Sea Science: by Jordanna Sheermohamed
Seeing is believing, and a human’s ability to observe with our eyes often dictates what we accept as truth. There are times, however, when observations can deceive the mind, playing on the eye’s limited abilities. Optical illusions, such as mirages, have been woven into historical accounts of visions on the ocean or in the deserts, sometimes leading sailors to their unfortunate demise.
Mirages occur when light travels through air masses of different temperatures and, therefore, densities. The light will ultimately bend toward the colder air, which is denser than warmer air because it contains more molecules in a given space. As the light enters and travels through the different layers, it takes multiple paths to the observer, and therefore may produce two images. Because the human eye generally perceives light to move in a direct line, the brain has a hard time distinguishing the optical illusion as being nonexistent.
Mirages can be further classified by the orientation of the air temperature. For instance, inferior mirages, also known as downward mirages, refer to the fact that the mirage appears below the actual object. This happens because the light passes through an unstable column of air, meaning colder air aloft and warmer air at the surface. This forces the light to bend upward, producing an image below the object. The manifestation of water in the hot and dry desert is a well-known result of this scenario, in which excessive sunlight heats the ground, producing the uneven temperature column. The even more common roadway shimmer seen on highways is also a type of inferior mirage.
A second type of mirage is known as a superior mirage, also called an upward mirage. The name is due to the bending light that displays the mirage above the object’s actual location. The sunlight passes through a warmer layer of air before entering into a relatively cooler layer of air near the surface. This forces the light to bend downward, producing the upward image. Superior mirage appearances generally occur near cold or icy landmasses and ocean surfaces. Tales of ghost ships and floating cities are examples of superior mirages, which are the most often type seen on the sea.
One of the most complex type of mirages, the notable “Fata Morgana,” forms as a result of several variations of air temperatures. This forces the light to bend multiple times on its downward trajectory, creating a rapidly changing image, which can appear vertically stacked, inverted, or even a series of the same object.
Other optical phenomena, such as rainbows, auroras, moon bows, and green flashes, also offer up beautiful reminders of atmospheric powers, but none as confusing and mysterious as the mirages that taunt our ability to discern reality from trickery.
Jordanna Sheermohamed is president and lead meteorologist of Weather Forecast Solutions, a weather-forecasting firm (WeatherForecastSolutions.com). Comments are welcome below.