Atmospheric Optics

White light: all visible wavelengths reach our eyes.

Black light: object(s) absorb all visible wavelengths.

Reflection: light bounces off a surface at same angle that it strikes the surface.

Scattering: light is deflected and sent in all directions (diffuse light).

Mie Scattering: cloud droplets (~20 um diameter) scatter all wavelengths of visible radiation, more or less, equally; clouds appear white; large clouds reflect more and less light can penetrate, making the underside darker. (Larger drops will also absorb rather than scatter; causing even darker cloud.)

Raleigh Scattering: selective scattering of visible radiation; air molecules much smaller than cloud droplets; O & N are selective scatterers; they scatter shorter waves (blue) much more effectively than longer waves (red); violet is scattered 16 times more than red; SKY IS BLUE! On Mars, dust turns the sky red at mid-day and purple at sunset; "Blue Haze" over mountains appears to be caused by tiny particles (hydrocarbons called terpenes) released by vegetation, which combine chemically with small amounts of Ozone to produce particles (0.2 um diameter) that selectively scatter blue light; fine dust & salt causes the sky to turn "milky white" because they are large enough to scatter all wavelengths of visible light; if humid enough, they can grow into haze particles, resulting in a "hazy day".

Crepuscular Rays: bright light beams radiating across the sky, usually when sunlight breaks through a layer of clouds, thus illuminating the dust, tiny water droplets or haze, which in turn scatter the sunlight; in England, they are called "Jacob's Ladder".

Sunrise & Sunset: when sun is 4 degrees above the horizon, the atmosphere is 12 times thicker than when the sun is directly overhead; in a clean atmosphere, the sun is bright, yellow-orange; fine particles a little larger than air molecules will give the sun a red-orange color; bigger particles (dust, salt) will cause a red color, especially dust and ash from a volcanic eruption; if there are large quantities of particles, the sun can be different colors, even at noon; a "blue sun" would be the result if the particles were of a size similar to the wavelength of visible light.

Twilight: depends on season and latitude; adds approximately 30 minutes of daylight at both sunrise and sunset; the atmosphere scatters and refracts light even when below the horizon; called a "White Night" in the northern latitudes during Summer.

Refraction: bending of a light path due to 1) density of material and 2) angle at which light enters the material; bends toward the normal if moving from less dense to more dense; bends away from the normal if moving from more dense to less dense.

Twinkling Stars: caused by starlight entering atmosphere and passing through regions of differing densities, each deflecting and bending the light, thus constantly changing the apparent position of the star; scintillation.

Planets: do not twinkle (unless close to horizon) because they are closer and appear larger (size is greater than the angle their light deviates when it enters atmosphere).

Mirage: an object appears to be displaced from its true position, caused by light passing through layers of air of differing densities; "puddles" on a roadway are actually blue skylight refracting up to our eyes; cool air over a hot road surface; an inferior image appears lower and inverted, whereas a superior image appears higher; Fata Morgana occurs when a fairly uniform horizon is transformed into a series of vertical walls and columns with spires.

Halo: a ring of light encircling and extending outward from the sun or moon; is caused by refraction of light by randomly oriented ice crystals (cirroform clouds), with a diameter <20 um; most common is the 22 degree halo (extend your arm, from tip of thumb to tip of little finger) and is usually a bright, white ring; the 46 degree halo forms when light is refracted through hexagonal column-type ice crystals (like tiny pencils), with a diameter of 15-25 um; tangent arc is a bright arc of light "tangent" to halo (upper & lower), caused by large hexagonal ice crystals falling with their long axis horizontal to the ground.

Dispersion: "selective" refraction of light can cause red tinge on inside of halo and a blue tinge on the outside.

Sundogs: "Mock Sun"; "Parhelia"; brightly colored spots on either side of the sun, caused by the refraction of sunlight by hexagonal, plate-like ice crystals (diameter > 30 um) falling slowly and becoming horizontally oriented; red on the inside and blue on the outside.

Sun Pillars: caused by reflection of sunlight by ice crystals; usually seen at sunrise or sunset as a vertical column of light upward and downward from the sun, caused by hexagonal plate-like ice crystals, with their flat bases horizontal to the ground and tilting from side to side, like a falling leaf ... or, they may fall vertically and rotate.

Rainbow: caused by the refraction and reflection of sunlight in falling raindrops with the sun to your back; light enters a raindrop and is slowed and bent as it is refracted (violet more than red); most of the light passes through the drop, but some of the light strikes the back of the drop, exceeding the critical angle of water (48 degrees) and is internally reflected; each light ray bends differently and emerges at a different angle (red at 42 degrees and violet at 40 degrees); creates a spectrum of colors, like a prism; the primary rainbow has red on top (outside) and violet on bottom (inside); the secondary rainbow has the reverse order of colors, due to a second internal reflection in each raindrop; two people standing next to each other will technically see two different rainbows.

Circumzenithal Arc: works like a rainbow, but it is not; sun must be less than 32 degrees above the horizon; sunlight enters the top of plate-like ice crystals, which are falling with their flat surfaces parallel to the ground and exits through one of the sides.

Corona: a bright ring of light around the moon (the sun is too bright to see), due to water droplets (altostratus or altocumulus) causing diffraction of the light (the bending of light around an object); a constructive interference pattern will be bright; a destructive interference pattern will be dark; are usually white, but if droplets are of uniform size, there can be color, with blue on the inside of the ring and red on the outside.; colors can repeat for each ring, getting fainter as one heads away from the center.

Iridescence: different size droplets can cause corona to become distorted and irregular and can cause patches of color (pastel shades of pink, blue or green).

Glory: colored rings around a shadow on water droplets of uniform size (diameter <50 um); an airplane shadow on a cloud ... or, around a person's head while sitting on a mountain top over an undercast; caused by diffraction around the object, followed by refraction and a "surface wave" reflection around the interior of of the cloud droplets and subsequent refraction out of the droplet back toward the original object.

Heiligenschein: a bright white light around the shadow of your head when standing over a dew covered ground, with the sun to your back; the light is "retroreflected", meaning that it is reflected back to your eye, but not along the exact same path.

Green Flash: a flash of green light seen near the upper rim of the rising or setting sun, caused by the refraction of the sunlight by the thick atmosphere near the horizon; we should see blue light, but since the atmosphere selectively scatters blue, we instead see a momentary "flash of green light"; there have been reports from Antarctica of the "Green Flash" lasting for as long as 35 minutes, as the sun "rolls" along the horizon.