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					                               Chapter 17. Atmospheric Optics

Chapter Overview: This chapter details optical phenomena which occur relative to atmospheric
conditions. Specific phenomena are described along with the atmospheric conditions necessary
for their development.

Chapter at a Glance: In addition to scattering and absorbing insolation, the atmosphere also
refracts energy. Refraction, or the bending of rays as they transmit through objects, causes
common atmospheric optical phenomena. Refraction occurs as radiation transmits through objects
of varying density. Due to decreasing atmospheric density with increasing altitude, the
atmosphere refracts radiation in an arc with the concave side oriented downward. The amount of
refraction differs from location to location and at various temporal scales depending on local
temperature profiles.

• Refraction and the Setting or Rising Sun - Refraction of solar radiation is greatest when the
solar angle is low. This causes the radiation to pass through an increasing amount of atmosphere
thereby increasing total refraction. At sunset and sunrise, this causes the Sun to be visible even
though the disk is actually below the horizon. Refraction also causes a twilight sky when the Sun
is further below the horizon. Because of these low angle/atmospheric relationships, twilight length
increases during the high Sun season and with latitude. Refraction also affects the Sun’s apparent
shape and color. Colored bands, reddish near the bottom, occur near dawn or sunset as longer
wavelength colors refract less than shorter ones. Occasionally, a bright green spot, or green flash,
caps the disk.

• Mirages - Refraction of radiation also causes mirages to appear when steep vertical temperature
gradients exist near the surface. Strong vertical temperature gradients lead to strong density
gradients, which increase with height. So radiation paths through the atmosphere curve upward.
Distant objects appear lower than they actually are. Further objects will appear distorted while
very far objects seemingly disappear. Inferior mirages may appear over asphalt roads due to low
albedo’s and strong surface heating during the day. In such a mirage a true image of an object
appears along with an inverted image directly below resulting in a “pond-like” image. A superior
mirage is similar but images are displaced upward. This results from radiation being bent concave
down due to decreasing density with increasing height.

• Cloud and Precipitation Optics - Radiation passing through clouds and/or precipitation causes
refraction leading to common optical phenomena.

               A. Rainbows - Rainbows appear when rain occurs at a distance from an observer
               with the Sun behind the observer. The brightest bow is the primary rainbow that
               extends along an 85o angle across the horizon. In a rainbow, light is refracted and
               reflected as it penetrates a raindrop. Longer wavelengths of energy are refracted
               less than shorter wavelengths. This causes the shortest wavelengths (violet, blue)
               to appear on the inside portion of the bow with longer wavelengths progressing
               towards the outside in order of wavelength. Secondary bows form in the same
               manner as primary ones but two reflections occur at the back of a raindrop resulting
               in a reverse color scheme than the primary bow.




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               B. Halos, Sun Dogs, and Sun Pillars - Halos are produced by solar (or lunar)
               radiation passing through cirrostratus clouds. The ice crystals that comprise the
               cloud refract the light energy at 22o or 46o producing a halo about the object.
               Platelike ice crystals produce a 22o halo while column-shaped ice crystals are
               responsible for the 46o halo. If the Sun is slightly above the horizon and behind
               platelike ice crystals, two bright spots, or sundogs, will appear to the right and left
               of the solar image. When light is reflected, instead of refracted, by platelike
               crystals, a sun pillar will form. In this image, apparent vertical columns appear
               above and below the Sun.

               C. Coronas and Glories - Bending, or diffraction, or light as it passes around
               water drops results in coronas and glories. A corona is a circular illumination of the
               sky that surrounds the moon, or sometimes, the Sun. Corona size is cloud drop
               size-dependent. A glory is a series of rings surrounding a shadow. Glories form as
               light enters the edge of a water drop and is refracted then reflected off the inside of
               the back of the droplet, and refracted again as it leaves the drop. This is typically
               seen from the air looking down upon clouds.

Related Web Sites:
Sunset/rise tables: http://aa.usno.navy.mil/faq/docs/RST_defs.html#top
Optics Photos: http://www.weather-photography.com/galleyr.php?cat=optics
Mirages: http://virtual.finland.fi/finfo/english/mirage.html#refr
Optics and Other Photos: http://www.polarimage.fi/

Key Terms:
atmospheric optics     mirage                 primary rainbow                 sun pillar
refraction             inferior mirage               secondary rainbow                diffraction
twilight               superior mirage               halo                             corona
green flash            rainbow                       sundog                   glory

Review Questions:
1. What is refraction and why is it related to variation in atmospheric density?

       Refraction is the bending of light rays as they pass through the atmosphere. The amount of
       refraction is dependent upon the density of the medium (air) it is traveling through. The
       denser the atmosphere, the slower the radiation. As radiation passes through air masses of
       different density speed alters, which in turn causes refraction.

2. Describe the way refraction alters the apparent position of the setting or rising Sun.

       Refraction is greatest when the Sun is low on the horizon as light passes through a thicker
       portion of the atmosphere. At sunset, refraction is sufficient to cause direct rays to be
       visible even after the Sun has dropped below the horizon. This is also true just prior to
       sunrise. Refraction also causes illumination of the sky (twilight) even though the Sun is
       actually below the horizon.




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3. Do longer or shorter wavelengths of light undergo greater refraction when passing through the
       atmosphere? How does the differential refraction cause an apparent banding of the Sun
       near the horizon?

       The Sun near the horizon typically appears to have horizontal bands of different colors
       with redder coloration near the bottom. This occurs because longer wavelength colors
       undergo less refraction than do shorter wavelength colors. Shorter wavelengths
       undergoing a greater amount of refraction concentrate near the top of the apparent sun
       while the longer wavelengths locate near the bottom.

4. Which type of vertical temperature gradients promote the appearance of superior and inferior
      mirages?

       A steep temperature gradient of the lower layer of the atmosphere causes light to refract
       more strongly than does air above it. This leads to a two image inferior mirage. A superior
       mirage forms when images are displaced upward as light rays are bent concave downward
       as a result of decreasing density with increasing height.

5. How do some mirages create the appearance of standing water on hot days?

       When light is refracted upward and reaches the viewers eyes after undergoing only a small
       amount of refraction it produces the regular image of the object. But some of the reflected
       light is also directed toward the ground where the steep temperature gradient causes very
       strong refraction. This produces a double image similar to an image duplicated by a pond.

6. Explain why the Sun must be behind you when you see a rainbow.

       Because light must be refracted by a drop then reflected through the drop back towards a
       viewer. For this to happen, the Sun must be behind the viewer with the drops located in
       front of the viewer.

7. Describe the difference in the way primary and secondary rainbows form.

       In a rainbow, light is refracted and reflected as it penetrates a raindrop. Longer
       wavelengths of energy are refracted less than shorter wavelengths. This causes the shortest
       wavelengths (violet, blue) to appear on the inside portion of the bow with longer
       wavelengths progressing towards the outside in order of wavelength. Secondary bows
       form in the same manner as primary ones but two reflections occur at the back of a raindrop
       resulting in a reverse color scheme than the primary bow.

8. How does the color pattern of a secondary rainbow differ from that of a primary rainbow?

       The color pattern is opposite that of the primary bow.

9. In addition to refraction, what process must occur within raindrops to produce a rainbow?




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       Reflection of the incident energy then refraction a second time as the energy leaves the
       drop.

10. Why is it that some halos at 22o angles and others at 60o?

       This is dependent upon the clouds in which the energy is passing through. Halos are
       produced by solar (or lunar) radiation passing through cirrostratus clouds. The ice crystals
       that comprise the cloud refract the light energy at 22o or 46o producing a halo about the
       object. Platelike ice crystals produce a 22o halo while column-shaped ice crystals are
       responsible for the 46o halo.

11. How are sundogs formed? Describe the color patterns associated with them.

       If the Sun is slightly above the horizon and behind platelike ice crystals, two bright spots,
       or sundogs, will appear to the right and left of the solar image.

12. Describe the formation of sun pillars. Does refraction play a role in their formation?

       When light is reflected, instead of refracted, by platelike crystals, a sun pillar will form. In
       this image, apparent vertical columns appear above and below the Sun.

13. Explain how coronas are formed around the Sun or Moon. What factor or factors determine
      their size?

       Bending, or diffraction, or light as it passes around water drops results in coronas and
       glories. A corona is a circular illumination of the sky, which surrounds the moon, or
       sometimes, the Sun. Corona size is cloud drop size-dependent.

14. What are glories, and how are they formed? Are they the result of refraction alone or is
      another process also involved?

       A glory is a series of rings surrounding a shadow. Glories form as light enters the edge of a
       water drop and is refracted then reflected off the inside of the back of the droplet, and
       refracted again as it leaves the drop. This is typically seen from the air looking down upon
       clouds.




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