You’re driving down the road on a hot day. In the distance you see a puddle, only when you get to that point in the road, the pavement is dry. Has this ever happened to you? What’s going on? Chief Meteorologist Alan Sealls explains.
Most of the time when we drive, we are too focused on traffic to notice… It looks like water on the pavement. Even the lights from oncoming cars seem to be reflected in it. It’s not water, it’s a mirage. What you think is straight in front of you is actually an upside down image of what’s above the road. The road absorbs and gives off so much heat that it causes light to bend or refract. Air density changes so quickly that you get the same result as when you look at a stick in water. The stick seems to bend. That’s refraction.
Over hot pavement, the light bends upward. Since the road is constantly giving off heat, the density fluctuates and makes the image shimmer. You can see that on anything that’s really hot like a roof, or even the top of a vehicle. Compare the shimmering over the pavement versus the grass. You’ve certainly seen this at a barbecue, when you look at heat coming off of a grill.
Another sunny, hot, dry day… I’m working up a sweat and it’s 90 degrees. But that’s the air temperature. Look at the pavement. My thermometer shows a ground temperature of over 125 degrees.
Get down lower to the ground and look. The effect is really dramatic. It’s not just about how hot the ground is. It’s about the rapid change in temperature or density of air just above the ground. Did you know you’ll find the same type of mirage above water? Look in the distance across Mobile Bay. The trees seem to float above the shore but it’s just the light in the distance bending upward. This kind of mirage is not in your mind, it’s in the air.
Alan Sealls, News 5
You will see these same mirages in the winter, and at night. The quickly changing air density is also what causes the stars to twinkle at night.
The dial thermometers used in the parking lot were kept shaded (one flat on the pavement and one just above the pavement) before the recording, to avoid increased heat from direct sunlight.
Extremely hot pavement is definitely something to consider when you take your dog for a walk on hot afternoons. Grass will be much less hot.
On average, drivers of cars will see more of a mirage than SUV or truck drivers simply because the lower you are to the ground, the more refraction or bending occurs in your line of sight.