Wednesday, October 24, 2012

Superior mirages

Superior mirage of a distant mountain.

Superior mirages are created under conditions that are opposite of those that create inferior mirages. They form in cold weather when the surface air is colder, and thus denser, than the air above. In a superior mirage, a distant object appears to be taller and closer to the observer than it really is. Sometimes it appears upside down. Superior mirages are most common in polar regions, where the air over a snow-covered surface is colder than the air several feet (about a meter) above.

For example, a mountain in the distance might appear taller and nearer than it actually is. The light from the mountaintop is reflected in all directions. Some of that light follows a gently sloping path downward. When that light enters the layer of colder air, it is bent toward the perpendicular; in this case, into a steeper downward path. When this refracted light reaches your eye, your eye follows the path of the light, at its refracted angle, back over the distance to the mountain. The image thus appears higher above the ground than it normally would.

Fata Morgana is most common in polar regions, especially over large sheets of ice with a uniform low temperature, but it can be observed almost anywhere. While in polar regions Fata Morgana is observed on cold days, in deserts and over oceans and lakes Fata Morgana is observed on hot days. In this form of mirage, images which would normally be concealed behind the horizon appear distorted in the sky. A fata morgana is caused by abrupt variances in air temperature; when air above the line of sight is warmer than the air below. This is known as temperature inversion, since it does not represent the normal temperature gradient of the atmosphere. Light traveling from the horizon is refracted towards the earth, away from the warmer air above. These are the conditions which cause a superior mirage - a Fata Morgana occurs only when light travels through multiple distinct temperature gradients. This results in an image which may be compressed in some sections and expanded in others. As air temperature changes over time so do the various angles of refraction, causing the image to expand or contract as the atmosphere returns to its standard temperature gradient.

The name ``Fata Morgana'' is Italian for ``fairy Morgan.'' According to mythology, Morgan, or Morgan le Fay, was the fairy half-sister of King Arthur. She lived in an underwater crystal palace and was capable of creating magical castles out of thin air. In the fifteenth century, Italian poets from the town of Reggio viewed a fantastic, castle-like mirage near the Strait of Messina (the waterway between Italy and Sicily). Unable to explain what they saw, they called it a ``Fata Morgana,'' and the name stuck.

A special type of superior mirage is called a Fata Morgana. A Fata Morgana takes the form of spectacular castles, buildings, or cliffs rising above cold land or water, particularly in polar regions. This type of mirage is produced by light that is refracted as it passes through air layers of various temperatures. A Fata Morgana requires that the air temperature over a cold surface increase with height. Specifically, the temperature rises slowly throughout the surface layer of air, then several feet above the surface the air temperature rises more quickly. In the next layer of air, the temperature rises slowly again.

No comments: