Why Do Roads Look Wet In The Distance?

It's one of the things we tend to focus on during long road trips on a hot summer day — a seemingly endless puddle in the distance that appears to vanish as we approach the spot — and then before figuring out why, we see another ahead and repeat the process, endlessly chasing vanishing puddles. It passes the time.

That's obviously no puddle, it's a mirage. While we associate them with being stranded in the desert and thinking there's water ahead, they're a regular feature of roads as well, and it's much more comfortable to encounter them while inside your air-conditioned car with a Big Gulp.

The familiar optical effect occurs on the road when the asphalt absorbs heat from the sun, warming the air directly above it and making it less dense than the cooler, denser air above it. As the light rays from the sky pass through the differing densities, it curves upwards (or refracts) and gives the eye the illusion of wet pavement.

When the light produces the wet road mirage, it's what's known an inferior image, which makes an object look inverted, in this case a bit of sky that appears to be settling on the ground. Your eyes and brain can't process all the bending that the light does when passing through the different densities, and instead can only see the illusion that they've bounced from the ground, interpreting that as its origin point. Because sand holds heat in the same way that asphalt does, the same type of mirage occurs and causes unfortunate souls to think that there's water in the distance that doesn't exist.

But mirages aren't only limited to hot roads and sand. If you were to find yourself wandering on snow-covered ground in Antarctica or a very cold body of water, you might encounter the opposite effect, known as a superior mirage. This occurs when warmer air sits above much colder air, bending the light downward and producing a quasi science-fiction effect of making objects appear higher in altitude than they actually are, like a boat floating in midair.