Abstract
There are many forms of optics that occur in the sky at different times. Some of the optics are more common and can be observed at almost any time while others are extremely rare. This paper examines atmospheric optics and how they relate to various phenomena that are observed in the skies. The article covers scattering of light, sky and cloud coloration as well as their causes. Additionally, it addresses the halo effect, the incidence of glory, rainbows and how they are formed, the Mirage and its various forms and crepuscular rays. Atmospheric optics is useful in many fields of ophthalmology to the mechanics of camera work. In addition to that, it helps us to understand natural phenomena and what causes them thus increasing our appreciation for the same.
Introduction
Have you ever been going on about your daily routine and had to stop what you’re doing because you needed to take a moment to admire and appreciate the beautiful orange skies of the sunset? Alternatively, have you ever found yourself fascinated by a huge beautiful rainbow that looks as if it extends from one side of the city to the other side? Well, both of these phenomena is in our atmosphere can be explained by taking a closer look at the optical elements in our atmosphere that causes them. Optical elements in the Earth’s atmosphere, such as scattering of light and the reflection and absorption of light, produce different optical phenomena. In addition, various optical phenomena that range from sky coloration to halos in the atmosphere are either caused by different or similar optical elements expressed differently.
The composition of the atmosphere plays a significant role in determining the types of optics that can occur in a given location. The atmosphere is primarily made up of air that consists of particles. The air has a varied density that depends upon a number of factors such as humidity, barometric pressure, altitude. Pressure and temperature impact the weight of the air that is influenced by the earth's gravitational pull and finally the air. When the air is hot, it expands while compression occurs with cold air. Hot air can absorb more water than cold air. In addition to this there is a continuous collision between air particles (Kokhanovsky 10).
Sky Coloration
Atmospheric optics comes about from the interaction between sources of light. The light can be from the sun and the moon with particles in the air such as ice crystals, dust, cloud droplets and raindrops; or atmospheric molecules (Liu 372). The piercing of the sun’s rays through the atmosphere results in refraction, diffraction, reflection, scattering or a combination of these. The observations made after differ depending on the type of interaction that took place.
Scattering of Light, a typical interaction takes place when a particle absorbs light and then immediately after it is discharged in another direction. Oxygen and nitrogen contained in air molecules have the diminutive size and therefore efficiently scatter shorter-wavelength light such as blue and violet as compared to longer wavelength light such as red. The ability to effectively scatter specific wavelengths of light is known as Rayleigh scattering, and it is the explanation of the blue color in the sky.
The Blueness of the sky is primarily as a result of the differences in the scattering of light in relation to their various wavelengths. Waves of violet and blue are the smallest lengths of light and, therefore, are more strongly refracted as compared to red light. Infrared waves effectively evade particles in the atmosphere while ultraviolet light tends to be stricken and scattered by these particles. The scattered light is reflected widely and, therefore, what we see is blue sky (Rayleigh 375).
Sunsets and Sunrises are a great phenomenon caused by differences in light rays. It is possible to distort heavenly bodies near the horizon if the earth’s atmosphere is viewed as a series of flat layers whose density increases with gravitational pressure. There is bending of rays of light bearing the sun’s image at the entry points to these layers resulting from alterations in pressure, composition and meteorological conditions. There is compression of air particles within the different layer through air pressure and the effect of the earth’s gravitational pull.
The compression causes a rise in density in the lower layers resulting in light waves bending upward toward, the less dense air. This means that refraction causes a different effect on the top half of the setting sun as compared to the bottom. The image of the top half is radiated truly while the bottom is distorted because of the thicker atmosphere that bends the light intensely giving a flat or squashed appearance. Therefore, there appears to be flattening of the sun on the horizon during sunrise and sunset because light rays from the lower half of the sun are bent more than those from the top. The fact that light at the horizon is sharply bent causes the sun to also appear higher.The timing of rising and setting is increased by two minutes more than it would be if there were no atmosphere (Raleigh, 384).
Dust particles in the air impact the color of the sky. Dust particles, as well as aerosols and moisture, lead to desaturation of the blue color of the sky culminating in a milky white. These light scatterers are comparative or larger than light wavelengths, so they scatter all colors fairly evenly. This is known as Mie Scattering (Wiscombe 1505). This occurs when light is scattered by water droplets found even in small clouds, haze and dust particles; all of which scatter visible light equally in all directions thus causing the clouds to appear to be white. Dust particles also absorb moisture and water resulting in different reflection wavelengths.
The Sahara desert is the source of some of the world’s largest and furthest travelling dust clouds. The dust clouds are in tonnes and can sometimes make it all the way across the Atlantic. The dust particles contribute to the observation of beautiful sunsets. The particles gather high in the atmosphere and scatter specific wavelengths of light which allows illumination of red hues.
Cloud Coloration
There are different aspects that affect the coloring of clouds. The color of the cloud is an indicator of what is happening within it. Clouds result from the rise of water vapour containing warm air that is lighter than the ambient air. As it rises, this warm water vapour containing air cools; condensing the water into micro-droplets that are relatively densely packed. Sunlight cannot penetrate too deep into the cloud before it gets reflected and resulting in the typical white color.
As the cloud grows, there is a combination of the micro-droplets or ice crystals into larger droplets that may reach a size eligible to fall as rain. During the process of growth, the space between droplets increases leading to deeper light penetration into the cloud. If the cloud gets to be of an adequately large size, the droplets have sufficient space between them that some of the light that penetrates is not reflected before it can be absorbed. Consequently, the size of the cloud will determine the range of color displayed from white to grey to black. In large clouds, the bottom of the cloud appears to be grey as limited light is being transmitted or reflected back (Deirmendjian 187).
The natural colors also seen in clouds include bluish grey that comes about from light scattering within the cloud. Blue and green are short wavelengths of visible light while red and yellow are the long wavelength. The short rays are scattered more easily by water droplets while the long wavelength colors are more readily absorbed. When the cloud is bluish in color, it shows that rain sized droplets within the cloud are scattering light.
Greenish hue in the sky is as a result of light scattered by ice crystals. When the water droplets condense, they form different sizes of ice crystals that scatter light in different wavelengths compared to water droplets. This is seen in cumulonimbus clouds, and it indicates the advent of strong winds, hail, rain or likely tornadoes. It is rare to see yellowish clouds, but their presence usually occurs in late spring or early fall due to forest fires. The yellow color indicates the existence of smoke. Sunrise and sunset can precipitate red, pink or orange clouds due to the scattering of sunlight by the atmosphere. The color does not indicate the actual appearance of the clouds rather they are a reflection of the long rays of sunlight that have not been scattered.
Halo
Halos are formed in the atmosphere because of the presence of ice crystals. The kind of optical phenomena seen is determined by the shape, size and distance from the earth’s surface of the ice crystals. It also depends on how light is refracted or reflected by the ice crystals. Reflection occurs when light bounces off the crystals while refraction requires the light to be absorbed and bent by the crystals. Ice crystals exist in the sky in any part of the world and are known to have a hexagonal molecular structure. They have a varying shape from flat to columnar. Halos as well as other optical phenomena come about due to the combination of the shape of the crystals and angle between their facets as well as their orientation (Tape). The angle can be 60° or 90°.
There are different types of Halos depending on how they are formed. There are two kinds of halos commonly seen from the earth’s surface. These are 22° and 46° halos. They are visible all through the year worldwide however they are more common in winter months since the cold creates conducive conditions for the formation of halo-generating ice crystals. 22° halos are formed when light passing through a crystal bends at a 22 degree angle and a similar scenario transpires with the 46° halos.
The glowing spots found around the sun are known as sun dogs, parhelion or mock sun and come about from the refraction of sunlight off the ice crystals on cirrus clouds that are plate shaped. They are some of the most frequently observed optical phenomena linked to 22° halos. Sundogs are at their most highly visible when the sun is near the horizon. Those parts of the sun dog closest to the sun will be red while those that are further away will manifest as green or blue. Scientists have conjectured that other planets also have sun dogs. Mars is said to have the possibility of sun dogs made from ice water and CO2 ice. Uranus, Neptune, Saturn and Jupiter can experience sun dogs produced from methane, ammonia and other substances (Tape).
Light Pillars manifest in cold arctic regions and are made of columns of light that are observed to be coming from either above or below a light source. They happen where natural, or artificial light is reflected from flat ice crystals in the atmosphere near to the surface of the earth. Since ice crystals are reflections of a light source, their color tends to be the same as that of the source of light (Tape).
Glory
The glory phenomenon is composed of a set of small luminous multicolored rings appearing around the observer’s antisolar point. When the glory is formed, light is backscattered by water droplets in clouds. There is a straight line from the sun, the observer and the observer’s shadow inside the glory. The color of the rings produced by this phenomenon is faint, and the luminous rings are small in size and vary in color. Sightings of glory had been rare before airplanes were invented. One of the incidences, when a glory can be seen, is during mountain climbing. Mountain climbers have had the chance to observe similar phenomena through his shadow projected on fog or clouds with the sun at his back (Laven 436).
Rainbows
Rainbows occur because of raindrop geometry. Clouds with rainbow colors are known as iridescent, and they occur when particularly tiny water droplets or ice crystals scatter light individually. When sections of cloud are thin and have droplets of similar size, diffraction of light will cause them to appear iridescent like a corona. Reflection of the sun from behind the observer and into the rain causes incident rays of light to penetrate the drop that is then refracted inwards. They are then reflected back to the surface of the drop and refracted as they leave the drop and are observed. The refraction is the reason the sunlight is split into its constituent parts (Laven, 444).
Double Rainbows result when sunlight is reflected by three times in one raindrop. In this instance, the sun’s light is reflected by the raindrop in a noticeably inward arc of the rainbow and is reflected out of a secondary bow causing a dark band to be observed between the bows. This is known as Alexander’s band, and it causes the sky beneath the lower rainbow and the one above the higher rainbow to be brighter as a result. A supernumerary rainbow is composed of more bands found in the inner arc of the primary rainbow and sometimes on the outer arc of the secondary rainbow. These pastel colored bands are a result of light wave interference (Laven 442).
Mirage
Despite the common misconception, a mirage is not an optical illusion but a real phenomenon caused by sharp bending of rays in layers with steep thermal gradients. A mirage contains at least one inverted image of an object. A mirage frequently has many images that alternate between erect and inverted and are classified according to how many and in what relative positions these images take. Mirages can be categorised as Inferior mirages, which there are two images with an inverted image below the erect one. Superior Mirage in which two images are arranged one inverted above the erect one. The three image mirages in which three images are arranged with the inverted image between the erect ones. Finally, the Fata Morgana in which there is an intricate alternation of distorted erect and inverted image where the images are many.
Green Flashes are known as colored phenomena that come about because of dispersion of atmospheric refraction. Individual images give out larger dispersion effects than most, and the green flashes are the most spectacular of these. The Novaya Zemlya phenomenon is an arctic mirage that was named for the archipelago. It was first observed in 1597 when William Barents carried out his third polar expedition. It resembles a large mirror in the sky. The sun could be below the horizon however its rays hit this layer and return down, becoming visible to observers. Therefore, the Novaya Zemlya effect presents a distorted image of the sun which looks like a square or a line.
Crepuscular Rays
Crepuscular rays also known as sunburst are seen when there is a partial shadow on the rays of the sun produced by objects such as mountain peaks or clouds. The light rays are parallel but due to the perspective they seem to converge on the sun. Their manifestation is typically orange in color. Rayleigh scattering by particles is easier for short wavelength light such as blue or green compared to long wavelength in red or yellow light. The sunset and sunrise rays also have to travel through the atmosphere when it has up to 40 times more air compared to midday rays.
The term crepuscular denotes ‘relating to Twilight’ because these rays are seen at sunrise and sunset. Crepuscular rays seem to span outward from the setting sun and can be seen only when there is a hazy atmosphere or through dust particles in order for the sunlight that is not shadowed to scatter toward the observer. The rays can also be occasionally seen underwater. The phenomenon is observed particularly in the Arctic areas and occurs when there are cracks in the ice as well as from ice shelves. The rays are almost parallel but appear to be diverging because of linear perspective.
There are different forms of crepuscular rays which include Jacob’s ladder. This type is formed when rays of sunlight penetrate through holes in low clouds. Another form of crepuscular rays results when there are beams of light that seem to be emerging in a divergent manner from behind a cloud. The third form of crepuscular rays seem to be radiating from below the horizon. The rays can have different hues such as red, pink or pale and are often thought to be light pillars which they are not.
Anticrepuscular rays are the opposite of crepuscular rays but generated in the same way. The rays typically extend across the sky and appear to converge at some point. The point of convergence is known as the antisolar point and is directly opposite the sun. Although the sunlight travels in straight lines, the Earth’s spherical atmosphere makes the projections appear as great circles causing the phenomenon to appear opposite the sun.
Conclusion
After looking at the various types of optics, it is evident that optical elements in the Earth’s atmosphere, such as scattering of light and the reflection and absorption of light, cause different optical phenomena. Atmospheric optics comes about from the interaction between sources of light that is from the sun and the moon with particles in the air such as ice crystals, dust, cloud droplets and raindrops; or atmospheric molecules. The type of atmospheric particle absorbing, deflecting or reflecting the light determines which optic will be formed. The different visual spectacles include sky coloration, halos, rainbows, glories, mirages and crepuscular rays. Some are a daily phenomenon while others may be seen once in a lifetime. The frequency with which the optics are formed depends on the availability of all the necessary components at the right time.
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