Mirages are nature's version of optical illusions. Variables like the path of light particles, the curvature of the Earth and air temperature can create false images that the eye is convinced are real. Mirages are the subject of many legends. So-called Fata Morganas, which make land and ships appear like they are floating in the air above the sea, have been unnerving sailors for centuries, while mirages involving oases have given false hope to many thirsty desert travelers.
Scientifically, most mirages can be explained by the fact that photons (particles of light) move faster through warm air than through cooler air because hot air is not as dense, according to Scientific American. This is why mirages are common in deserts, oceans and other places with hot or extremely varied temperatures.
Here are nine different types of mirages, and a look at how, why and where they occur.
1. Fata Morgana
For superstitious sailors, the phenomenon known as the Fata Morgana must be terrifying. The illusion occurs on oceans and seas, and it makes distant objects such as other ships or shoreline appear to be floating in the sky. Some even attribute the legend of the ghostly Flying Dutchman ship to Fata Morgana mirages. This is not only an ocean phenomenon; there are accounts of such illusions on the Great Lakes as well.
Like most mirages, Fata Morganas - named after Morgan le Fay, the sorceress in the legend of King Arthur - appear when the light is refracted (or "bent") by contrasting air temperatures. In oceans and seas, the air near the surface is sometimes cooled by the water, so the temperature is warmer at higher altitudes. Light passes through hot air more easily, so it reaches the eyes of a far-off viewer after refracting above the cooler air. The viewer’s brain expects that light travels in a straight line, so it's fooled by the refraction and perceives that the far-off object is floating above the water.
2. Sundog
A sundog (sometimes written sun dog) is an atmospheric phenomenon that causes bright spots to appear on either side - often both sides, as pictured here - of the sun. The mirage is usually seen when the sun is rising or setting. Sundogs may also have a faint halo that seems to arc around the sun. No matter where in the world the lights are seen, they appear 22 degrees away from the sun.
The meteorological name for a sundog is a parhelion, and they're caused when light passes through ice crystals. The ice is contained in high, thin cirrus clouds or, in colder climates, in lower clouds. It is refracted through the crystals and appears as completely separate light sources. A nocturnal version of the mirage, called moondog, has also been documented.
3. Desert mirage
Like Fata Morganas, desert mirages occur because light bends to move through warmer, less dense air. In the desert, refraction-caused illusions are known as inferior mirages (as opposed to Fata Morganas, which are superior mirages). “Superior” and “inferior” refer to where the mirage takes place. Superior means it's above the horizon, while inferior means it's below. This is why inferior desert mirages usually show up as water-like images on the ground.
In the desert, the air is at its hottest near the surface, and it cools as it rises. This is why the light refracts downward, causing the eye to see sky-like (or water-like) colors below the horizon. A similar illusion is very common on hot highway pavement. You have probably noticed that the road often appears wet or covered with puddles in the distance on an especially sunny day. This is caused by the same phenomenon that creates fake desert oases.
4. Brocken spectre
The Brocken spectre, named after the highest peak in Germany’s Harz Mountains, has the potential to be the spookiest mirage that a person will ever encounter. Mountain climbers were the first to experience this visual phenomenon. They were confronted with ghostly human-like figures apparently looking at them through the high-altitude haze. In reality, people who see a Brocken spectre have no need to feel frightened because they are seeing their own shadow.
The spectre occurs when the sun is behind the observer. The light casts a shadow, not on the ground, but on clouds or fog that occur most often at high altitude. The sunlight that shines around the observer creates a halo-like glow. When the clouds move, the figure may appear to move as well. This phenomenon requires a bright light source shining at a low angle, so it can occur at ground level on foggy days with a strong artificial light such as the “high beams” of a car’s headlights.
5. Magnetic hill
A magnetic hill or gravity hill has more in common with artificial optical illusions than light-based mirages. One of the most well-known magnetic hills is located in the Indian province of Ladakh [pictured above]. Tourists on the Srinagar-Leh highway will come across a stretch of road that appears to run up a hill. However, if you put your vehicle in neutral, you will actually continue to move forward instead of rolling backward (downhill).
This illusion has nothing to do with gravity or magnetism. Instead, it has to do with the landscapes surrounding the road. The adjacent hills are sloped in such a way that it appears that the road going up an incline. However, if you were able to block out the surrounding visual cues, you would see that the road ahead is sloping downward. The illusion is especially pronounced in Ladakh, but there are many documented examples of gravity or magnetic hills around the world.
6. Light pillars
Light pillars can be caused by both natural and artificial light. The phenomenon is characterized by unnatural beams that seem to shoot up into the sky or down to the ground. This is caused when light bounces off ice crystals in the air. Because ice is involved, light pillars that occur close to the ground and are caused by an artificial light source are commonly seen during winter in cold climates, according to National Geographic.
Light pillars can sometimes be caused by light from the sun (they are referred to as solar pillars). When this occurs, the ice crystals are usually in high clouds. The shape of the crystals that create a light pillar is important. The crystals are usually flat and they fall more-or-less horizontally, making it easier for them to continuously catch the light.
7. Ice blink and water sky
Ice blink is a phenomenon that creates an unnaturally bright underside on low clouds. The unusual brightness comes from the daytime light reflecting off of ice below the cloud. Often, the ice field will be too far for the naked eye to see, says the National Snow and Ice Data Center. Inuit peoples and early European explorers learned to use ice blink to predict the presence of ice so that they could avoid it on trips through Canada’s dangerous Northwest Passage.
Water sky [pictured above] is a similar phenomenon. Instead of light clouds, however, the reflection of the water creates unnaturally dark spots. Early polar travelers also used water sky as a navigational tool. It helped them chose their route while traveling overland.
8. Green flash
Green flashes occur right before sunset or just after sunrise. The name “flash” is quite apt. The phenomenon, usually a green spot above the normal circular rim of the sun, rarely lasts for more than a few seconds. Though the image appears and disappears quickly, it does not “flash” across the whole sky, according to San Diego State University.
Green flashes are caused by the way light reacts with earth’s atmosphere. Because of the short duration, the phenomenon is difficult to see. You can increase your chances by finding a level horizon, such as on the ocean.
9. Omega sun
Omega suns appear to make the shape of their namesake Greek letter when they are just above the horizon. The legs (bottom) of the omega are created by warm water heating cooler air just above the surface. The omega shape can be quite pronounced if the water is calm.
Like other ocean horizon mirages, omega suns are caused by light refracting through warmer air (in this case, near the surface of the water), according to San Diego State University. Because the water, especially in an ocean, sea or large lake, is more constant than the air when it comes to temperature, omega suns are common in colder climates during the winter. In some places, such as Japan, seeing an omega sun is considered a good omen.
Top gif image: Superior mirage of Point Reyes National Seashore as seen from San Francisco. Credit: Brocken Inaglory/Wikimedia Commons.
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