ARCTIC MIRAGES
Though popular lore has it that mirages are connected with deserts, surprisingly, the most interesting and varied mirages are found in the polar zones often during times of intense cold. Polar mirages have fooled many an explorer and sent them off looking for non-existent mesas and shimmering mountains.
To understand how mirages work, consider that light rays traveling horizontally through the atmosphere bend slightly because the atmosphere acts like a weak lens. In a 'normal' mid-latitude atmosphere the radius of curvature of the light ray is five times larger than that of Earth, so the bending effect is so small it is usually not evident. However, if the temperature at the surface is very low, and the amount of clouds and water vapor also small, as they often are in the polar regions, than strong radiative temperature inversions set up, increasing the strength of the atmosphere lens.
For example when the temperature inversion is 6.3 degrees F per 100 ft (11 degrees C per 100 m) the radius of curvature of light rays equals that of the earth and in principle a horizontally-travelling light ray would encircle the earth and you would see the back of your own head! Of course this never happens Nonetheless, spectacular optical effects do occur from such curvatious rays!
In Fairbanks, we often have strong temperature inversions (called 'inversion' because the temperature increases, rather than the more normal situation of lowering with height). Sometimes they are so strong that by walking up a set of stairs at the UAF campus, about 100 feet high, you can detect a distinct warming as you climb!! During winter oftentimes the temperature in the nearby hills can be 20 or 30 degrees F higher than in the valley and when driving I've found variations of 10 degrees or more caused from up and down swells in the roadway.
Sometimes the temperature structure in the polar atmosphere can introduce such spectacular variations in atmospheric refractive index that the atmosphere can 'reflect' rays and invert the image.
Several examples of reflections of small Buttes in the Tanana Valley flats south of Fairbanks by atmospheric thermoclines are shown below.
Most people associate mirages with scenes in deserts. These are caused also by light bending, but in the opposite sense to the polar mirages. In this case there is heating at the surface and a very rapid fall off of temperature with height. In fact the fall-off is so great that the atmosphere becomes unstable and 'turns over' or 'convects', causing a shimmering effect such as this image that shows the shimmering mirage photographed at heights of 1 meter and 2 meter above the roadway:
Here are a few mirages of the Alaska range, 90 miles across the Tanana Valley from my office window in Fairbanks
