1. Chapter 14 Light and Sound in the Atmosphere
Weather Studies Introduction to Atmospheric Science American Meteorological Society
Chapter 14
Light and Sound in the Atmosphere
Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

2. 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
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

3. 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

4. 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

5. 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

6. 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

7. Atmospheric Optics
Scattering of sunlight by air molecules is responsible for the blue sky. Scattering of sunlight by tiny ice crystals and water droplets in clouds is responsible for the white of clouds.
The setting sun turns the horizon red on a clear evening.

8. 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

9. Atmospheric Optics
Light rays may be refracted as they travel from one transparent medium into another
Light ray is refracted as it travels from air to water
Light rays that enter the water at a 90° angle are not refracted
Refracted light can be deceptive

10. Atmospheric Optics
This type of refraction produces a 22-degree halo centered on the sun or moon
This type of refraction produces a 46-degree halo centered on the sun or moon

11. 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

12. 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

13. Atmospheric Optics
Refraction of solar rays by raindrops plus double reflection within raindrops produces a dimmer secondary rainbow above the primary rainbow
A rainbow appears to an observer who has his or her back to the sun and faces a distant rain shower

14. 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.

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
Iridescent clouds

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

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.

18. 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

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.

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

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

22. 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

23. 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)

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

25. 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

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