The word mirage comes from the French mirer, to look at; or se mirer, to be reflected. The phenomena includes the appearance of water in arid deserts, heat shimmers on hot asphalt roads and landing fields, as well as the images of ships, islands, and icebergs, frequently seen as if inverted and suspended in the atmosphere in the Polar Regions. Affects can also Include the Polar Morgana, and "looming" as witnessed in mists or fogs.

The atmosphere is really a mixed bag of temperature and density variations; each with its own distinct layer. Temperature variations can pile up air in the atmosphere like layers in a wedding cake. Each layer has its own refractive index, which is the amount that light is distorted as it streams through from above; the way a straw is distorted when you stick it in a glass of water. The amount of distortion between atmospheric layers 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. Hot thick air can sometimes pile up over, or under a cooler region like another layer, or the ground. When this happens the boundaries between highly irregular temperature layers can sometimes act like a mirror or lens. If you are standing below, and look up at this atmospheric mirror, you will see whatever is reflected there.

A common mirage is the appearance of an isolated lake frequently seen in hot sandy deserts and on superheated black-top roadways. Sometimes extreme temperature boundaries can cause thick dense air to pile up near the ground. In this case you could be standing in, or slightly above the reflective part of the layer. You might see isolated mirror effects in terrain which is slightly below your viewpoint. A reflected bit of sky on an inversion layer becomes a shimmering lake.

As the ground is heated by the hot sun, the air nearby expands in the heat. As it expands the air thickens, and its refractive index plummets. Higher up where the air is cooler the density and refraction normalizes. The vast difference in temperatures and density between the two layers can cause a mirror effect to occur. The sky appears as a body of water; mountains or trees 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.

Curious Fact. 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."

 

Art: 2000. R Ausbourne

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