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

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

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

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

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

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

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.

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

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

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

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

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

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

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.

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

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

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.

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

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.

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

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

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

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)

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

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

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