There’s a scene that’s appeared in so many movies and TV shows that it’s become a cliché. You know the one: it’s where this guy is staggering, eventually crawling, through the desert. Cut to a shot of the sun glaring down at him. Cut to a close-up of his parched lips. Cut to a wide shot of the desert. Cut back to the sun. Etc., etc., etc.
How the scene plays out depends on the plotline of the show you’re watching, of course, but in more than a few instances, the next scene involves the poor fellow seeing water in the distance, staggering through the sand panting, and finally collapsing in a spot just as dry as everywhere else. The water he thought he saw wasn’t there: it was a mirage.
Images of oasis complete with tents and camels owe as much to the hallucinatory effect of too much heat and not enough water on the human mind as they do to any physical phenomenon, but that doesn’t explain all mirages, because even if you’re motoring down the highway with the air conditioner cranked up to “Arctic,” listening to cool tunes and sucking on a Big Gulp, you’ll probably see a mirage this time of year: the appearance, dead ahead on the pavement, of what looks like a sheet of water, complete with reflections from the power poles lining the road.
That’s the simplest form of mirage, and it’s related to another common phenomenon this time of year: the shimmering you see above sun-heated objects.
Actually, that effect’s not confined to the summer: it can be even more noticable in the winter, when “heat waves” can be clearly seen rising off, say, the surface of a black car parked in the sunlight on a cold day.
What’s happening is that air of two drastically different temperatures is mixing. The air rising off of the sun-heated object is much hotter than the air that’s just hanging around. Although we think of air as perfectly permeable to light, the fact is that light travels at a slightly different speed through air of different densities. Air expands when it’s heated, which means it’s less dense; light travels through it slightly faster than it does through air that’s cooler and more dense. This change in speed also produces a slight change in direction The result is that light passing through a place where the air temperature changes bends slightly, a phenomenon called “refraction.”
Just above a hot object, the warm, rising air hasn’t yet mixed completely with the cooler air surrounding it, so you get discrete patches of air at various temperatures. Light passing through those patches is bent, first one way, then the other as the air swirls about. The result is a shimmer that’s very visible to the naked eye. (It’s even more visible in the shadows on the ground, where you can see a very clear image of the air roiling above the heated-object.)
This refraction of light passing through air at different temperatures is also what causes mirages, from the simple “water-on-the-road” effect all the way up to the wonderfully named “fata morgana”–about which, more in a moment.
The “water-on-the-road” effect occurs because, on a sunny day, the road gets very hot. Just above the road, the air is also very hot, but a short distance further up, the air is still relatively cool. The path of light rays passing from one temperature zone to the next is so sharply bent that the light reflecting off the surface of the road no longer reaches our eyes at all, and so the road is no longer visible. The light rays that actually reach our eyes originated in the sky, so instead, we see a patch of silvery blue. In effect, that “water on the road” is a piece of sky. An object alongside the road may also appear “reflected” in the “water” because the light waves bouncing off of it down onto the road are also sharply bent out of their normal path and into our eyes. Because images are displaced downward, this is called an “inferior” mirage.
Optics dictates that the mirage’s image will always be displaced toward the hotter, less-dense air, so when the when the atmospheric situation is reversed, and there’s a layer of cool air beneath a much cooler layer of air–such as over water, or in the winter when very warm air blows in over snow cover–you can get a “superior” mirage, where distant objects–even objects normally out of sight–appear, usually much distorted, above the horizon. This effect is not uncommon in Saskatchewan, where you can sometimes find yourself apparently driving through a giant bowl, as if the horizon had been slightly tilted up on all sides. It’s a good bet the weather’s about to change when that happens.
That’s a really cool sort of mirage, and I get a kick out of it every time I see it, but it’s not the best kind of all: that’s the fata morgana, something I’d never heard of until I started researching this column.
The fata morgana was first described in the Straits of Messina in Italy, but it’s been reported over other bodies of water, including Lake Geneva and the Great Lakes…and I’m pretty sure I’ve seen it here, too. It’s a complex combination of an inferior and a superior mirage; the precise atmospheric conditions that produce it weren’t described anywhere I could find, which usually means nobody has figured them out yet. The result is a looming, fantastic “castle” hanging in the sky. Its exotic name comes Morgan le Fay, the sorceress of the King Arthur legend.
So the next time you’re crawling across the parched earth, panting for water, the sun beating down on you, don’t be fooled by that shimmer in the distance. It’s not the liquid refreshment you crave, but only light that’s been bent out of shape; you’ll have to suffer with your thirst a while longer.
There’s usually water somewhere around the 14th hole, anyway.
