Download AMS Weather Studies

AMS Weather Studies
Introduction to Atmospheric Science, 4th Edition
Chapter 14
Light and Sound in the
Atmosphere
© AMS
Case-in-Point
 Meteorology provides a scientific explanation
for myth and legend
– Fata Morgana
 Optical phenomenon: a mirage is caused by refraction
of light rays as they travel through the atmosphere
 Causes objects to distort vertically
 Most common on the horizon after sunrise
–
–
–
–
© AMS
Following a clear calm night
Extreme radiational cooling would have occurred
Low-level temperature inversions
Occur in coastal areas, high mountain valleys in winter, and
over the frozen Arctic Ocean
2
Driving Question
 What is responsible for the various optical
phenomenon observed in the atmosphere and
how does sound propagate through the
atmosphere?
– This chapter will examine:
 The cause and meteorological significance of various
atmospheric optical phenomena
 Propagation of sound through the atmosphere
 Sounds of meteorological origin
© AMS
3
Atmospheric Optics
 Sun’s rays are scattered, reflected or
refracted by cloud droplets, ice crystals or
raindrops as they travel through the
atmosphere
Creates optical phenomena
– Refraction (bending of light) occurs when solar
radiation travels from one transparent medium
into another, as well as when traveling through
air of different densities
© AMS
4
Atmospheric Optics
 Visible Light and Color Perception
Sunlight (visible light) is only a small portion of the
electromagnetic spectrum
 Sir Isaac Newton determined that visible light is
polychromatic (composed of several primary colors)
Color is sensed by cones in the human eye
 Discrete wavelengths excite the cone
 Relatively high temperature objects emit visible
radiation
– Example: a hot piece of metal
 Most objects on the earth do not emit visible radiation
– Their perceived color is that of visible light wavelengths
reflected by the object; all other wavelengths are absorbed
© AMS
5
Atmospheric Optics
 Red Sun, White Clouds, And Blue Sky
Scattering of sunlight responsible for blue sky
 Scattering occurs when light waves are sent in different directions by
particles in the atmosphere
 Rayleigh effect: amount of scattering varies with wavelength
 Human eye is more sensitive to blue light rather than violet light, blue is
scattered more than most colors, and violet light is diluted by all the other
scattered colors
– Why is the Sun red?
 Length of atmosphere crossed by solar beam is 40 times longer at sunset
 All other colors have been scattered out due to longer wavelengths
Why are clouds white?
 Large particles scatter visible radiation equally at all wavelengths
 Water droplets/ice crystals (compose clouds) are sufficiently large, causing
clouds to appear white
Particles that are about the same size as the wavelength of light are
responsible for Mie Scattering
© AMS
6
Atmospheric Optics
© AMS
Atmospheric Optics
 Halo
A halo is a whitish (sometimes slightly colored) ring of
light surrounding the Sun or Moon
Forms when tiny ice crystals that compose high, thin
clouds refract the Sun’s rays
 Refraction: the bending of light as it passes from one transparent
medium into another
© AMS
8
Atmospheric Optics
Light rays may be
refracted as they
travel from one
transparent medium
into another
A. Light ray is refracted as it
travels from air to water
B. Light rays that enter the
water at a 90° angle are not
refracted
C. Refracted light can be
deceptive
© AMS
Atmospheric Optics
This type of refraction produces a 22-degree
halo centered on the Sun or Moon
© AMS
This type of refraction produces a 46-degree
halo centered on the Sun or Moon
Atmospheric Optics
 Halos, continued
Sundogs appear as bright
colored spots on either side of
the Sun
 Occur when sunlight is refracted
by plate-like crystals falling
through the atmosphere
Sun pillars are bright light
shafts appearing above or
below the setting or rising sun
 Caused by reflection off
horizontal ice crystals
© AMS
11
Atmospheric Optics
 Rainbow
A circular arc of concentric colored bands caused by a combination
of refraction and reflection of sunlight by raindrops
Solar ray is refracted on entering a raindrop, reflected internally,
and then refracted when exiting
Sun must be shining, and no higher than 42° above horizon to see
a rainbow
© AMS
12
Atmospheric Optics
A rainbow appears to an observer
who has his or her back to the Sun
and faces a distant rain shower
© AMS
Refraction of solar rays by raindrops
plus double reflection within
raindrops produces a dimmer
secondary rainbow above the
primary rainbow
Atmospheric Optics
 Fog Bow
– Forms in a similar fashion to a
rainbow
– Broader than a rainbow and
appears almost white
– Caused by diffraction of reflected
light by very small cloud droplets
– Washed out appearance caused by
interference of wavelengths of
visible light with one another
© AMS
Atmospheric Optics
 Corona
Series of alternating light and dark concentric rings
surrounding the Moon or sun (less common)
 Caused by diffraction of light around similarly sized water
droplets that compose a thin veil of clouds
Diffraction: slight bending of a light wave as it moves
along the boundary of an object, with production of an
interference pattern as light waves bend behind the
obstruction
 Constructive interference: crests of one light wave
coincide with another. Results in a larger wave.
 Destructive interference: crests of one wave coincide with
troughs of another. Waves cancel each other; results in
dark band.
© AMS
15
Atmospheric Optics
 Corona, continued
Diffraction involved in iridescent clouds
 Thin clouds with nearly uniformly sized cloud droplets having
bright spots, bands, or borders of delicate colors
 Typically appear up to about 30° from the Sun
© AMS
16
Atmospheric Optics
 Glory
Observer sees concentric rings
of color centered about the
shadow of his/her head
 Seen in bright sunshine about a
warm cloud or fog layer, with the
Sun casting the observer’s
shadow on the clouds below
 Usually seen below aircraft
Depends on the size of
reflecting and refracting
particles, and the direction of
reflected and refracted light
Reflected light due to small
droplet sizes
© AMS
17
Atmospheric Optics
In the special optics that produce a glory,
both the incident and returning solar rays
are diffracted slightly toward the surface
of the cloud droplet. Consequently, the
incident and returning rays are parallel.
© AMS
Atmospheric Optics
 Mirage
Optical phenomenon in which an image of a distant
object appears to be displaced from its normal view
Caused by refraction of light rays within the lower
atmosphere
 Light rays bend as they pass through a substance of varying
density
Superior mirage: light rays reflected from a distant
object bend more sharply than usual, causing the object
to appear higher then normal
Inferior mirage: rays refracted less than normal, the
object appear lower
All mirages are displacements or distortions of
something real
© AMS
19
Atmospheric Optics
 Sunrise-Sunset and Twinkling Stars
Image of setting/rising sun is slightly higher in the sky
then it would be without the atmosphere
 Due to refraction of sunlight by the atmosphere
Sun/Moon near horizon appears distorted from usual
circular disk
 Occurs when strong atmospheric stratification is present
 Rays from lower rim are lifted more than rays from upper rim
Twinkling stars caused by fluctuations in air density
 Causes rapid changes in brightness as light from stars passes
through the atmosphere
 Known as scintillation. Noticeable on cold, clear nights.
© AMS
20
Atmospheric Optics
 Twilight
Period following sunset or before sunrise when the sky
is illuminated
 Caused by scattering of sunlight
 Length of twilight varies with latitude and time of year
Divided into three sequential stages:
 Civil twilight: center of Sun’s disk is 6° below horizon; no
need for artificial lighting
 Nautical twilight: center of Sun’s disk is 12° below horizon;
can distinguish outlines of distant objects
 Astronomical twilight: center of Sun’s disk is 18° below
horizon; sixth magnitude stars visible directly overhead
© AMS
21
Atmospheric Optics
 Twilight, continued
Crepuscular rays
 Occur at the beginning of
evening twilight
 Appear as beams of
sunlight radiating from
the Sun
 Alternating light and dark
bands that diverge in a
fanlike pattern from sun’s
position, visible because
of scattering
© AMS
Atmospheric Optics
 Twilight, continued
The green flash
 Thin, green rim that appears
briefly at the upper edge of the
Sun
 Best seen on a distant horizon
when the atmosphere is very
clear
 Consequence of refraction and
scattering of light from a low
Sun
 Rayleigh scattering causes
green color
© AMS
Atmospheric Acoustics
 Sound Waves
Compressional wave, consisting
of alternate compressions and
rarefactions of air
 Transmission of sound energy
via alternating increases and
decreases in air pressure
produced by waves that radiate
outward from a source
Wave frequency: number of
oscillations per second
 Measured in hertz (Hz)
 Audible range for most humans:
20 – 20,000 Hz
 Intensity (loudness) of sound
measured in decibels (dB)
© AMS
24
Atmospheric Acoustics
 Sound Waves, continued
– Wind and air temperature affect the speed of
sound waves
 Travel faster in warm air than in cold
– A change in temperature gradient with distance
alters the speed of sound waves causing
refraction of the waves
 In normal atmospheric conditions, sound waves
refract upward (away from warm surface)
– Opposite with a thermal inversion; waves reflected
downward
 Refraction may produce acoustic shadows, areas
where sound is not heard
© AMS
Atmospheric Acoustics
 Thunder
– Sharp clap or rumbling following lightning discharge
 Lightning heats the air along conducting path
 Rapid rise in temperature is accompanied by a tremendous
increase in air pressure, which generates a shock wave
 Shock wave propagates outward, producing a sound wave
– Storm cells that are more than 20 km (12 mi) away are
too distant for thunder to be audible, but lightning is still
observed
 Known as heat lightning
– Light travels about a million times faster than sound
 Reason for seeing lightning before hearing thunder
 This flash-to-bang method can be used to estimate the
distance to a thunderstorm
© AMS
26
Atmospheric Acoustics
 Sonic boom
– Caused by aircraft traveling at speeds that exceed the
speed of sound (Mach 1)
– Noise propagates in all directions faster then speed of
the aircraft
– A narrow, conical zone of compressed air in the form of
a shock wave is produced
 Aeolian sounds
– Produced by winds blowing over obstacles, creating
humming, singing, or whistling sounds
 Turbulent eddies responsible for sounds
© AMS