Light, Color, and Atmospheric Optics

Atmospheric optics: results from the interaction between sunlight (moonlight), and particles (cloud droplets, raindrops, ice crystals, dust) or molecules in the atmosphere. As sun's rays travel through the atmosphere, they undergo scattering, refraction, reflection, diffraction, or any combination of these.

Scattering: a process whereby light is actually absorbed by a particle and then quickly emitted in another direction. Since air molecules (oxygen and nitrogen) are small in size, (smaller than wavelengths of visible light), they are more effective at scattering shorter wavelengths (blue and violet) of visible light than the longer wavelengths (red light). This property of being more effective at scattering particular wavelengths of light is named as Rayleigh scattering, which is responsible for the blue appearance of sky.

Figure 19.5, page #552 (Ahrens)

Table 19.1, page #551

Another form of scattering called Mie Scattering, is responsible for the white appearance of clouds. Mie scattering occurs when the wavelengths of visible light are approximately, equally scattered. Water droplets, in even small clouds, as well as dust and haze particles effectively scatter all wavelengths of visible light in all directions, thus making clouds appear white.

Figure 19.3, page #551 (Ahrens)

Table 19.1, page #551 (Ahrens)

Crespuscular rays,bright light beam due to Mie scattering by haze and dust. It is mostly seen during sunrise and sunset.

Figure 19.8, page #553 (Ahrens)

Refraction: a process whereby light bends as it passes from one transparent medium (such as air) into another transparent medium (such as water or ice). It depends on i) the density of medium, and ii) the angle at which the light enters the medium. light that travels from a less-dense to a more-dense medium decreases its speed and bends toward the normal, while light that enters a less-dense medium increases its speed and bends away from the normal. If light enters a medium at 90 angle (normal to medium), refraction does not occur.

Refracted light can be deceptive. When light enters a less-dense medium, an object within this medium appears to be further away that it really is, and vice versa is true when light enters a more-dense medium.

Figure 19.12, page #556 (Ahrens)

Table 19.2, page #555 (Ahrens)

As starlight enters the atmosphere, it often passes through regions of different air density undergoing repetitive refractions. As a result, the star appear to twinkle or flicker, a condition known as scintillation.

Figure 19.13, page #556 (Ahrens)

The sun appears to flatten on the horizon during sunrise or sunset since light rays from lower portion of the sun (or moon) are bent more than those from upper part. Also, since light is bent more on the horizon, the sun and moon both appear to be higher than they really are. In fact, they both rise two minutes earlier and set two minutes later than they would if there were no atmosphere.

Figure 19.14, page #556 (Ahrens)

Twilight is the time at the beginning of the day immediately before sunrise and at the end of the day when sky remains illuminated. It increases with increasing latitude, especially in summer. During the summer in middle latitudes, twilight adds about 30 minutes of light to each morning and evening for outdoor activities.

A green flash light may be seen near the upper rim of a rising or setting sun since light penetrates a thick section of the atmosphere, which bends blue light the most, red light the least. Since the atmosphere selectively scatters blue light, green light reaches us instead. The green flash usually lasts about a second, but could be longer in polar region as reported for 35 minutes in the south polar region.

Figure 19.15, page #557 (Ahrens)

Halo is a ring of light encircling and extending outward from the sun or moon. It form when light from the moon or sun is refracted by ice crystals of a cirroform cloud. Halos are typically seen as bright, white rings, however, refractive effects can cause a halo to have color.

There are two different types of halos: i) a sunlight (or moonlight) is refracted 22 as it passes through ice crystals of diameters less than 20 mm from one side to another side, ii) a sunlight is refracted 46 as it passes through ice crystals of diameters between 15 and 25 mm between a side and top (or bottom). The angle between two adjacent side of a hexagonal-shaped column is 60, while the angle between a side and bottom (or top) is 90. The second type of halos are quite rare.

Figure 19.19, page #559 (Ahrens)

Figure 19.20, page #560 (Ahrens)

Tangent arc is a bright arc of light at the top of 22 degree halo. It is formed by refraction of light passing horizontally-oriented pencil-shaped hexagonal ice crystals.

Figure 19.21, page #560 (Ahrens)

Sundogs (parhelia) are a pair of brightly colored spots, one on either side of the sun. It form as sunlight is refracted by hexagonal plate-like ice crystals of diameters larger than 30 mm that are slowly falling with a horizontal orientation.

The ice crystals behave like tiny prisms, bending and dispersing the sunlight into components colors as it passes through them. The more energetic violet end of the visible spectrum is refracted the most, while less energetic red end of the visible spectrum is refracted the least. Therefore, violet portion of a sundog is farthest from the sun, red portion of a sundog is closest to the sun.

In order for sundogs to be visible, the sun must be near the horizon and on the same horizontal plane as the observer and the ice crystals.

Figure 19.23, page #561 (Ahrens)

Figure 19.24, page #561(Ahrens)

Mirage is a refraction phenomenon that makes an object appear to be displaced from its original position. It occurs when sunlight passes through and is being bent by air layers of different densities. Such changes in air density are usually associated by sharp changes in air temperature. When the air temperature lapse-rate is steeper near the Earth's surface, the object appears taller and lower, often inverted. the phenomenon known as inferior mirage.

Figure 19.17, page #558 (Ahrens)

When the vertical temperature gradient decreases with altitude and the air temperature increases with altitude (inversion) , an object appears elevated but shorter than it really is, the phenomenon known as superior mirage.

Figure 19.18, page #558

Fata Morgana is a special type of superior mirage. It is observed where temperature increases with height, slowly at first, then more rapidly, then more slowly again. Fata Morgana is frequently seen in polar regions where large bodies of cold water are present.

Figure 1, page #559 (Ahrens)

Reflection: a process whereby light bounces off a surface at an angle equal to the angle at which it initially strikes the surface.

Sun Pillar occurs when sunlight is reflected off the surfaces of falling tiny ice crystals. It is usually seen during sunrise and sunset as a vertical shaft of light extending upward and downward from the sun. Sun pillars can be observed when ice crystal (cirroform) clouds are present and the sun is low in the sky.

Figure 19.1, page #550 (Ahrens)

Figure 19.26, page #562 (Ahrens)

A Rainbow is caused by a combination of refraction and reflection of sunlight by raindrops. It occurs when light strikes the back of the spherical raindrop at an angle greater than 48.

Like a prism, raindrop refraction disperses sunlight into its components. Primary rainbow, which is a combination of two refraction and one internal reflection, has colors of red, orange, yellow, green, blue, and violet from outer to inner end. The angle between the incoming sunlight and the emerging light varies between 40 (violet edge) and 42 (red edge). The rainbow angle decreases with increasing size of raindrops since raindrops flatten above 1 mm in diameter.

Figure 19.28. page #563 (Ahrens)

Figure 19.29, page #563 (Ahrens)

Figure 19.30, page #564 (Ahrens)

Secondary rainbow, which is a combination of two refraction and two internal reflection, has colors in a opposite direction. It is 8 higher and also fainter than the primary rainbow.

Figure 19.31, page #564 (Moran and Morgan)

Figure 19.33, page #564 (Ahrens)

A rainbow appears to an observer who has his/her back to the sun and is facing a distant shower. It never form when sky is completely cloud covered: the sun must be shining. Since the weather systems usually move west to east, rainbow indicates upcoming showers in the morning and clearing in the evening.

Circumzenithal arc is a refraction of light by plate-like ice crystals as they fall with their flat surfaces parallel to the ground. It forms about 46 degree above the sun in the presence of abundant ice crystals (cirroform cluds). It is similar to sundog but the sunlight enters the top of the crystal and exits one of its sides. It is short-lived and only seen when the sun is lower than 32 degree above the horizon. Unlike rainbow, it is not associated with rain.

Figure 2, page 565 (Ahrens)

Figure 3, page 565 (Ahrens)

Diffraction: the process by which an object in the atmosphere interferes with the movement of a light wave. It occurs when sunlight bends slightly along the boundary of the object (e. g. water droplets, ice crystals).

When sunlight (or moonlight) encounters a cloud droplet, light waves are altered and interact one another. When the crests of one wave coincide with the crests of another wave, interference is constructive and a large wave results; the light appears brighter. When the crests of one wave coincide with the troughs of another wave, interference is destructive and the waves cancel each other; the light will appear either darker or dissapear.

A corona is a series of alternating dark and bright rings of light surrounding the moon or less often, the sun. It occurs when the moon is seen through a thin altostratus or altocumulus that comprised of tiny water droplets. Corona results from the diffraction of light around the droplets.

If cloud droplets are of uniform size, a corona is colored with blue-violet on the inside and red on the outside. These color rings may repeat themselves as fainting further from the moon. Corona is only a few degrees in radius, hence, is far smaller than a halo.

Figure 19.34, page #566 (Ahrens)

Cloud iridescence is the area within 20 degree of the sun where corona distorted producing patches of color. It occurs when the drops in a cloud have wide range of sizes which distorts the corona. It is associated with cirrocumulus or altocumulus clouds.

Figure 19.36, page #566 (Ahrens)

Glory, which is a combination of two refraction and one internal reflection, differs from a primary rainbow such that sunlight strikes water droplets (with diameters less than 50 mm) in clouds and is refracted back in the same direction as it enters the drop, resulting from diffraction of sunlight by cloud droplets of uniform size.

Glory appears as color rings around the shadow of the aircraft. The appearance of color in glories depends on which color reaches our eyes from each droplet and what angle each color is reflected from the droplets.

Figure 19.37, page #567 (Ahrens)

Figure 19.38, page #567 (Ahrens)

Heiligenschein (German for halo) occurs when sunlight falls on nearly spherical dew drops and refracted back toward the sun in the same direction as it enters. It appears as bright white light around the shadow of your head on a dew-covered lawn.

Figure 19.39, page #567 (Ahrens)