The Highest Mirage in North America

Is the Great One a grand illusion? Is the tallest mountain in North America a mirage?

A friend recently told me that the Mount McKinley we see as a huge lump on the southwest Fairbanks horizon is actually an impostor, an optical illusion that really isn't there. She said that because of the curvature of the Earth, we shouldn't be able to see the mountain from Fairbanks or from Anchorage.

Her argument made sense. Because the Earth is a sphere, sailors at sea only can view other ships to a distance of about 13 miles before those ships seem to disappear into the horizon. Mount McKinley is 160 miles from Fairbanks as the raven flies and 135 miles from Anchorage, so seeing it from either city should be impossible. Perhaps the sailor's formula is not applicable because the mountain rises 20,320 feet, almost four miles, into the sky.

Any good scientist would test the Mount McKinley mirage theory with observation and applied knowledge. I ran for help.

I enlisted the calculator of Tom Hallinan, a Geophysical Institute professor of geophysics who studies the aurora and who doesn't share my fear of spherical trigonometry. Using an equation into which he plugged the earth's radius, the altitude of Mount McKinley, and the distance from the Geophysical Institute to the mountain, he figured that we indeed should be able to see the top sixth of the mountain---about 3,300 feet---from the roof of the eight-story Elvey Building, which houses the institute.

But when viewed from south-facing windows in the building, the mountain looks much larger than a 3,300-foot hill. According to Neal Brown, a Geophysical Institute assistant professor of geophysics, this is where refraction comes in.

Changes in air density cause light rays to refract, or bend. These light rays, which carry the image of the mountain to our eyes, always bend toward the densest air. Mount McKinley is so tall it penetrates air of vastly different densities; thin air surrounds the summit and a dense layer of air lies at the base. When light rays carrying the image of the summit hit the denser air, they bend down to our eyes, causing the mountain to appear much taller than it actually is. The effect of the atmosphere on the way we view Denali can be intensified by local cold and warm air layers, such as those that occur in a temperature inversion. Light rays bend down toward denser, colder air, which can distort light that reaches our eyes to the point where mountains flatten out on top or seem to float in the air.

This type of arctic mirage differs from "inferior" mirages, those that occur in deserts or over hot road surfaces, because the air layers are reversed. In a desert or on hot blacktop, heated air lies at the surface, covered by a layer of cooler air. Since light rays always bend toward cooler, more dense air, objects appear to be lower than they actually are, often upside-down. The illusion of shimmering water on the highway ahead is actually the image of blue sky bending up to our eyes as it passes through different densities of air.

The fata morgana is another spectacular optical illusion that seemingly transforms Alaskan mountain ranges, sea ice and other objects on the horizon into walls, spires, and skyscrapers. In Celtic legend, Morgana was a fairy half-sister of King Arthur who lured sailors to their deaths by creating mirages of castles in the air, which they would mistake for a harbor. A fata morgana occurs when many alternating layers of cold and warm air force light rays to take a complicated path.

So the Mount McKinley we see on the Fairbanks horizon actually is an illusion, according to Brown. Atmospheric density changes and local air masses act like fun- house mirrors, causing the mountain to look distorted and stretched out. It never looks exactly the same twice.