What causes a superior mirage?

A question from a visitor. He writes, “Last summer while in my sailboat in southeast Alaska I could see what appeared to be icebergs floating in the sky. They appeared about 15 miles away. Icebergs were present, and the weather was calm, clear and warm. What makes this occur?”

What you saw is what’s called a superior mirage — the kind of mirage that gives the illusion that an object is above its actual position. We see any object by virtue of the fact that rays of light are reflecting from it. When you see a mirage, the light rays have been first reflected — and then bent by the atmosphere.

A superior mirage happens where there’s a layer of cold air below, and warm air above. Cold air is denser than warm air, but when the density is very different and the change happens over a very short distance, then you might see this sort of mirage. It makes sense that you might see it while boating in Alaska during the summer. These sorts of mirages are common in that part of the world around then. The air around the icebergs, near the water, would have been very cool — in contrast to the warmer air above.

The result – flying icebergs.

Excellent Description – The Arctic Mirage: Aid to Discovery (Weather, People, and History from IslandNet.com)

Dr. Segnan offered this anaology for a superior mirage: Imagine lines of soldiers marching in muddy terrain. Let’s say they come upon a concrete surface at an angle. They have been given orders to keep rank and stay in line. In order to do this the soldiers who meet the concrete first must not only slow down, they must veer at an angle to keep in line with the soldiers who are still in the mud. These lines of soldiers are like the bending rays of light.

Recommend: Image gallary of atmospheric effects (Dept. of Physics at the University of Florida)

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