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Optical Effects Questions Answered

What is the Fata Morgana?

Weve all seen optical effects in the atmosphere such as a rainbow. But one of the strangest and most famous is the Fata Morgana. It is a vast unusual mirage, which appears over the water in just a few locations scattered around the globe.

It is most noted for its appearance in the Strait of Messina off the coast of Italy. Friar Minasi, an eighteenth century Italian poet, wrote a classic description of the phenomena. It has been described as a town in the sky with beautiful palaces and towers with people walking through the streets.

No one is sure what the mirage reflects, but it probably manifests itself by using multiple distortions of images. This is thought to be the result of distant objects such as cliffs and buildings being magnified and distorted into shapes resembling wondrous castles.

Why does the moon appear large in the distant horizon?

During clear skies, the moon can appear as a giant glowing orb when it is low on the horizon. The moon is actually not any closer than usual. What we are seeing is an optical illusion. This is known as the ponzo illusion or the railroad track illusion.

If two blocks of equal size are inserted between railroad tracks, the distant block will appear larger. Due to the appearance of converging parallel lines, in this case railroad tracks, we perceive the narrowing upper portion of the tracks as being farther away as compared to the wider and nearer tracks. The tighter tracks makes the more distant block look larger. When we focus our attention to the horizon, the sky seems much more distant than it is overhead.

Have you ever noticed how the horizon turns a bright red and orange color during fall sunsets?

During Fall we experience some of the most visually spectacular sunsets the Great Lakes region has to offer.

Lake effect clouds play an important role in creating these sunsets as they form off of the Great Lakes. Lake clouds differ from clouds found in large scale storms, which usually produce overcast skies across an entire area.

In lake effect storms there can be breaks in the overcast on the horizon as lake effect can be extremely localized. As the sun sets, the rays of light coming from the sun shine underneath these low level clouds on the horizon. Due to the path of light coming through the atmosphere the observer only sees the longer wavelengths of red and orange on the underside of the clouds.

Is there such a thing as "Snowbows"?

Rainbows are unique optical phenomena in the atmosphere that form when the suns light gets refracted through tiny liquid drops of water in the atmosphere. The formation that you speak of occurs in cold weather with air temperatures below 32 degrees, especially in the upper atmosphere.

When you see Rainbow-like formations in the fall, winter, or early spring the formation is actually called a halo. Light gets reflected differently through ice crystals as compared to liquid water. So, Snow-halo may be a more accurate term to describe youre sighting. Sometimes the best Snow-halos form when a thin layer of Cirrus clouds are present between the sun and moon and youre line of sight.

What causes double rainbows?

We have discussed how the refraction of light entering a raindrop creates the different colors of a rainbow. However, the inside of the drop is more complicated than you may think.

The light can also reflect off the back of the raindrop and then exit the raindrop in a different location; so not all the light exits at that point, but some of this visible light is reflected a second time. After the second reflection, the light will leave the drop at a different angle. This causes the creation of a second rainbow. The first one (on the inside) is called the primary rainbow and the outside one is called the secondary rainbow.

It is even possible for the light to be reflected many more times within a raindrop, but one rarely ever sees more than two rainbows.

What caused the colors that I saw in the clouds?

We have received several inquires about what appears to be rainbows which have been spotted in thin cirrus clouds.

Most folks think of just one type of rainbow, but there are many types and categories including cloud bows, primary bows, secondary bows, red bows, reflection bows, sun dogs or moon bows (to name just a few).  I believe the folks inquiring may have spotted an iridescent cloud.

This optical effect can produce bright spots, color bands or borders of colors.  This can occur when visible light from the sun is diffracted or bent by water droplets or ice crystals in the cloud. Sometimes iridescence clouds can be seen far from the sun. However, it is most frequently viewed when the sun is at a closer angle to the cloud. Remember to block the suns direct rays behind a building or tree to view iridescence safely.

How come the moon sometimes has a halo around it?

Remember that the highest clouds in the atmosphere are called Cirrus clouds. These clouds have refractive effects on visible light that can actually produce a pattern that appears like a halo. The angle of light incidence necessary to produce this spectacular effect is around 22 degrees.

Much smaller colored rings sometimes surround the moon also. These are not a halo but a corona produced by the diffraction of light by the water droplets of clouds. And of course moonlight can create a rainbow effect, although to the unaided eye it is usually hard to observe. Good viewing on a cold late February night.

I saw a faint rainbow, but no rain. Is this possible?

It seems like just about anything is possible when it comes to the weather.

Although we typically see a rainbow during a rain shower when the sun peaks through the clouds, it is possible to see a rainbow when no rain is falling. Remember a rainbow occurs when the light from the sun enters a drop of water and is bent (refracted) and reflected back to your eye. But the drop of water doesnt necessarily have to be a large raindrop; tiny droplets in a cloud can also reflect sunlight to produce a rainbow.

It should be noted that there may have been rain evaporating before reaching the ground.  The clouds high in the atmosphere may also be in the form of ice crystals, which can also produce a rainbow.

What is the green flash that occurs near sunset?

The green flash is an interesting and rare atmospheric phenomenon that can occur at sunset or sunrise.  If conditions are right the flash is seen for less than a second and is not always green.  At times the flash is blue.  Just as the sun sets or rises at the horizon, the upper edge of the sun can be seen. The upper or lower edge of the sun will usually take on the appearance of a green (or blue) rim that is too difficult to be seen by the human eye, unless the rest of the sun is obstructed.  When the sun breaks the horizon, and a majority of the sun is covered, the green or blue flash will be caused by the dispersion of light.

Why is the sky so blue in September?

On a clear day light entering our atmosphere collides with various particles, mainly nitrogen and oxygen. These collisions then scatter shorter wavelengths turning the skies blue.  The sky appears a deeper blue on days when there are fewer particles in the atmosphere to scatter the suns light, such as a clear day with low humidity.

On the other hand, compare that to a hot summer day when it is very humid. There are more water vapor particles for the sunlight to be reflected off of, making the sky appear milkier, or hazy.

If you have ever visited the Rocky Mountains, you may have noticed the brilliant blue skies. There is less atmosphere between the ground and the sun at higher elevations, equating to less sunlight being reflected.

What is the Aurora Borealis?

Occasionally we are treated to an amazing display in the nighttime northern sky. We are talking about the Northern Lights. Also known as the Aurora Borealis, these optical events are much more common in the northern latitudes, but during periods of high solar activity, Western New York can also experience them.

The aurora forms as highly energized particles from the sun, known as the solar wind, impacts the Earths magnetosphere. This collision moves high energy particles from the Earths magnetosphere into the Earths upper atmosphere where they excite gas atoms and molecules. The excited gases in the Earths upper atmosphere emit visible radiation and start to glow.

The best possible conditions for the aurora borealis are clear, calm, and dry nights.  Look away from any artificial light sources for best viewing.

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