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HOW COME? When the road shimmers

On a hot day, why does the air over a road sometimes shimmer? asks a reader.

What do hot asphalt, candle flames and your kitchen toaster (lever pressed down) have in common? All can create that shimmery effect, where the air just above and the objects seen through it appear to tremble. It's like looking at swimmers standing in a backyard pool; as the water moves, so do their legs seem to waver underwater, like plants under the sea.

But while the pool water sloshes back and forth because of the movements of other swimming, splashing bodies, the currents in the air above a road are caused by rising heat. Parcels of air, heated by sun-baking asphalt, rise and swirl. Result: a fun-house view of objects in the distance. Telephone poles tremble; approaching cars shimmer and shake.

The visual effects come courtesy of bendy light waves, which change course as they pass from cooler air into hotter. The bending, called refraction, is the result of a sudden shift in speed.

How does it work? Light zips through a vacuum, such as the space between stars and planets, at about 186,000 miles a second. But a light wave slows when it's traveling through matter, such as water, glass or air. So as light streams through air, it slows in denser, colder air, in which gas molecules are more closely packed. Then when it hits a pocket of thinner, hotter air, light speeds up.

Each shift in speed also causes a shift in direction of the beam. As light waves pass down from the cooler air above to the heated air above hot asphalt, the traveling beams shift, and shift again. And as the randomly bending light waves reach our eye, we see that shimmery effect. The effect is easiest to see when light has farther to travel. Which is why the view just above a summer road looks normal up close but wavery in the distance.

On a grander scale, the shimmering effect is also known as atmospheric boil, and it's what makes stars twinkle. All day long, the ground heats in the sun; all night long, the ground radiates stored heat back into space. As the heat rises, air just above the ground warms and rises, too. The heated air tumbles, rather like water boiling in a pan, mixing with cooler air as it ascends into the sky.

Starlight, streaming down through the atmosphere, zips through cooler, denser air into the warmer, thinner, swirling air nearer to the ground. Just like the sunlight bending this way and that above a summery road, the starlight constantly changes speed and direction on its way to your eyes. And so as you look at the (steadily shining) stars, they seem to waver and twinkle.

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