Everyone has at some point been tricked by the optical illusion of water on a road in front of them or the appearance of a lake in the middle of the desert. Known as mirages, these phenomena represent one interesting example of important scientific principles at work. While these features don’t exist, our brain fools us into believing they are. So what’s really happening? Is there a reason our eyes play these tricks on us? Let’s find out.
To understand the physical process behind this illusion, we must start with an understanding of how temperature behaves in the atmosphere. As you move upward from the surface, air temperature decreases with height. Therefore under average atmospheric conditions, the higher above the ground, the colder the air. The rate at which temperature decreases is known as the lapse rate and is a very useful value within meteorology. Lapse rates help us identify how air will move vertically, analyze the potential for thunderstorms, and, in this case, understand how features such as mirages work.
The strength of lapse rates vary greatly throughout different sections of the Earth’s atmosphere. For example, in the stratosphere (the second lowest layer of the atmosphere), the temperature actually increases with height. This situation is called an inversion and is the opposite of what we typically observe in the troposphere (the lowest level of the atmosphere where our weather forms). If the lapse rate is small, meaning that the decrease happens gradually as you move vertically upward, air is not likely to rise and weather conditions are typically much calmer. When the lapse rate is a high value (i.e. the temperature decreases very quickly with height), air rises rapidly and the potential for clouds and precipitation increases. It is important to note that while lapse rates are important, they are only one of several factors that contribute to the potential for active weather.
In the case of a mirage, we have a very high lapse rate really close to the surface. A hot road for example, underneath cooler air directly above it can create an extreme difference in temperature over a very short distance. Since light travels through warm, less dense air more quickly and wants to take the shortest path possible to any particular point, light travelling near the ground will bend upward in a U-shape through the warm air. To a viewer, this makes light from the sun appear as though it traveled in a straight line directly from the surface. We see what appears to be water because water and this large temperature difference bend light in a very similar fashion. Thus our brains misinterpret this bending of light as the presence of water on the ground. While the water itself does not really exist, mirages are interesting optical illusions that truly trick our brains into seeing something that differs from reality!
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©2019 Weather Forecaster Dennis Weaver