Aquarium, Computer, and Alaska Range Mirages

F my house in Fairbanks, Alaska, I see Mt. McKinley 250 km to the southwest. One hundred fifty kilometers to the south is Mt. Hayes, and 200 km to the southeast is Mt. Hadjukovich. But in the winter it is often hard to know what these Alaska Range mountains really look like. They appear as mirages, distorted into towering images, squashed images, inverted images, or floating images (see Figs. 1 and 2a). These mirages are due to severe temperature inversions1—cold air beneath warmer air—together with the favorable topography of the Tanana Flats between my house and the mountains. In this paper I will use aquarium mirages and computer mirages to understand the natural mirages. A mirage occurs when light rays bend in air having varying density.2 The viewer interprets the rays as being straight and therefore sees a displaced image (Fig. 3). A superior mirage occurs when a dense layer lies below a less dense layer. Light bends towards the denser air, and the image is therefore displaced upward. The mirages in this paper are superior mirages, but readers may be more familiar with inferior mirages, such as the highway mirage. Inferior mirages occur when a dense (cooler) layer lies above a less dense (warmer) layer. The light bends upward towards the denser air, and the viewer sees an image displaced downward. In a highway mirage, what appears to be water on the pavement is in fact an image of the sky.