"Mirages: Seeing Things that are not There"
Teagan Calahan, Undergraduate Geographer/EIUWC Weather Analyst/WEIU Skywatch Forecaster
January 27, 2012
It usually happens when someone is stranded in the middle of the desert, desperate for water and refuge. Out of nowhere, off in the distance, it’s there: water. But as the story usually goes, it is only a mirage. Of course seeing water in a desert is not the only kind of mirage that exists.
On a hot sunny day when you look into the distance it can sometimes seem like there is water on the road (Figure 1). This is actually considered a mirage, usually known as a highway mirage. Whether you are driving on a highway or lost in the desert, the mirage that you see is most likely one of two main types: inferior or superior.
An inferior mirage is what usually comes to mind when we hear the word mirage, it is the one that makes it seem like there is water where there really isn’t. This happens when light rays pass from warmer air near the ground, to cooler air above, which causes the light ray to bend upward. We do not notice that the light ray is bent upward though because our eyes perceive light rays as straight lines, not being bent. This then makes it appear that the sky is on the ground, hence seeing ‘water’ on pavement.
On the other hand, there are also superior mirages, which are the opposite of inferior. When a superior mirage occurs, this means that the ground is cooler and the surrounding air above is warmer, also known as a temperature inversion. The light rays in this situation then are bent downward. With the combination of the light ray being bent downward and the curvature of the earth, the result is seeing an image in the distance upside down, or a mixture of upside down and right side up (Figure 2). Superior mirages are much less common, and are mostly seen in the polar regions of the earth.
So the next time that you see what looks like water on a hot summer day, you can be sure of two things: it is not really water, and you have just seen a mirage!
Figure 2. Superior mirage. Photo from NOAA.