A mirage is an optical illusion that occurs when light rays are bent, producing a displaced image of distant objects. A mirage is caused by changes in atmospheric density in the lower air layers of the atmosphere which depend upon air temperature with height in those layers. There are two main types of mirages: inferior and superior mirages. An inferior mirage which takes the form of a pool of clear water over the pavement of a road is actually the refracted light rays of the sky above caused by the changes in air density near the surface of the road. Understanding the cause of a mirage can enable us to admire better the phenomenon.
How mirages form:
Light travels through the air in a straight line if the air is at the same temperature, cold or hot; however, if a temperature gradient exists, light will take the minimum path when traveling from one point to another. As a result light will travel through hot air faster than it does through cold air because hot air is less dense than cold air. A flat surface absorbs the sun´s energy and becomes hot. This causes the air close to the surface to become hotter and less dense. The air density gradually increases with height.
The index of refraction of air varies depending on the density of air. Air density depends on its temperature and water vapor content and is proportional to its pressure (density increases as pressure increases) and inversely proportional to its temperature (density decreases as temperature increases). Cold air is denser than hot air; therefore, cold air has a greater refractive index. When light passes from a mass of hot air to a mass of cold air, the light rays will bend toward the direction of the temperature gradient.
Light refractions occur in the atmosphere when light passes through air layers of varied densities. An inferior mirage forms when light rays are bent upwards from their path when passing through a warm layer of air. Inferior mirages most commonly form when the air close to the ground is hotter than the air above it, refracting light in a parabolic shape. Initially, the mind interprets the light rays as coming from a straight path, which in this case is the ground, thus, we see a bluish patch in the distance which may take the form of a body of water. Normally an inferior mirage will form on the ground when the temperature gradient is above 4.4° C per meter.
When the temperature gradient is constant, the image will not be distorted, but if the temperature gradient changes in the air layers, the image will be distorted in various ways. The image may appear taller than it really is or stretched (towering) or it may appear shorter (stooping). Complex refractions may produce complex formations of both towering and stooping mirages. Different views of the mirage can be viewed by changing your angle of sight either up or down.
A superior mirage forms when cold air gathers beneath a mass of warm air (temperature inversion). In this state, light rays get refracted toward the colder denser air. This causes an image to appear as if it is above its actual position. The temperature gradient affects how light travels from the object to our eyes, so an image may appear visible even though it is located below the horizon or lifted above its actual position (looming), for instance, the image of a boat in the clouds. The image may appear taller or closer than it actually is or smaller (stooping) and further away than it actually is. Most mirages occur over large bodies of water which are colder than the air above them or over snow surfaces.
One common form of superior mirage is the rising or setting of the Sun. When the Sun is directly over the horizon, all or part of it is actually below the horizon; however, the refraction of the sun´s rays by the atmosphere make it appear as if it is actually over the horizon. This occurs at sunrise and sunset each day. The effect is due to an increase in air density derived from an increased in atmospheric pressure. The same effect may alter the viewed position of the planets, moon and stars.