We come to know our world through five senses. But we understand our perception is selective; we can’t possibly process all the details of external stimuli. We say seeing is believing but when we look around what we see is really a type of dream residue clinging to our memories like streamers from a party. What we remember achieves a certain truth in our mind, yet is only a brand of falsehood, much less than the full story. This makes us human, but it also makes us vulnerable.
An astonishing effect allegedly took place below the Denmark Strait during 1941, according to Ludovic Kennedy in his book Pursuit: The Chase and Sinking of the “Bismark.” The Bismark, one of the largest battleships ever built by Germany, was being pursued by two British cruisers when it suddenly disappeared into the sea mist. Within a matter of seconds, the ship appeared to come steaming at high speed out of the mist directly toward the two British ships. In alarm the cruisers separated, anticipating an imminent attack. Then the German battleship fluttered, grew indistinct and faded away again. It just flat disappeared! During these events, radar confirmed that the Bismark had in fact made no change to her original course. Was it a hallucination? Had the British sailors been eating magic mushrooms? Mirages have been fooling man from the beginning of our time on earth.
As mirages go, the kind Kennedy described above was a complex one, known as a Fata Morgana (more on this later); however, everyone has been fooled by that pool of water on a distant roadway, right? Or maybe you were stopped in your tracks while traveling through the desert in West Texas by a huge lake appearing out of nowhere? These are not hallucinations—the images are real enough to be photographed. Many so-called optical illusions have little to do with the physics of light—they are tricks of the mind. But mirages are true optical illusions, because their fundamental explanation lies in the science and behavior of light.
There’s a particular recipe for whipping up a mirage. All you need is a difference in air density caused by a difference in air temperature. Light rays move more slowly through cool dense air than they do through warm air. This means the light is broken up into separate paths on its way to your eyes (refraction). The light bouncing toward your eyes at different angles makes it seem like you’re seeing a mirror image of what’s ahead. Your brain interprets this in various ways which don’t always represent reality. So, put these two air densities together simultaneously, stir in enough distance for the optics to work, and you have the ingredients of a mirage. Anything that heats or cools the surrounding air can spawn a mirage.
Heat haze is a common mirage that most of us have experienced. It’s also one of the simplest and happens when viewing objects through a mass of heated air. The image can be anything but because of the rising heated air, it appears to “shimmer.” It includes objects viewed above hot tarmac, masonry rooftops on hot days, above and behind heat sources (campfires, candles, patio heaters), or through heated exhaust gases (diesel trucks, jet engines, etc.). What we see is based on the interpretative faculties of the human mind. Images on land are very easily mistaken for the reflections from a small body of water. The illusion moves into the distance as we approach. This is a form of inferior mirage.
An “inferior” mirage means the reflected image appears below the horizon. It happens when you have a dense layer of cold air sitting above your line of sight, with a less dense warmer layer below. When rays of light reach the warm air near the ground, they bend the light of the blue sky back up toward your eyes, making the illusion appear below the horizon. Since warm air rises and cool air sinks, the air layers are likely to mix, destroying the mirage. Inferior mirages don’t last long unless there is continued heating by the sun. Inferior mirages happen less frequently over the ocean because the ocean surface usually doesn’t get extremely hot. However, it can occur over the ocean when very cold arctic wind blows over warmer unfrozen water.
For context, you can’t have an “inferior” mirage without a “superior” one. This time, circumstances are reversed. Not only does the mirage image appear above the horizon but the chilly air is closest to the surface while the warm air is on top. Normal temperature gradients in the atmosphere during daytime carry warmer air under cooler air, so when they’re switched like this, it’s called a temperature inversion. Regardless, when light travels through an inversion, the light rays are bent down, making the image appear above the true object. Interestingly, a superior mirage can be right-side up or upside-down, depending on the distance involved and the temperature gradient. Often, the image appears as a distortion of up and down parts. Crazy.
Unquestionably, superior mirages produce the most bizarre, unbelievable images. Recall the so-called Fata Morgana mirage in the opening? The Novaya Zemlya mirage is a type of Fata Morgana that was named after a place above the arctic circle after a ship became stuck in the ice there. This mirage was documented in 1596 (and again in 1894) when searching for the Northeast passage. To the sailors’ astonishment, the Sun rose two weeks earlier than it was supposed to. As it happened, the real Sun was below the horizon, but its light rays followed the curvature of the Earth. The inversion layer must have just the right temperature gradient over the whole distance to make this possible—something called an atmospheric duct. But the effect can be mind-blowing. In the same way, ships that are so far away they’re below the sight horizon may appear on or even above the horizon as Fata Morgana mirages (like the infamous Flying Dutchman warship).
Fata Morgana mirages are most common in polar regions, especially over large sheets of ice that have a uniform low temperature. They also can occur at more moderate latitudes but tend to be weaker and less stable. As an example, a distant shoreline may appear to loom and look higher (and, therefore, closer) than it really is. These complex mirages can include a variety of stretched and compressed images that look like they’re stacked on top of each other. Fata Morgana is also a fast-changing mirage. Because of turbulence, there can appear to be dancing spikes and towers—all kinds of dramatic effects. Fata Morgana mirages can be seen from any altitude, including from mountaintops or airplanes. The name Fata Morgana is Italian for Morgan le Fay, the fairy, shapeshifting half-sister of King Arthur. Makes sense, doesn’t it?
Mirages can also be seen at night, especially for astronomical objects (the Sun, the Moon, the planets, bright stars, very bright comets) and mirages of lights from moving vehicles, aircraft, ships, buildings, etc. The most observed are sunset and sunrise mirages. A green flash happens at sunset when the sun’s rays travel through more of the Earth’s atmosphere than at other times of the day. The atmosphere acts as a prism and separates the color spectrum in such a way that only the green photons appear briefly before the sun falls below the horizon.
The Brocken specter is a wildly haunting mirage. It occurs most often at high altitudes. In this case the sun needs to be behind the observer and there needs to be clouds or fog present. It was first observed by mountain climbers as a ghostly human-like figure inside a glowing halo which appears to be watching them. In reality, they are seeing their own shadow cast upon the moisture in front of them. When the clouds move, the figure appears to move as well. This mirage can also occur at ground level on foggy days with strong artificial light, such as high beams of a car’s headlights. A Very unnerving experience!
Omega suns have the appearance of their namesake letter of the Greek alphabet when the sun is setting just above the waterline. The Sun appears to melt into the water near the bottom, making an omega shape. The “legs” of the omega image are created by warm water heating cooler air just above the surface. The effect can be quite pronounced if the water is calm. Omega suns are common in colder climates during the winter.
The multitude of mirages exceeds the space I have here, but you can see they are visual misunderstandings. It has been said that wisdom is the ability to discern what is real, so we can’t believe everything we see. A mirage is here before our eyes, ever changing, evolving throughout a vast canvas of emptiness, and then it is gone. Like us, a mirage is what it is, and then it becomes something else.
By Larry Gfeller