Monday, August 08, 2016

Illusions on Rainy Lake

I spent part of my vacation with my son and other family and friends fishing. We were staying on the shore of a very large body of water on the Minnesota/Canadian border called Rainy Lake. The weather, fishing, and companionship were all fantastic. One morning the winds were very light and  the sun was shining over the chilly lake water. I noticed something strange on the horizon. There appeared to be trees floating in the sky.

This did not look like the typical mirage. I recalled something about a type of mirage that is often seen over ice or cold bodies of water. I kept a watch on the horizon as we pulled one walleye after another into our boat. I had my Cannon HD video camera that can take pretty good 8 MB low-light stills. Over the next hour or so I took pictures of some other strange sights on the horizon.

Some small islands and parts of the shoreline joined the trees in the sky. We were witnessing the formation of superior mirages. These form when there is a layer of cold air below a layer of warm air. The cold lake water was keeping the air near it cooler than the air above, forming a temperature gradient known as an inversion layer. Light from distant objects was bending down as it encountered the less dense warmer air. The path of the light from the objects can follow the curvature of the Earth, allowing objects beyond the horizon to be seen. This effect can be demonstrated by partially dissolving sugar or corn syrup in an aquarium. If done carefully, the density gradient of the sugar mimics that of the inversion layer, causing the path of a laser beam to bend down.

Inversion layers are stable, allowing the mirages to be seen for long periods. They also lack the shimmering of inferior mirages often seen on the road on a sunny day. This makes superior mirages seem more real and provoked an eerie feeling. Some of the images I had a hard time explaining. This one shows a superior mirage forming in front of an island.

On returning from my trip I did some reading about mirages to learn more about what I had seen. Superior mirages have been reported throughout history and have sometimes altered it. Early explorers trying to find the Northwest Passage turned around when they saw a mountain range looming in the distance. Later explorers returning to the same place saw no mountains. They had probably been discouraged by a complex superior mirage known as a Fata Morgana. Another stranded group of arctic explorers saw the polar night end two weeks early when the sun formed a superior mirage over the ice. In fact, anytime you watch the sun rise or set its image is being refracted around the curve of the Earth by the density gradient always present in the atmosphere. This is technically not a superior mirage because the density gradient is pressure, not temperature-based but the effect is the same.

There are other fascinating stories of superior images and Fata Morgana sightings that are worth reading. I found the Wikipedia article about mirages and Fata Morgana useful, as well as this website and this one. I am now better prepared for the next time I observe this unusual phenomenon. Another illusion on Rainy Lake was self created. When posing with my 23-inch walleye, we found we could make it look a lot bigger with some creative posing. If anyone from the Fish and Wildlife service is reading this, we did throw it right back in the water.

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