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Sunlight bending through ice crystals in cirriform clouds produces bands of color called sundogs,or parhelia, on both sides of the sun on this cold winter day in Minnesota.Photo © 2002 STAR TRIBUNE/Minneapolis-St. Paul11622_19_ch19_p516-538.qxd 1/11/06 8:56 M Page 516517Light, Color, andAtmospheric OpticsCONTENTSWhite and ColorsWhite Clouds and Scattered LightBlue Skies and Hazy DaysRed Suns and Blue MoonsTwinkling, Twilight, and the Green FlashThe Mirage: Seeing Is Not BelievingFOCUS ON AN OBSERVATIONThe Fata MorganaHalos, Sundogs, and Sun PillarsRainbowsFOCUS ON AN OBSERVATIONCan It Be a Rainbow If It Is NotRaining?Coronas, Glories, and HeiligenscheinSummaryKey TermsQuestions for ReviewQuestions for ThoughtProblems and ExercisesQuestions for ExplorationThe sky is clear, the weather cold, and the year, 1818. Near BaffinIsland in Canada, a ship with full sails enters unknown waters. Onboard are the English brothers James and John Ross, who are hoping tofind the elusive “Northwest Passage,” the waterway linking the Atlanticand Pacific oceans. On this morning, however, their hopes would bedashed, for directly in front of the vessel, blocking their path, is a hugetowering mountain range. Disappointed, they turn back and report thatthe Northwest Passage does not exist. About seventy-five years laterAdmiral Perry met the same barrier and called it “Crocker land.”What type of treasures did this mountain conceal—gold, silver, precious gems? The curiosity of explorers from all over the world hadbeen aroused. Speculation was the rule, until, in 1913, the AmericanMuseum of Natural History commissioned Donald MacMillan to lead anexpedition to solve the mystery of Crocker land. At first, the journey wasdisappointing. Where Perry had seen mountains, MacMillan saw only vaststretches of open water. Finally, ahead of his ship was Crocker land, butit was more than two hundred miles farther west from where Perry hadencountered it. MacMillan sailed on as far as possible. Then he droppedanchor and set out on foot with a small crew of men.As the team moved toward the mountains, the mountains seemed tomove away from them. If they stood still, the mountains stood still; ifthey started walking, the mountains receded again. Puzzled, they trekkedonward over the glittering snow-fields until huge mountains surroundedthem on three sides. At last the riches of Crocker land would be theirs.But in the next instant the sun disappeared below the horizon and, as ifby magic, the mountains dissolved into the cold arctic twilight.Dumbfounded, the men looked around only to see ice in all directions—not a mountain was in sight. There they were, the victims of one of nature’s greatest practical jokes, for Crocker land was a mirage.This icon, appearing throughout the book, indicates an opportunity toexplore interactive tutorials, animations, or practiceproblems available on the MeteorologyNow Web siteat http://now.brookscole.com/ahrens811622_19_ch19_p516-538.qxd 1/11/06 8:56 M Page 517The sky is full of visual events. Optical illusions (mirages) canappear as towering mountains or wet roadways. In clearweather, the sky can appear blue, while the horizon appearsmilky white. Sunrises and sunsets can fill the sky with brilliantshades of pink, red, orange, and purple. At night, the sky is black,except for the light from the stars, planets, and the moon. Themoon’s size and color seem to vary during the night, and the starstwinkle. To understand what we see in the sky, we will take acloser look at sunlight, examining how it interacts with the at-mosphere to produce an array of atmospheric visuals.White and ColorsWe know from Chapter 2 that nearly half of the solar radiationthat reaches the atmosphere is in the form of visible light. Assunlight enters the atmosphere, it is either absorbed, reflected,scattered, or transmitted on through. How objects at the sur-face respond to this energy depends on their general nature(color, density, composition) and the wavelength of light thatstrikes them. How do we see? Why do we see various colors?What kinds of visual effects do we observe because of the in-teraction between light and matter? In particular, what can wesee when light interacts with our atmosphere?We perceive light because electromagnetic waves stimulateantenna-like nerve endings in the retina of the human eye.These antennae are of two types—rods and cones. The rods re-spond to all wavelengths of visible light and give us the abilityto distinguish light from dark. If people possessed rod-type re-ceptors only, then only black and white vision would be possi-ble. The cones respond to specific wavelengths of visible light.Radiation with a wavelength between 0.4 and 0.7 micrometers(µm) strikes the cones, which immediately fire an impulsethrough the nervous system to the brain, and we perceive thisimpulse as the sensation of color. (Color blindness is caused bymissing or malfunctioning cones.) Wavelengths of radiationshorter than 0.4 µm, or longer than 0.7 µm, do not stimulatecolor vision in humans.White light is perceived when all visible wavelengths strikethe cones of the eye with nearly equal intensity.* Because thesun radiates almost half of its energy as visible light, all visiblewavelengths from the midday sun reach the cones, and the sunusually appears white. A star that is cooler than our sun radi-ates most of its energy at slightly longer wavelengths; therefore,it appears redder. On the other hand, a star much hotter thanour sun radiates more energy at shorter wavelengths and thusappears bluer. A star whose temperature is about the same asthe sun’s appears white.Objects that are not hot enough to emit radiation at visi-ble wavelengths can still have color. Everyday objects we see asred are those that absorb all visible radiation except red. Thered light is reflected from the object to our eyes. Blue objectshave blue light returning from them, since they absorb all vis-ible wavelengths except blue. Some surfaces absorb all visiblewavelengths and reflect no light at all. Since no radiationstrikes the rods or cones, these surfaces appear black.Therefore, when we see colors, we know that light must bereaching our eyes.White Clouds and Scattered LightOne exciting feature of the atmosphere can be experiencedwhen we watch the underside of a puffy, growing cumuluscloud change color from white to dark gray or black. When wesee this change happen, our first thought is usually, “It’s goingto rain.”


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