Understanding Weather and Climate

3rd Edition

Edward Aguado and James E. Burt

Anthony J. Vega

Part 7. Special Topics and Appendices

Chapter 17

Atmospheric Optics

IntroductionThe atmosphere scatters and absorbs incoming solar radiation

This has an important effect on the energy obtained by the surface and the atmosphere

The atmosphere also refracts solar radiation• Refraction is the bending of rays as they pass through the atmosphere

– Occurs whenever radiation travels through a medium whose density varies

– Or whenever it passes from one medium to another with a different density

– Occurs because radiation speed varies with density

– The denser the atmosphere, the slower the radiation

– Typically, atmospheric density decreases with height

– Causes radiation to refract slightly, forming an arc with the concave side oriented downward

– Rate of density changes varies from place to place and day to day because of differences in the temperature profile above the surface

– The amount and direction of refraction varies with atmospheric conditions

• Refraction and the Setting or Rising Sun– Refraction is greatest when the solar angle is low

– Causes radiation to pass through an increasing amount of atmosphere, which increases total refraction

– At sunset/sunrise this causes the Sun to be visible even though the disk is actually below the horizon

– 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

Refraction causes a distortion of images

Effects of refraction on the solar 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

– Radiation paths through the atmosphere curve upward

– Distant objects appear lower than they actually are

– Farther objects appear distorted while very far objects seemingly disappear

– Inferior mirages may appear over asphalt roads due to low albedos and strong surface heating during the day

– 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

– Results from radiation being bent concave down due to decreasing density with increasing height

A steep drop in temperature would cause slight distortion of Figure A, while one would not see the legs of Figure B, and Figure C would disappear

An inferior duplicate mirage

• Rainbows– Rainbows are sweeping arcs of light that exhibit changes in color from the

inner part to the outer part

– Appear when rain is falling some distance away with a clear sky above and behind the viewer with the Sun behind the viewer

– Primary rainbows are the brightest and most common

– Always the same size so that at the horizon the angular distance from one end to the other extends about 85o of angle

– Shortest wavelengths of visible light (violet and blue) appear at the innermost portion of the ring with longer wavelengths (orange and red) on the outside

– The primary bow is typically surrounded by a secondary rainbow that covers 100o of arc and has the reverse color scheme

– If the precipitation shaft is not large enough or is too far in the distance, a partial rainbow will appear

– Bows form through the refraction and reflection of sunlight as it enters and penetrates a raindrop

– Light passing through a medium of varying density will refract

– Longer wavelengths refract less than shorter ones

– Refracted light penetrates the raindrop with most exiting the opposite side

– A small portion of light hits the back of the raindrop and is reflected back

– This penetrates the droplet once again, and is refracted a second time as it exits the front of the droplet slightly lower than the position of entry

– The upper drop directs red light toward the viewer while the lower drop directs violet light

– This is due to the fact that each wavelength is refracted differentially

– The red light is refracted 42.3o while violet light is refracted 40.6o

– The ring is only 1.7o wide

– Secondary bows are formed in the same manner as the primary bow, except that two reflections occur at the back of the raindrop

– Results in a reverse color scheme

Rainbow formation

A secondary bow requires tworeflections

• Halos, Sundogs, and Sun Pillars– Cirrostratus clouds produce circular bands of light that surround the Sun

or Moon

– These halos, with radii of 22o or 46o, occur when ice crystals are between the viewer and the Sun or Moon

– Sunlight (moonlight) passes through the sides of the column-shaped and plate-like ice crystals in which each of the six edges form a 60o angle

– The crystals act as a prism that refracts the light at a 22o angle

– Because ice crystals are so numerous and randomly aligned within the cloud, a sufficient number will direct light toward the observer, making the halo visible

– A 46o halo forms when column-shaped ice crystals are oriented lengthwise toward the incoming light

– Plate-like crystals cannot form a 46o halo

– Sundogs appear as whitish spots in the sky, but sometimes they exhibit color differentiation, with redder colors located on the side of the sundog nearest the Sun

– They form when ice crystals larger than about 30 m align horizontally

– With the Sun slightly above the horizon and behind the crystals, bright spots appear 22o to the right and left of the Sun

Formation of a sundog

Formation of a halo

– Sun Pillars are caused when plate-like crystals between a low Sun and an observer reflect sunlight off their tops and bottoms

– The many ice crystals are aligned almost, but not exactly, horizontally, with each reflecting a portion of incoming light differently to produce apparent columns stretching upward and downward from the Sun

• Coronas and Glories– Coronas and glories are optical phenomena resulting from the bending

of light as it passes around water droplets (diffraction)

– A corona is a circular illumination of the sky immediately surrounding the Moon, or in rarer instances, the Sun

– Clouds having uniform droplet sizes cause highly circular coronas that concentrate shorter wavelength colors on their innermost portions

– If the cloud contains a wide assortment of droplet sizes, the illumination appears white and irregularly shaped

– Corona size is also related to droplet size, with larger drops producing smaller coronas

– Glories occur when sunlight entering the edge of a water droplet is first refracted, then reflected off the inside of the back of the drop then refracted again as it exits the drop

– Very similar to a rainbow

– A glory, however, requires that the combination of these processes redirect the incoming light a full 180o

– A viewer sees the returned light from the top of the cloud with the Sun behind

– Glories are typically seen from aircraft flying over a cloud deck

Refraction and reflectionassociated with a glory

End of Chapter 17

Understanding Weather and Climate

3rd Edition

Edward Aguado and James E. Burt