Why Does the Road Look Wet on Hot Days?

iStock / baona
iStock / baona

Reader Robert wrote in to say, “As I drive across the Panhandle of Texas, I am wondering why the highway looks wet and shiny off in the distance but normal as one gets closer.”

For the same reason that cartoon characters lost in the desert often think they see an oasis: a mirage caused by refraction. 

First, a quick physics lesson. Light moves slower through denser mediums and faster through less dense ones. As it travels through a given medium—say, air—it moves in a straight line. When it passes from one medium to another, though, and there’s a difference in density—say from air to water—the light waves change speed, which causes them to also change direction or refract, and then continue in a straight line on their new path.

An easy way to see refraction in action is to put a straw into a glass half-filled with water. From the top, it looks like the straw is bent or broken. From the side, depending on where in the glass the straw is, it might look like it grows wider below the water line or even detaches from the part above the water line. 

Remember that you see objects because light reflects off of them and then travels to your eye. What’s happening here is light from the straw is reflecting and traveling to your eye through two different mediums—the air and the water. Above the water line, light travels directly from the straw to your eye through the air and doesn’t refract (technically it travels through air into the glass and back into air, but the refraction into and out of the glass causes little enough deviation to not matter). Below the water line, though, the light reflecting off the straw has to travel through the water into glass and then into air. This light changes medium and speed, so it refracts or bends on its way to you. Your eye and brain don’t account for refraction when looking at the straw (stupid brain), and assume the object to be where the light waves appear to originate from along a straight line. The top and bottom parts of the straw are in line with each other, but the light from them comes along two different lines, making the straw look broken after your visual system gets done with it.

What Robert is describing is also the work of refraction. Maybe you were driving around one day and thought you saw a puddle on the pavement a little ways down the road. Once you got to the spot where you thought you saw the water, it was gone. Looking farther down the road, you see another puddle, but that one also disappears as you get closer to it. You can chase the puddles all day, but you’ll never actually find one. 

Light refracts not just when it moves through two different mediums like air and water, but also when it moves through different layers of the same medium that have different densities. As the sun beats down on the blacktop, it heats it up. The road, in turn, heats the air immediately surrounding it, keeping the air just above it warmer and less dense than the air farther up. 

As light from the sky travels downward toward the hot road, it moves through these increasingly warm and less dense layers of air, changing speed and refracting as it moves through each one. It winds up taking a sort of u-shaped path down toward the road, then parallel to it and finally back up into the sky—where it may meet the eye of someone standing up the road. 

When this refracted light reaches you, your brain and eye—like they did with the straw in the water—don’t account for all the bending it did along the way. They trace it back along a straight line and interpret that point as its origin and the location of the object. What you see, then, is a little bit of sky that appears to be sitting on the ground—an inferior mirage where the mirage is under the real object. Even as your brain and eye try to quickly make sense of what you’re seeing, the brain knows that sky on the ground doesn’t make sense, so you often wind up perceiving the mirage as water on the road reflecting the sky. Turbulence of the air also distorts the mirage, strengthening the effect. 

Sand, like highways, is really good at holding onto heat and warming up the air near it, so these types of watery mirages often happen in deserts and can fool people into thinking there’s water nearby.

Science Finds a Better Way to Calculate 'Dog Years'

thegoodphoto/iStock via Getty Images
thegoodphoto/iStock via Getty Images

Anyone who has ever owned a pet is likely familiar with the concept of “dog years,” which suggests that one year for a dog is like seven years for a human. Using this conversion metric, a 2-year-old dog is akin to a high school freshman, while a 10-year-old dog is ready for an assisted living facility.

If that seems rather arbitrary, that’s because it is. But now, researchers at the University of San California, San Diego have come to a more data-based measurement on dog aging through DNA.

The paper, published on the preprint server bioRxiv, based the finding on DNA methylation, a process in which molecules called methyl groups attach themselves to DNA and serve as an indicator of aging. Generally speaking, the older living beings get, the faster the rate of methylation. In the study, 104 Labrador retrievers were examined, with subjects ranging from 1 month to 16 years old. The results of their DNA methylation were compared to human profiles. While the rate of methylation tracked closely between the two—young and old dogs had similar rates to young and old people—adolescent and mature dogs experienced more accelerated aging.

Their recommended formula for comparing dog and human aging? Multiply the natural logarithm of a dog’s age by 16, then add 31. Or, just use this calculator. Users will see that a 2-year-old dog, for example, wouldn’t be the canine equivalent of a 14-year-old. It would be equivalent to 42 human years old and should probably start putting money into a 401(k). But because methylation slows considerably in mid-life, a 5-year-old dog is approximately a 57-year-old human, while a 6-year-old dog is nearing 60 in human years—a minor difference. Things level out as the dog gets much older, with a 10-year-old dog nearing a 70-year-old human.

Different breeds age at different rates, so the formula might not necessarily apply to other dog breeds—only Labs were studied. The work is awaiting peer review, but it does offer a promising glimpse into how our furry companions grow older.

[h/t Live Science]

Sssspectacular: Tree Snakes in Australia Can Actually Jump

sirichai_raksue/iStock via Getty Images
sirichai_raksue/iStock via Getty Images

Ophidiophobia, or fear of snakes, is common among humans. We avoid snakes in the wild, have nightmares about snakes at night, and recoil at snakes on television. We might even be born with the aversion. When researchers showed babies photos of snakes and spiders, their tiny pupils dilated, indicating an arousal response to these ancestral threats.

If you really want to scare a baby, show them footage of an Australian tree snake. Thanks to researchers at Virginia Tech, we now know these non-venomous snakes of the genus Dendrelaphis can become airborne, propelling themselves around treetops like sentient Silly String.

That’s Dendrelaphis pictus, which was caught zipping through the air in 2010. After looking at footage previously filmed by her advisor Jake Socha, Virginia Tech Ph.D. candidate Michelle Graham headed for Australia and built a kind of American Ninja Warrior course for snakes out of PVC piping and tree branches. Graham observed that the snakes tend to spot their landing target, then spring upward. The momentum gets them across gaps that would otherwise not be practical to cross.

Graham next plans to investigate why snakes feel compelled to jump. They might feel a need to escape, or continue moving, or do it because they can. Two scientific papers due in 2020 could provide answers.

Dendrelaphis isn’t the only kind of snake with propulsive capabilities. The Chrysopelea genus includes five species found in Southeast Asia and China, among other places, that can glide through the air.

[h/t National Geographic]

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