Why do Things Look Wavy in the Heat?

While most of us have experienced the waviness and lack of clarity that occurs above a hot surface (such as a highway in the summer) many of us don’t understand just what happens to produce this phenomenon. The heart of the matter lies in a process called refraction. Light that enables us to see travels in waves. When these are “bent” for some reason whatever we see will change in appearance.

Temperature differences in the air are also a major factor in producing the waves we see. One simple explanation states that light waves from the sun pass through warm air faster (it’s less dense) and that causes it to turn up again quickly. This change in direction is the primary reason things look wavy above a hot surface. People traveling in the desert experience this same phenomenon when they see a mirage.

A mirage is defined as a phenomenon in which an “object appears displaced above, below or to the side of the real object.” The word “mirage” comes from the French for “to look at” and from Latin for “to wonder at.”

While that may seem to describe the activity adequately, it might help to consider that, in addition to the action of the light, the air itself is no stable. Cool air from above sinks toward the surface, as is natural for cooler air. But as it comes close to a super-heated surface its temperature increases, causing it to rise. Instability and the reaction of light waves produce an almost moist view of objects behind the waves.

Scientists have divided the illusion into two types of “mirage.” One is known as an inferior mirage. This occurs when the unclear or unreal image is lower than the object. A superior mirage occurs when the unclear or unreal image is higher (above) the real object. This is best seen when the sun has passed below our horizon but we still see some of the red/orange just above the horizon line.

Some discussion has taken place about the idea of a mirage or even the wavy lines we see above a hot road surface. Some have wondered if this is hallucination. The answer is most likely “no.” Hallucinations are thought to exist only in the mind of the person experiencing it, while a mirage is real in the sense that many people can see the same thing. It is actually taking place because of the unique properties of light and heat.

Other examples of the wavy-line phenomenon are a cooking grill in use or the hood of an automobile that has been driven for a length of time. In these situations, the heated air around the grill or engine is rising. It is unstable compared to the cooler air around it. A mirage is a similar phenomenon but on a much larger scale. The large area of hot, open sand and the cooler air above it contribute to this curious view.