What causes mirages?

  • Simplified diagram demonstrating how light is refracted when travelling across a thermal boundary.

    Inferior mirage photographed across the Egyptian Sahara Desert. Image: Michael Gwyther Jones.

    An inferior mirage on a highway. Image: Michael Gil.

    A Fata Morgana seen in the Badlands National Park in December 2000. Image: NOAA

    The Fata Morgana seen on the Norwegian Coast. Image: Gerd A.T. Mueller

  • What causes mirages?
    25.07.2013 17:39

    You’ve probably noticed how dry surfaces can sometimes appear wet on hot, sunny days, giving the illusion of distant puddles on the road or a sparking lake in the desert. These are examples of mirages, and chances are you’ve spotted one during the recent heatwave. But what causes these optical phenomena?

    All mirages are produced by the process of refraction, the bending of a light ray as it passes between mediums of different densities. In the atmosphere these density boundaries are produced by steep thermal gradients, with temperatures either rapidly increasing or decreasing with height. Light travels more slowly in colder, denser air than it does in warm air. Thus as light reflected from some distant object passes from colder air aloft to warmer air below, the lower part of the wave speeds up first and causes the light to bend away from the surface. Likewise, when warm air is resident above cold, light can be bent towards the surface.

    In the case of the wet road illusion, heat from the sun is transferred from the road to the air by conduction, albeit only in the lowest layers as air is a poor conductor of heat. The resulting strong temperature gradients mean that light rays that would otherwise be headed for the surface are bent upwards to the eye, producing an image of the sky on the ground. Our eyes expect the light to have travelled in a straight line, and interpret the image as the more familiar sight of water. Furthermore, as light rays from distant objects cross each other en route to the eye via refraction in the different temperature layers, the images appear upside down. These are known as inferior mirages as the image is below the object. The shimmering effect that often accompanies these mirages is the result of turbulence, caused by the warm air rising from the surface.

    Over large bodies of water or ice sheets, air close to the surface can be cooler than that above, causing light to be bent downwards. These temperature inversions can also be present over the land, particularly overnight and during early the morning when warm daytime air has been transferred away from the surface.  If the temperature inversion is sufficiently strong, an atmospheric duct is formed in which the downward curvature of the light rays exceeds that of the Earth. The eyes again perceive the light to have travelled in a straight line, and crossing of rays produces a superior mirage in which an inverted image appears above the erect one. This can give the illusion of boats floating upside down above the horizon.

    A complex form of superior mirage is the fata morgana, in which multiple ray crossings in an atmospheric duct lead to a number of stacked and often rapidly changing images. Thus distant mountains, coastlines and islands can be distorted into unrecognisable shapes, while walls of water can appear to rise out to sea above the horizon.

    By: Billy Payne