkle each evening in the night sky.
Sometimes extreme refractive variations can occur due to irregular heating. When this happens the boundaries between highly irregular air temperatures can sometimes act like a mirror or lens.
A common mirage is the appearance of an isolated lake frequently seen in hot sandy deserts and on superheated black-top roadways. As the ground is heated by the hot sun, the air nearby expands in the heat. As it expands, its density becomes lower and its refractive index also becomes lower. Higher up where the air is cooler the density and refraction normalizes in a higher layer. The vast difference in temperatures and density between the two layers can cause a mirror effect to occur. The sky appears as a shining lake; mountains or palms may be similarly reflected, but the images are inverted. Similar atmospheric conditions sometimes occur in the air over large bodies of water on cold autumn mornings.
Another type of mirage, frequently observed at sea in the northern latitudes, is the illusive appearance of ships and icebergs inverted and floating in the clouds. This is due to a layer of hot air high above sea level. At this altitude the abnormal boundary is large and is curved around the earth. It is so big that it becomes a lens through which distant objects are magnified. We may see inverted ships and even dry land in the clouds, although nothing is visible on the horizon. In 1909 polar explorer Robert E. Peary witnessed a mirage and thought it was a distant land. So fooled was Peary that he named the new land “Crokerland” after an investor and later sought funding to explore and settle the new territory.
Robert Peary discovers “Crokerland” in 1909.
Mirages associated with water are sometimes called Fata Morganas. They act the opposite of desert mirages. In this case the water is much colder than the air above and creates a boundary layer. A famous Fata Morgana often seen in the Straights of Messina creates a lens effect that can distort distant objects vertically. Sometimes conditions in misty or foggy weather, often at sea, can create atmospheric lens effects which magnify distant objects both horizontally and vertically. This causes the phenomena known as “looming.”