ATSC 113 Weather for Sailing, Flying & Snow Sports


Optical Phenomena

Learning Goal 10f: Recognize and explain optical phenomena over the sea, including mirages, fata morgana, and the green flash.


Sailors have claimed to see all sorts of strange things on the horizon: a flash of green light, floating castles, boats on top of boats, etc. Many of these ‘bizarre sightings’ were actually the result of optical phenomena, an interaction between light and the atmosphere. These are not illusions; they are real images that can be photographed. Here are a few optical phenomena to enjoy:

Mirages

Mirages are created when light passes through air of different temperatures. Two types of mirages are inferior and superior.

An inferior mirageoccurs when you have a dense layer of cold air sitting on above of your line of sight, with a layer of less dense warmer air below your line of sight.  This happens frequently on hot summer days when the sun shines on a black asphalt highway, and the hot ground heats the bottom centimeters of air.  When rays of light coming down from the sky get close to the hot air near the ground, the light rays are bent (refracted) back up toward your eyes. 

ray path for inferior mirage

Source: R. Stull, 2017: Practical Meteorology. 


This is your typical ‘oasis in the desert’ mirage, where you look down toward the highway or desert sands and see what appears as a pool of blue water.  In reality, it is light the blue sky bending back up toward your eyes.   Since warm air rises and cool air sinks, the air layers are likely to mix and inferior mirages will not last long unless there is continued heating by the sun. 

Inferior mirages happen less frequently over the ocean because the ocean surface usually doesn't get extremely hot.  However, it can occur over the ocean when very cold arctic blows over unfrozen water.

Note in the image below, an inferior mirage makes it look like there is water on the pavement, when in fact there is not.

Inferior mirage

Source: U-ichiro Murakami (Murambo) - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=11630764


A superior mirage is the opposite: a layer of warm air sits above your line of sight with a cool layer beneath it. This is also known as a temperature inversion. Light bends down towards the denser air, but because our eyes assume the light we see travels straight, the object we are seeing appears higher than it actually is. Therefore we see the image, or inversions of it, above where it actually is.

If another ship sails away from you, eventually they will disappear below the horizon.  However, if conditions create superior mirages, then you can still the other ship further away from you even though the ship is a short distance below the horizon. Similarly, with the refraction of a superior mirage you can see mountain tops sooner as you sail toward a mountainous island or coastline, even though the mountain top might still be slightly below the geometric horizon. Also, boats on the distant horizon sometimes appear to be floating in the air above the water. A superior mirage can also act like a lens make objects appear smaller or larger, and closer or further away, than they actually are.


mirages

For a superior mirage, the mountain appears higher than it actually is.  This means that you can see the mountain top from further away, if the air is clear.  Source: This image was created during "DensityDesign Integrated Course Final Synthesis Studio" at Polytechnic University of Milan, organized by DensityDesign Research Lab in 2015.
Image is released under CC-BY-SA licence. Attribution goes to "Ludovica Lorenzelli, DensityDesign Research Lab". - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=37081455


Summary of Mirages
  • Inferior mirage.  Rays of light are bent upward.  Looking down below the horizon, you see objects (or the sky) that are actually above the horizon.
  • Superior mirage.  Rays of light are bent downward.  Looking up above the horizon, you see distant objects that might be lower or even below the horizon.

Fata Morgana

When the temperature inversion is not even, you may see a Fata Morgana.  This could be caused by several layers of warm and cold air that cause a combination of superior and inferior mirages.  Because of the uneven inversion, light is refracted in strange ways, creating multiple segmented reflections of the original image, with some sections towering high above the water. The image created by a Fata Morgana is usually much less recognizable than that in a mirage, and often appears as floating walls or castles.

fata morgana sketch

Source: R. Stull, 2017: Practical Meteorology. 


The name of this mirage comes from Fairy Morgan (or Fata Morgana in Italian) of the King Arthur legend, an enchantress/magician who could create illusions of floating castles.


Superior mirage

Source: Brocken Inaglory, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=17778586
Caption: Fata Morgana


Fata Morgana

Source: Gerd A.T. Mueller - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=312887
Caption: A Fata Morgana of a boat


Green Flash

A green flash is a rare optical phenomenon that only lasts a few seconds immediately after sunset or before sunrise. As the sun sets below the horizon, the sun’s rays travel through more of the Earth’s atmosphere than when the sun is directly overhead at mid-day. The atmosphere acts like a prism, splitting white light from the sun into its various colours by wavelength. The shorter wavelengths (green, blue, violet light) are the last to set below the horizon. Blue and violet light typically get scattered by particles in the atmosphere, leaving green as the last light you see before all light from the setting sun disappears below the horizon.


green flash sketch

Source: R. Stull, 2017: Practical Meteorology. 


Mirages can amplify a green flash by exaggerating the separation of light into its different colours, and making the band of green light look wider.

Green flashes only occur under the perfect atmospheric conditions, making them a relatively rare sighting.  Residents of islands in the Caribbean Sea report seeing green flash many times each year.  The image below shows a progression of a green flash during a sunset.

Green flash

Source: Brocken Inaglory - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6527401


Additional Resources: (non-required material)

An Introduction to Mirages: http://www-rohan.sdsu.edu/~aty/mirages/mirintro.html

How can I see a green flash? – EarthSky.org: http://earthsky.org/earth/can-i-see-a-green-flash

The math behind atmospheric optical phenomena (mirages, rainbows, halos, etc.) Stull 2017: "Practical Meteorology: An Algebra-based Survey of Atmospheric Science".  Chapter 22 Atmospheric Optics


Keywords: mirage, inferior mirage, superior mirage, inversion, Fata Morgana, green flash


Image credits: are given near the images.