Now the word superior has nothing to do with the quality of the mirage, but its location relative to the object it represents. Superior mirages appear above the object's true position, while the inferior mirage appears below.
Superior Mirage: Scale is greatly exaggerated.
We don't hear or know as much about the superior mirage, so many think it is a rare occurrence. However, one form is so common that most of us do not recognize it as a mirage. Have you ever seen the sun lying on the horizon? What, do you think, was the sun's actual position at that time?
In actual fact, when you saw the full solar disk sitting on the horizon, it was actually below the horizon. We think the sun is actually on the horizon because the solar light's path is bent (refracted) by the atmosphere. This refraction occurs at both sunrise and sunset and adds over four extra minutes of daylight each day.
Mirages form when light passing through the atmosphere is refracted by air layers of different densities. The amount of refraction -- the degree of bending -- is directly proportional to the air density, and air density is proportional to its pressure (density increases as pressure increases), and inversely proportional to its temperature (density decreases as temperature increases). The effect on the setting sun's perceived position primarily results from the increase in atmospheric density (due to pressure) as the solar rays travel from outer space to the Earth's surface.
In the surface atmospheric layer, air density differences are more dependent on the temperature change with altitude than pressure changes. Therefore, if there is a large change in temperature over a short vertical distance -- known as the vertical temperature gradient -- in the lower atmosphere, the path of light rays can bend significantly from a straight line. The bend is always toward the region of higher density, i.e., colder temperature.
When cold air lies below warm air (an inversion as shown in the above figure), light rays are bent downward toward the surface, thus tricking our eyes into thinking an object is located higher than it actually is. Thus, to see a superior mirage, the air at the surface must be colder than that above it, conditions most common over snow, ice, and cold water surfaces.
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