Atmospheric Optics Great Web Sites: Les Cowley http://www.atoptics.co.uk Claudia Hinz: http://www.meteoros.de/ Finnish Halo Blog: http://www.ursa.fi/blogit/haloreports/ The Phenomena Are Produced by Mirages Refraction Crepuscular Rays Scattering and Perspective Sky and Sun Colors Scattering and Absorption Halos Refraction and/or Reflection Rainbows Refraction and Reflection Coronas and Iridescence Diffraction Glories Scattering Oil Slicks Interference All atmospheric optical phenomena are produced when the path of light is obstructed. This can happen in several ways. Light is… Scattered (splattered) in all directions when it passes microscopic aerosol particles or air molecules. Reflected (bounced) from the surface of large particles such as raindrops and ice crystals. Refracted (bent) gradually as it travels through the atmosphere, or abruptly as it passes between water and air. Diffracted into a field of patterned waves as it skirts around tiny cloud droplets. Absorbed by matter and extinguished. Optical phenomena are colored when the obstruction of light varies with wavelength. Rainbows, Halos Mirages, Rainbows, Halos Crepuscular Rays, Sky Light and Color, Glories Coronas, Iridescence The Green Flash and Chinese Lantern Effect Pekka Parviainen This is a form of mirage caused by a combination of unusually large refraction and Rayleigh scattering (preferential scattering of short light waves by tiny particles) when temperature increases in a narrow height range of height above a cold surface (a near-surface temperature inversion). REFRACTION OF LIGHT: MOTHER OF MIRAGES, HALOS, RAINBOWS Change of direction of the light path (bending) when one part of a wave crest or trough moves faster than another. For light, this occurs when light moves obliquely from one medium (air) to another (water) or when the density of the medium varies. Spectral colors appear when light passes through a prism because each wavelength is refracted by a different angle. This is called dispersion. Natural substance with the largest refraction and largest dispersion? Diamond!! Waves always bend toward the region where they move slowest. Red is usually refracted or bent least of all colors Enrique Hita Villaverde Facultad de Ciencias – Universidad de Granada Superior Mirage – Victoria, British Columbia, Canada In this photo, air near the ground is much colder and denser than the warmer air above so light waves travel slower. When the light waves pass through the temperature inversion, they speed up and are refracted or bent downwards. Looking up, we actually see light coming from below. When air near the ground is much warmer and less dense than colder air above, light waves near the ground travels faster than light waves above and thus are refracted or bent upwards. Looking down on hot, dry ground, we actually see shimmering skylight above that appears like water. The Superior Mirage When temperature increases with height, inverted and possibly magnified images of objects appear higher than in reality because light waves are bent down as they pass through elevated inversions. Sometimes called Fata Morgana, superior mirages are common in the Arctic and have fooled explorers, who named islands that never existed. RUN MIRAGE And make the image appear up in the sky warm Light Beams Bend in the Elevated Inversion Light beams below the inversion do not bend 1. Rays bend toward region where waves move slowest (where air is cold) 2. Rays bend most where temperature gradient is the largest cold T Crepuscular Rays (Divine Rays) Caspar David Friedrich Tetschen Altar or Cross in the Mountains 1807-08 Crepuscular rays are produced when light is scattered by aerosols. Crepuscular Rays Crepuscular Rays are sunbeams illuminated by light scattered toward the viewer by aerosols and air molecules. They appear by contrast with the surrounding shaded air. The closer the crepuscular ray to the observer, the wider it appears. But since sun rays are parallel, the apparent spreading results from linear perspective. Crepuscular rays generated by laser light passing through artificial fog. Compliments of Michael Vollmer Light and Colors of the Sky Preferential scattering of short waves makes the sky blue. But note that the sky is whiter near the horizon Why is the Sky Blue? Skylight is sunlight that has been scattered in all directions by air molecules and aerosol particles. The sky is usually blue because particles such as air molecules that are much smaller than the wavelength scatter short waves much more efficiently than long waves, just as our bodies reflect the ripples that strike it in the bathtub but have no effect on ocean waves that pass us on their way to shore. This is called Rayleigh Scattering after Lord Rayleigh who proved that the amount of scattering by tiny particles varies inversely with the 4th power of the wavelength. Thus, air molecules scatter the shortest violet waves [7/4]4 10 times more efficiently than the longest red waves. Even though violet light is scattered most efficiently, the Sky is blue and not violet because air molecules scatter significant amounts of blue and green light, and progressively smaller amounts of yellow, orange, and red light. Under normal daytime conditions, the mixture of skylight averages out to a pale blue that whitens toward the horizon. Aerosol particles are larger than air molecules and scatter the different wavelengths of light less selectively and by smaller angles. This makes hazy skies whiter and brighter, especially near the sun. WHITE LIGHT APROACHES TINY PARTICLE BLUE LIGHT IS SCATTERED IN ALL DIRECTIONS RED LIGHT PASSES DIRECTLY, UNDISTURBED RUN SCATTER Pure Air Hazy Air Aerosols whiten and brighten the sky around the Chariot of the Sun God, Apollo (Madrid) The Sun’s and Sky’s Coat of Many Colors Skylight is sunlight that has been scattered in all directions. Sunlight is direct light that penetrates the atmosphere without being scattered. Outside the atmosphere, direct sunlight is white. At ground level, the Sun is slightly yellow when it is high in the sky and the sky is blue because 30% of the shortest violet waves and only 3% of the longest red waves have been scattered. But as the Sun nears the horizon it must penetrate almost 40 times as much air through the wafer-thin atmosphere to reach the ground as when it is overhead! Then 99.9999% of the violet and blue waves, and 70% of the longest red waves have been scattered out of the sunbeam, so the Sun turns red or orange. The lower the Sun in the sky the longer its path through the wafer thin atmosphere so more light waves (especially short waves) get scattered and less penetrate. Thus, the more air a light beam penetrates, the redder it gets. V = 0% R = 30% V = 70% R = 97% % of light that penetrates the atmosphere. Most violet and almost all red light penetrate when the Sun is overhead but virtually no violet penetrates when the Sun lies on the horizon. Post Volcanic Crimson Twilights and Blue Moons At twilight, when the troposphere lies in the shadow of night, the stratosphere is sunlit. Major forest fires and large volcanic eruptions produce crimson twilights because they inject the stratosphere with enormous numbers of aerosol particles and sulfurous droplets 1 to 2 m in diameter that scatter long waves (red) more than short waves. These particles also turn the Sun or Moon blue because they allow the shorter, blue waves to penetrate further. But that only happens “once in a blue moon”. Crimson twilight skies last a few months to a few years after major eruptions. Slowly the particles fall out of the stratosphere and are rapidly washed to the ground by rain or snow. But while they remain in the stratosphere they cool Earth’s climate. Claude’s Fantastic Sunset Paintings made Turner Jealous Until Tambora blew in 1815 and produced fantastic twilight skies HALOS – Celestial Circles: Aerial Arcs 22 Halo Halos are produced when sunlight is refracted and/or reflected by ice crystals. The 22 halo is one of many halos. Features of the halos are determined by 5 factors, 1: crystal shape, 2: crystal orientation, 3: light path through or on the crystals, 4: Sun’s height in the sky, 4: cloud optical thickness. The 22° Halo and the Path of Light through the Ice Crystals The 22 halo is a circular ring of light whose inside appears 22 from the Sun or Moon. The inside is reddish because red light is refracted least of all the colors of the spectrum. It is one of the most common halos but its colors are washed out. The halo is the classic warning sign of an approaching winter storm. Crystal Form – Usually a Column Light enters a rectangular face and exits an alternate (not opposite or adjacent) rectangular face To produce the 22° halo, crystals must be randomly oriented to bend sunbeams up, down, left, right, and diagonal and form a complete circle around the Sun or Moon. Many different halos are possible. In the next slide the tangent arc at the top of the 22º halo forms when pencil crystals fall with their long axes horizontal so that light refracted by alternate rectangular faces is bent mainly up or down. The sundogs at the sides of the halo form when plate crystals fall horizontally (with rectangular faces vertical) so that light refracted by alternate rectangular faces is bent either right or left. http://www.atoptics.co.uk Les Cowley’s Fantastic website 22 halo with Sundogs to left and right, sun pillar and upper tangent arc above. Bright spots are light from individual nearby crystals Path of Light for the 46° Halo The 46 halo is a circular ring of light whose inside appears 46 from the Sun or Moon. It is seldom seen and even at its best, is never as bright as the 22 halo. However, the circumzenithal and circumhorizontal arcs that are produced when light takes this path through horizontal plate crystals can be extremely bright and have vivid spectral colors. Crystal Form – Usually a Thick Plate Light enters a rectangular face and exits a hexagonal face (or vice-versa). Circumhorizontal Arc The circumhorizontal arc forms parallel to the horizon at least 46 below the Sun when the Sun is at least 58 above the horizon. The colors are purest when the Sun is 68 above the horizon. It is produced when light enters a vertically oriented rectangular side of a plate crystal and exits the bottom, hexagonal face. Since it is only produced when the Sun is high in the sky it can only be seen near noon at any season in the tropics or around the summer solstice in the mid latitudes, but never near the Poles. I. Halos and the Forms of Crystals Simple crystals such as plates and columns produce the brightest and most spectacular halos. Most crystals have a complex form with many facets that confuse the light and produce weak halos. The simple crystals above left were collected during the South Pole Halo Display that includes the 22º halo, the parhelia (sun dogs), parhelic circle, upper tangent arc and Parry arc to the 22º halo, the infralateral and supralateral arcs to the 46º halo and the spectral circumzenithal arc. The Clustered and Rimed Crystals below are so complex that they cannot Produce Halos http://www.its.caltech.edu/~atomic/snowcrystals/ http://www.kidsgeo.com/geography-for-kids/0112-bergeron-process.php South Pole Halo Display Jarmo Moilanen II. Halos and the Number of Crystals * ** * ** * ** * ** * ** * * ** ** * *** ** * ** * * *** ** * *** * ** * ** * * * * * * * * * * * ** * ** ** * **** * ** * * * * * ** * ** * * * * * * * * * * * * * * * ** * ** * ** ** * * * ** ******* ** * ** * **** * * * * * * * * * ** * ** * * ** * * * *** *** * * * * * * * * * **** *** ***** ***** * ** * * * * * * * * * * ** * ** ** * ** 22° 22° Cloud Thickness The fraction of light that penetrates a medium without being scattered or absorbed is equal to e-t where t is called the optical thickness. The brightest halos are produced by clouds with optical thickness between about 0.04 and 2. Optically thinner clouds have too few crystals to refract much sunlight while clouds that are optically thicker scatter the sunlight so many times before it can emerge that the result is an incoherent gray color. At t = 1 (t = 2), only 1/e 36.8% (1/e2 13.5%) of the light penetrates the cloud without being scattered. When t < 0.02 a cloud is optically too thin to see, but because ice crystals focus light, halos may be visible. When t > 10 the Sun cannot be seen. Impact of Cloud Thickness on Halo Brightness Cloud Thin – Sun Strong, Sky Blue and Halo Weak Sun Moderate, Sky Pale, Halo Bright Thick Cloud – Sun Weak, Sky Gray, Halo Weak RAINBOWS Rainbows, coronas, and glories are produced when sunlight strikes drops Rainbows, Coronas and Glories and Cloud Droplets and Raindrops Bright, vividly colored rainbows are produced when sunlight strikes raindrops between 0.1 and 2 mm in diameter because such drops are spherical and geometric optics applies. Larger drops flatten and oscillate. Smaller droplets do not refract light well but scatter or diffract it in more complex ways, and produce white fog or cloud bows and glories opposite the sun (around an observer’s shadow) or coronas and iridescent clouds around the sun. EXPLANATION OF THE PHENOMENA Sun Beam Sunlight strikes everywhere on the drop but only the ‘chosen’ point leads to the rainbow Refraction #1 Raindrop Reflection #1 Refraction #2 Enrique Hita Villaverde Facultad de Ciencias – Universidad de Granada The Optics of the Rainbow is similar to the Optics of the Eye because Raindrops and Eyeballs are spheres. Right eye (horizontal cross section) temporal 5° Visual Axis Light can only enter the eye through the pupil. nasal Optical Axis Effective Pupil of Drop Raindrops don’t have pupils but the rays that produce the bow strike the drop only in a narrow zone. Descartes’ Illustration of the Rainbows A swath of many drops is needed to make a rainbow. The next slides show how the number of drops affects rainbow brightness. Too few sunlit drops make a faint bow but too many brighten the background and swamp the bow. Forecasting Weather with Rainbows To see a rainbow look opposite the Sun i. e., to the West in the morning and to the East in the afternoon. Since storms move from West to East outside the Tropics, the morning bow heralds an approaching storm and the late afternoon bow indicates a departing storm. In the tropics storms move from East to West so the late afternoon bow means trouble. Afternoon Sun in West, Bow in East Morning Sun in East, Bow in West Coronas, Glories and Fogbows are produced when light waves are diffracted (etc.) as they pass through and around tiny droplets. Coronas appear around the Sun or Moon. Glories appear opposite the Sun, around the observer’s shadow. Fogbows appear opposite the Sun in a larger arc like rainbows, but are broader than rainbows and have little color. Cloud droplet Sun A Fogbow at Saguenay Fjord, Quebec, Canada Coronas and Iridescence form around or near the Sun Paul Neiman: Corona over Nederland, Colorado Glories are most often seen when flying – around the shadow of the plane. Do Professors fly first class?