The Earth Story

5 Optical phenomena you will probably never see.

The interaction between the atmosphere, particulates, light and water can create an array of amazing optical phenomena to feast our eyes on- some are more difficult to witness first hand than others. But, at least we have the facility of the internet to show everyone what they are missing, or provide an opportunity to the lucky few witnesses to gloat. Here are five of the rarest optical phenomena you are (un)likely to see, accompanied by a ‘what you need’ supplement to inform you when to have a camera ready and aimed in order to capture these awesome sights. May the odds be forever in your favour!

Number 5: The Fire Rainbow.

Technically, it is known as a circumhorizontal arc, but that’s not quite as much fun to say. This phenomenon is caused by clouds which contain water and ice droplets of nearly uniform size. These clouds diffract sun light which separates the light into different wavelengths, which we perceive as different colours. Although they look similar to rainbows in their colour arrangement; the mechanism by way the light is scattered to produce them is different. Rainbows are the result of refraction and reflection. When light is refracted, it is bent by passing through mediums of different densities, such as water. Reflected light bounces off a surface at an angle equal to the angle it hit the surface at. Diffraction, however, involves light waves being scattered into a pattern; creating what you see here.

You will need:

-Cirrus clouds at least 20,000 feet in the air. -Optimum amount and distribution of ice crystals. -The sun angled so light hits the clouds at precisely 58 degrees.

Number 4: The Moonbow

The principles behind the formation of a Moonbow are quite the same as a rainbow; light is refracted as it passes through water droplets in the sky. These refractive properties of the droplets cause light to be split into a band of colours, more specifically the light spectrum. The only exception here is the light source is no longer the Sun, but the Moon (yes, it is technically still sunlight).

While most of us will have seen a rainbow at least once in our life time, a much fewer number will have had the pleasure to encounter the lunar equivalent. This is because, as the Moon is more variable than the sun, many conditions need to be just right.

You will need:

Number 3- Night Shining Clouds

These rare, mystifying clouds are formed under very restrictive conditions and are only seen in the summer, at latitudes north of 50 degrees. They originate in the layer called the mesosphere; making them the highest cloud formations in our atmosphere. While ordinary tropospheric clouds get their source of dust from things like desert storms, this is not a viable medium in the mesosphere where this dust simply cannot reach. Accordingly, it is speculated that these cloud formations utilise dust particles from outer space, making them extra awesome. Normally, they are far too faint to be seen, but they can be visible when illuminated by sunlight below the horizon.

You will need:

-Latitude north of 50 degrees - Moisture -Space Dust

Number 2: The Green Flash:

The famed Green Flash is a meteorological phenomenon that occurs at sunset and sunrise. The green flash is viewable because refraction bends the light of the sun. The atmosphere acts as a weak prism, which separates light into various colours. When the sun's disk is fully visible above the horizon, the different colours of light rays overlap to an extent where each individual colour can be seen by the naked eye- for a fraction of a second!

You will need:

-1 sun set (or rise) -A long, uninterrupted horizon. The ocean horizon works well. -A very clear day -The ability to not blink helps considerably.

Number 1: The Red Sprite

First noted by scientist in 1989, sprites are bursts of electrical energy that form around 50 miles (80 kilometres) above the Earth. From what is known, sprites send pulses of electrical energy up toward the edge of space (the electrically charged layer known as the ionosphere) instead of down to Earth’s surface. It is speculated that ions and electrons floating about the atmosphere are heated by this field and glow red in response. Why are they so hard to see? Well, they last just a few milliseconds and since they are commonly associated with thunderstorms; clouds block the view from the ground. For this reason, many pictures of sprites have come from flights in orbit.

You will need:

-1 thunderstorm -1 Space Station (optional)

-Jean

All images courtesy of Wikimedia Commons

THIS IS SPARTA!!!

No really, it is.

This photo shows an enchanting pink Sun adorning a speck of green on its upper rim near (you guessed it) Sparta, Greece. The green rim is a result of atmospheric dispersion which can produce separate images of each spectrum colour and result in what is referred to a ‘mock green flash’. The flash appears in the uppermost rim of the Sun as it rises. The flash is caused by a thermal inversion; cool air overlain by warmer air. When the Sun is close to the horizon, the light at the bottom of the Sun is refracted more noticeably than is the top portion due to the denser atmosphere. As a result, if we are lucky, we see a slight distinction between the colors red/yellow (long wavelength) and green (shorter wavelength).

-Jean

Photo courtesy of P. Nikolakakos

The Green Flash

In the movie Pirates of the Caribbean: At Worlds End, the green flash signifies the return a soul from the afterlife.

This is where most of us first heard about the green flash. While the fantasy about the green flash in the movie is rife, there is some truth to it. No, not the part about souls coming back.

The green flash is actually an optical phenomenon that occurs in the last seconds of sunset or the very first moments at sunrise. As you can imagine, not many have seen it.

The movie exaggerated the flash by lighting up the entire horizon. The reality is quite different. In the dying seconds of a sunset, the sun is visible as a green spot for no more than a second or two. This is the green flash.

This phenomenon is not altitude dependent but is easiest seen over the ocean however some have seen it occur over mountain tops. There have also been reports of the green flash being seen from aircraft.

The light from the sunset/sunrise needs to be seen by the observer without being scattered in order to effectively view the flash.

Very rarely, blue light is visible instead of the green, this results in a blue flash.

♞Renesh T

Image credit: Thomas “Hank” Hogan, http://home.centurytel.net/Arkcite/grnray.htm

Physics Today December 1999

The Green Flash and Other Low Sun Phenomena D. J. K. O'Connell and C. C. Kiess, Reviewer National Bureau of Standards

http://www.physicstoday.org/resource/1/phtoad/v12/i3/p52_s1?isAuthorized=no

References:

http://mintaka.sdsu.edu/GF/ http://mintaka.sdsu.edu/GF/papers/Zenit/glance.html http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html

5 Optical phenomena you will probably never see. The interaction between the atmosphere, particulates, light and water can create an array of amazing optical phenomena to feast our eyes on- some are more difficult to witness first hand than others. But, at least we have the facility of the internet to show everyone what they are missing, or provide an opportunity to the lucky few witnesses to gloat. Here are five of the rarest optical phenomena you are (un)likely to see, accompanied by a ‘what you need’ supplement to inform you when to have a camera ready and aimed in order to capture these awesome sights. May the odds be forever in your favour! Number 5: The Fire Rainbow. Technically, it is known as a circumhorizontal arc, but that’s not quite as much fun to say. This phenomenon is caused by clouds which contain water and ice droplets of nearly uniform size. These clouds diffract sun light which separates the light into different wavelengths, which we perceive as different colours. Although they look similar to rainbows in their colour arrangement; the mechanism by way the light is scattered to produce them is different. Rainbows are the result of refraction and reflection. When light is refracted, it is bent by passing through mediums of different densities, such as water. Reflected light bounces off a surface at an angle equal to the angle it hit the surface at. Diffraction, however, involves light waves being scattered into a pattern; creating what you see here. You will need: -Cirrus clouds at least 20,000 feet in the air. -Optimum amount and distribution of ice crystals. -The sun angled so light hits the clouds at precisely 58 degrees. Number 4: The Moonbow The principles behind the formation of a Moonbow are quite the same as a rainbow; light is refracted as it passes through water droplets in the sky. These refractive properties of the droplets cause light to be split into a band of colours, more specifically the light spectrum. The only exception here is the light source is no longer the Sun, but the Moon (yes, it is technically still sunlight). While most of us will have seen a rainbow at least once in our life time, a much fewer number will have had the pleasure to encounter the lunar equivalent. This is because, as the Moon is more variable than the sun, many conditions need to be just right. You will need: - 1 full moon, and more particularly before and after the gibbous phase is favourable. -Plenty of moisture in the air. -The Moon to be no greater than 42 degrees above the horizon (otherwise no bow will form) Number 3- Night Shining Clouds These rare, mystifying clouds are formed under very restrictive conditions and are only seen in the summer, at latitudes north of 50 degrees. They originate in the layer called the mesosphere; making them the highest cloud formations in our atmosphere. While ordinary tropospheric clouds get their source of dust from things like desert storms, this is not a viable medium in the mesosphere where this dust simply cannot reach. Accordingly, it is speculated that these cloud formations utilise dust particles from outer space, making them extra awesome. Normally, they are far too faint to be seen, but they can be visible when illuminated by sunlight below the horizon. You will need: -Latitude north of 50 degrees - Moisture -Space Dust Number 2: The Green Flash: The famed Green Flash is a meteorological phenomenon that occurs at sunset and sunrise. The green flash is viewable because refraction bends the light of the sun. The atmosphere acts as a weak prism, which separates light into various colours. When the sun's disk is fully visible above the horizon, the different colours of light rays overlap to an extent where each individual colour can't be seen by the naked eye- for a fraction of a second! You will need: -1 sun set (or rise) -A long, uninterrupted horizon. The ocean horizon works well. -A very clear day -The ability to not blink helps considerably. Number 1: The Red Sprite First noted by scientist in 1989, sprites are bursts of electrical energy that form around 50 miles (80 kilometres) above the Earth. From what is known, sprites send pulses of electrical energy up toward the edge of space (the electrically charged layer known as the ionosphere) instead of down to Earth’s surface. It is speculated that ions and electrons floating about the atmosphere are heated by this field and glow red in response. Why are they so hard to see? Well, they last just a few milliseconds and since they are commonly associated with thunderstorms; clouds block the view from the ground. For this reason, many pictures of sprites have come from flights in orbit. You will need: -1 thunderstorm -1 Space Station (optional) -Jean All images courtesy of Wikimedia Commons

The green flash. The famed and elusive Green Flash is a rare meteorological phenomenon that occurs at sunset and sunrise. During these times, the sun's light travels through more of the earth's atmosphere to reach your eye, creating a prism effect. Yeah, the explanation is definitely more dull than many of the maritime legends surrounding the phenomenon, but consider yourself very lucky if you're able to witness this event. To increase your chances, watch the sun set (or rise) over a long and uninterrupted horizon on a very clear day. The ocean horizon works well for this, as will a prairie, or the horizon line while inside an airplane. The flash lasts only a fraction of a second, so don't blink!