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.

Why Does Humidity Make Us Feel Hotter?

Tomwang112/iStock via Getty Images
Tomwang112/iStock via Getty Images

With temperatures spiking around the country, we thought it might be a good time to answer some questions about the heat index—and why humidity makes us feel hotter.

Why does humidity make us feel hotter?

To answer that question, we need to talk about getting sweaty.

As you probably remember from your high school biology class, one of the ways our bodies cool themselves is by sweating. The sweat then evaporates from our skin, and it carries heat away from the body as it leaves.

Humidity throws a wrench in that system of evaporative cooling, though. As relative humidity increases, the evaporation of sweat from our skin slows down. Instead, the sweat just drips off of us, which leaves us with all of the stinkiness and none of the cooling effect. Thus, when the humidity spikes, our bodies effectively lose a key tool that could normally be used to cool us down.

What's relative about relative humidity?

We all know that humidity refers to the amount of water contained in the air. However, as the air’s temperature changes, so does the amount of water the air can hold. (Air can hold more water vapor as the temperature heats up.) Relative humidity compares the actual humidity to the maximum amount of water vapor the air can hold at any given temperature.

Whose idea was the heat index?

While the notion of humidity making days feel warmer is painfully apparent to anyone who has ever been outside on a soupy day, our current system owes a big debt to Robert G. Steadman, an academic textile researcher. In a 1979 research paper called, “An Assessment of Sultriness, Parts I and II,” Steadman laid out the basic factors that would affect how hot a person felt under a given set of conditions, and meteorologists soon used his work to derive a simplified formula for calculating heat index.

The formula is long and cumbersome, but luckily it can be transformed into easy-to-read charts. Today your local meteorologist just needs to know the air temperature and the relative humidity, and the chart will tell him or her the rest.

Is the heat index calculation the same for everyone?

Not quite, but it’s close. Steadman’s original research was founded on the idea of a “typical” person who was outdoors under a very precise set of conditions. Specifically, Steadman’s everyman was 5’7” tall, weighed 147 pounds, wore long pants and a short-sleeved shirt, and was walking at just over three miles per hour into a slight breeze in the shade. Any deviations from these conditions will affect how the heat/humidity combo feels to a certain person.

What difference does being in the shade make?

Quite a big one. All of the National Weather Service’s charts for calculating the heat index make the reasonable assumption that folks will look for shade when it’s oppressively hot and muggy out. Direct sunlight can add up to 15 degrees to the calculated heat index.

How does wind affect how dangerous the heat is?

Normally, when we think of wind on a hot day, we think of a nice, cooling breeze. That’s the normal state of affairs, but when the weather is really, really hot—think high-90s hot—a dry wind actually heats us up. When it’s that hot out, wind actually draws sweat away from our bodies before it can evaporate to help cool us down. Thanks to this effect, what might have been a cool breeze acts more like a convection oven.

When should I start worrying about high heat index readings?

The National Weather Service has a handy four-tiered system to tell you how dire the heat situation is. At the most severe level, when the heat index is over 130, that's classified as "Extreme Danger" and the risk of heat stroke is highly likely with continued exposure. Things get less scary as you move down the ladder, but even on "Danger" days, when the heat index ranges from 105 to 130, you probably don’t want to be outside. According to the service, that’s when prolonged exposure and/or physical activity make sunstroke, heat cramps, and heat exhaustion likely, while heat stroke is possible.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

This article has been updated for 2019.

Chimpanzees Bond by Watching Movies Together, Too

Windzepher/iStock via Getty Images
Windzepher/iStock via Getty Images

Scientists at the Wolfgang Kohler Primate Research Center in Germany recently discovered that, like humans, chimpanzees bond when they watch movies together, the BBC reports.

In the study, published in Proceedings of the Royal Society B, researchers stationed pairs of chimpanzees in front of screens that showed a video of a family of chimps playing with a young chimp. They found that afterward, the chimps would spend more time grooming and interacting with each other—or simply being in the same part of the room—than they would without having watched the video.

They gave the chimps fruit juice to keep them calm and occupied while they viewed the video, and they chose a subject that chimps have previously proven to be most interested in: other chimps. They also used eye trackers to ensure the chimps were actually watching the video. If you’ve ever watched a movie with friends, you might notice similarities between the chimps’ experience and your own. Drinks (and snacks) also keep us calm and occupied while we watch, and we like to watch movies about other humans. Since this study only showed that chimps bond over programs about their own species, we don’t know if it would work the same way if they watched something completely unrelated to them, like humans do—say, The Lion King.

Bonding through shared experiences was thought to be one of the traits that make us uniquely human, and some researchers have argued that other species don’t have the psychological mechanisms to realize that they’re even sharing an experience with another. This study suggests that social activities for apes don’t just serve utilitarian purposes like traveling together for safety, and that they’re capable of a more human-like social closeness.

The part that is uniquely human about this study is the fact that they were studying the effect of a screen, as opposed to something less man-made. The chimps in question have participated in other studies, so they may be more accustomed to that technology than wild apes. But the study demonstrates that we’re not the only species capable of social interaction for the sake of social interaction.

[h/t BBC]

SECTIONS

arrow
LIVE SMARTER