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Highway mirage is an optical illusion

I was driving back from Bend over the weekend on a long straight road by Crater Lake when I kept seeing water on the road ahead. It had been raining off and on and the road is very flat, so seeing water wasn't surprising.

But every time I got close enough to the water to really see it, it would disappear. I've heard this called a "highway mirage" but was wondering how it really works?

— Kasey G., Ashland

Isn't it too bad, Kasey, that just when you're trying your hardest to keep your eyes focused on the road ahead of you, the road starts playing tricks on you?

The common phenomenon known as a "highway mirage" is nothing new, though depending on where you normally travel and what the weather is like, some drivers may see it more than others.

What you saw is an inferior mirage, and is most noticeable on hot sunny days or sunny winter days, when it looks like a pool of water on the road several hundred feet away.

Really, the fake pool of water you were seeing isn't that different than the desert mirages depicted in books and movies over the years.

Only now, a lot more of the world is covered in highways, with thick black asphalt increasing the ground's heat and upping the chances you'll see a mirage.

The mirage forms when the air near the ground is much warmer than the air above it — the larger the difference in temperature, the stronger the mirage appears.

Light rays from the sun passing through the relatively warmer area are bent upwards in a phenomenon called refraction.

The optical illusion of a pool of water is actually a distorted image of the clouds and sky above.

When we get closer, our eyes adjust and the refracted light isn't as noticeable, causing the pools of water disappear.

Send questions to "Since You Asked," Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by email to youasked@mailtribune.com.