Castles in the air

We're used to thinking of mirages as oases in the desert, surrounded by palm trees. In real life, our most likely encounter with a mirage is on the highway, where water may seem to shimmer in the distance. Mirages rely on a property of the atmosphere called refraction – the bending and spreading of light rays as they pass through air layers of different densities. On a bright, sunny day, heating at the surface – especially above a quick-to-heat surface like pavement or sand – lowers the density of the air nearest the ground, where usually it's the most compressed. The light rays bend upward as they hit this less dense layer near the ground, so that the light from something in the distance (like the sky) may actually be coming from below the horizon when it hits our eyes. A bright patch may thus appear on our visual image of the highway. Tiny variations in air temperature and density give the mirage its shimmering, waterlike texture. If the vertical profile is reversed (warm air on top of cool), a surface-based feature may appear above ground. Such is the case with the fata morgana. Sometimes used as a synonym for any mirage, it is in fact a particularly rare one usually seen at sea. The fata morgana is named after Morgan le fay, the shape-changing half-sister of legend's King Arthur. This enchantress lived in an undersea castle and lured sailors to their death by reflecting her castle's image into the sky. Likewise, the fata morgana stretches ground-based features upward so that the sea surface becomes a row of mountains or a bank of turreted castles. In 1906, when Robert Peary thought he saw a mountainous land just beyond reach in the Arctic Ocean, it was almost certainly the fata morgana that fooled him. It took seven more years for another explorer, Donald MacMillan, to reach the site and discover that the so-called Crocker Land was only an illusion. Henry Wadsworth Longfellow saluted the famed fata morgana in his poem of the same name:”I approach and ye vanish away, I grasp you, and ye are gone/But ever by night and by day, The
melody soundeth on.”