M Page 516 Photo 2002 STAR TRIBUNE Minneapolis St Paul 11622 19 ch19 p516 538 qxd 1 11 06 8 56 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 11622 19 ch19 p516 538 qxd 1 11 06 8 56 M Page 517 Light Color and Atmospheric Optics he sky is clear the weather cold and the year 1818 Near Baffin CONTENTS Island in Canada a ship with full sails enters unknown waters On White and Colors White Clouds and Scattered Light Blue Skies and Hazy Days Red Suns and Blue Moons Twinkling Twilight and the Green Flash The Mirage Seeing Is Not Believing T board are the English brothers James and John Ross who are hoping to find the elusive Northwest Passage the waterway linking the Atlantic and Pacific oceans On this morning however their hopes would be dashed for directly in front of the vessel blocking their path is a huge towering mountain range Disappointed they turn back and report that the Northwest Passage does not exist About seventy five years later Admiral 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 had been aroused Speculation was the rule until in 1913 the American Museum of Natural History commissioned Donald MacMillan to lead an expedition to solve the mystery of Crocker land At first the journey was disappointing Where Perry had seen mountains MacMillan saw only vast stretches of open water Finally ahead of his ship was Crocker land but it was more than two hundred miles farther west from where Perry had FOCUS ON AN OBSERVATION The Fata Morgana Halos Sundogs and Sun Pillars Rainbows FOCUS ON AN OBSERVATION Can It Be a Rainbow If It Is Not Raining Coronas Glories and Heiligenschein Summary Key Terms Questions for Review Questions for Thought Problems and Exercises Questions for Exploration encountered it MacMillan sailed on as far as possible Then he dropped anchor and set out on foot with a small crew of men As the team moved toward the mountains the mountains seemed to move away from them If they stood still the mountains stood still if they started walking the mountains receded again Puzzled they trekked onward over the glittering snow fields until huge mountains surrounded them 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 if by 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 to explore interactive tutorials animations or practice problems available on the MeteorologyNow Web site at http now brookscole com ahrens8 517 11622 19 ch19 p516 538 qxd 1 10 06 4 23 PM Page 518 518 Chapter 19 he sky is full of visual events Optical illusions mirages can appear as towering mountains or wet roadways In clear weather the sky can appear blue while the horizon appears milky white Sunrises and sunsets can fill the sky with brilliant shades of pink red orange and purple At night the sky is black except for the light from the stars planets and the moon The moon s size and color seem to vary during the night and the stars twinkle To understand what we see in the sky we will take a closer look at sunlight examining how it interacts with the atmosphere to produce an array of atmospheric visuals T White and Colors We know from Chapter 2 that nearly half of the solar radiation that reaches the atmosphere is in the form of visible light As sunlight enters the atmosphere it is either absorbed reflected scattered or transmitted on through How objects at the surface respond to this energy depends on their general nature color density composition and the wavelength of light that strikes them How do we see Why do we see various colors What kinds of visual effects do we observe because of the interaction between light and matter In particular what can we see when light interacts with our atmosphere We perceive light because electromagnetic waves stimulate antenna like nerve endings in the retina of the human eye These antennae are of two types rods and cones The rods respond to all wavelengths of visible light and give us the ability to distinguish light from dark If people possessed rod type receptors only then only black and white vision would be possible The cones respond to specific wavelengths of visible light Radiation with a wavelength between 0 4 and 0 7 micrometers m strikes the cones which immediately fire an impulse through the nervous system to the brain and we perceive this impulse as the sensation of color Color blindness is caused by missing or malfunctioning cones Wavelengths of radiation shorter than 0 4 m or longer than 0 7 m do not stimulate color vision in humans FIGURE 19 1 For a ray of light striking a flat smooth surface the angle at which the incident ray strikes the surface the angle of incidence or 1 is equal to the angle at which the reflected ray leaves the surface the angle of reflection or 2 This phenomenon is called Snell s law White light is perceived when all visible wavelengths strike the cones of the eye with nearly equal intensity Because the sun radiates almost half of its energy as visible light all visible wavelengths from the midday sun reach the cones and the sun usually appears white A star that is cooler than our sun radiates most of its energy at slightly longer wavelengths therefore it appears redder On the other hand a star much hotter than our sun radiates more energy at shorter wavelengths and thus appears bluer A star whose temperature is about the same as the sun s appears white Objects that are not hot enough to emit radiation at visible wavelengths can still have color Everyday objects we see as red are those that absorb all visible radiation except red The red light is reflected from the object to our eyes Blue objects have blue light returning from them since they absorb all visible wavelengths except blue Some surfaces absorb all visible wavelengths and reflect no light at all Since no radiation strikes the rods or cones these surfaces appear black Therefore when we see colors we know that light must be reaching our eyes White Clouds and