Several years ago a friend and I did a snowshoe hike from Stemple Pass to Flesher Pass (near Helena) on the Continental Divide Trail (CDT). The temperature was pleasant, the snow was perfect, and it was sunny with no wind - but the best part of the hike is that we were able to witness an unusual atmospheric phenomena known as Fata Morgana.
It's a trap! . . .
“Fata Morgana” is so-named because it is the Italian name for the Arthurian sorceress Morgan le Fay, and it was believed that she created these illusions of distant castles or land to lure sailors to their deaths. In reality all mirages are due to refraction (bending, redirecting) of light from distant objects. What we observed during the hike was the result of light from distant mountains being refracted as it traveled through layers of air that had different temperatures. The temperature inversion that blanketed the area that week was the major factor that allowed the sorceress to do her handiwork.
Inversion? . . .
During winter months, the mountain valleys of western Montana are prone to inversions. They're called inversions because they are upside-down situations. The temperature of the atmosphere NORMALLY gets colder as you get farther away from the surface. However during inversions, air near the surface is colder than the air above. Inversions tend to form during stretches of clear, calm, very cold weather. Without clouds, heat given off by the earth escapes easily into space, causing a layer of cold air to develop at the surface. An especially strong inversion was present in the Helena-Lincoln area when the Fata Morgana photos were taken - There was a layer of very cold air in the valleys with warmer air above.
Illusionary beauty . . .
Fata Morgana is a type of mirage known as a superior mirage. All mirages are caused by the refraction (curving or redirecting) of light as it passes through layers of air with different temperatures and densities, creating false images, like seeing "water" on a hot road or distant objects appearing to float. This happens because hot air (less dense) and cool air (denser) refract light differently, making the brain perceive bent light as if it traveled in a straight line from a different location (diagram). This bending or redirecting occurs because light travels at slightly different speeds when passing through different mediums. When there are stark changes in air density due to layers of air having different temperatures, this can create the illusion of distant objects or water. For instance, on a hot day, the air near on a paved highway is much warmer and less dense than the air above it, causing light rays to bend upwards, which creates the illusion of water on a road’s surface - this is an inferior mirage. The intricate nature of Fata Morgana involves multiple curved light rays converging towards the observer's eye. This convergence creates an illusionary effect, making objects appear inverted or smeared upwards into towering cliffs as shown in the photo atop this page.
Term: refraction
CLICK HERE to learn more about Fata Morgana (Wikipedia)
CLICK HERE to access my blog and photo tour of the hike we were doing when we witnessed the Fata Morgana.
CLICK HERE to watch a short YouTube video that demonstrates refraction.
Below: The situation we witnessed was more complicated than the one illustrated below, but the diagram may help readers understand how refraction is involved.
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