Presentation on theme: "Chapter 14 Light and Sound in the Atmosphere"— Presentation transcript:
1 Chapter 14 Light and Sound in the Atmosphere Weather Studies Introduction to Atmospheric Science American Meteorological SocietyChapter 14Light and Sound in the AtmosphereCredit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants
2 Case-in-PointMeteorology provides a scientific explanation for myth and legendFata MorganaOptical phenomenon: a mirage is caused by refraction of light rays as they travel through the atmosphereCauses objects to distort verticallyMost common on the horizon after sunriseFollowing a clear calm nightExtreme radiational cooling would have occurredLow-level temperature inversionsOccur in coastal areas, high mountain valleys in winter, and over the frozen Arctic Ocean
3 Driving QuestionWhat 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 phenomenaPropagation of sound through the atmosphereSounds of meteorological origin
4 Atmospheric OpticsSun’s rays are scattered, reflected or refracted by cloud droplets, ice crystals or raindrops as they travel through the atmosphereCreates optical phenomenaRefraction (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 spectrumSir Isaac Newton determined that visible light is polychromatic (composed of several primary colors)Color is sensed by cones in the human eyeDiscrete wavelengths excite the coneRelatively high temperature objects emit visible radiationExample: a hot piece of metalMost objects on the earth do not emit visible radiationTheir 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 skyScattering occurs when light waves are sent in different directions by particles in the atmosphereRayleigh effect: amount of scattering varies with wavelengthHuman 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 colorsWhy is the sun red?Length of atmosphere crossed by solar beam is 40 times longer at sunsetAll other colors have been scattered out due to longer wavelengthsWhy are clouds white?Large particles scatter visible radiation equally at all wavelengthsWater droplets/ice crystals (compose clouds) are sufficiently large, causing clouds to appear whiteParticles that are about the same size as the wavelength of light are responsible for Mie Scattering
7 Atmospheric OpticsScattering 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 moonForms when tiny ice crystals that compose high, thin clouds refract the sun’s raysRefraction: the bending of light as it passes from one transparent medium into another
9 Atmospheric OpticsLight rays may be refracted as they travel from one transparent medium into anotherLight ray is refracted as it travels from air to waterLight rays that enter the water at a 90° angle are not refractedRefracted light can be deceptive
10 Atmospheric OpticsThis type of refraction produces a 22-degree halo centered on the sun or moonThis 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 sunOccur when sunlight is refracted by plate-like crystals falling through the atmosphereSun pillars are bright light shafts appearing above or below the setting or rising sunCaused 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 raindropsSolar ray is refracted on entering a raindrop, reflected internally, and then refracted when exitingSun must be shining, and no higher than 42° above horizon to see a rainbow
13 Atmospheric OpticsRefraction of solar rays by raindrops plus double reflection within raindrops produces a dimmer secondary rainbow above the primary rainbowA 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 cloudsDiffraction: 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 obstructionConstructive 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 cloudsThin clouds with nearly uniformly sized cloud droplets having bright spots, bands, or borders of delicate colorsTypically appear up to about 30° from the sunIridescent clouds
16 Atmospheric Optics Glory Observer sees concentric rings of color centered about the shadow of his/her headSeen in bright sunshine about a warm cloud or fog layer, with the sun casting the observer’s shadow on the clouds belowUsually seen below aircraftDepends on the size of reflecting and refracting particles, and the direction of reflected and refracted lightReflected light due to small droplet sizes
17 Atmospheric OpticsIn 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 viewCaused by refraction of light rays within the lower atmosphereLight rays bend as they pass through a substance of varying densitySuperior mirage: light rays reflected from a distant object bend more sharply than usual, causing the object to appear higher then normalInferior mirage: rays refracted less than normal, the object appear lowerAll 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 atmosphereDue to refraction of sunlight by the atmosphereSun/moon near horizon appears distorted from usual circular diskOccurs when strong atmospheric stratification is presentRays from lower rim are lifted more than rays from upper rimTwinkling stars caused by fluctuations in air densityCauses rapid changes in brightness as light from stars passes through the atmosphereKnown as scintillation. Noticeable on cold, clear nights.
20 Atmospheric Optics Twilight Period following sunset or before sunrise when the sky is illuminatedCaused by scattering of sunlightLength of twilight varies with latitude and time of yearDivided into three sequential stages:Civil twilight: center of sun’s disk is 6° below horizon; no need for artificial lightingNautical twilight: center of sun’s disk is 12° below horizon; can distinguish outlines of distant objectsAstronomical 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 twilightAppear as beams of sunlight radiating from the sunAlternating 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 sunBest seen on a distant horizon when the atmosphere is very clearConsequence of refraction and scattering of light from a low sunRayleigh scattering causes green color
23 Atmospheric Acoustics Sound WavesCompressional wave, consisting of alternate compressions and rarefactions of airTransmission of sound energy via alternating increases and decreases in air pressure produced by waves that radiate outward from a sourceWave frequency: number of oscillations per secondMeasured in hertz (Hz)Audible range for most humans: 20 – 20,000 HzIntensity (loudness) of sound measured in decibels (dB)
24 Atmospheric Acoustics Sound Waves, continuedWind and air temperature affect the speed of sound wavesTravel faster in warm air than in coldA change in temperature gradient with distance alters the speed of sound waves causing refraction of the wavesIn normal atmospheric conditions, sound waves refract upward (away from warm surface)Opposite with a thermal inversion; waves reflected downwardRefraction may produce acoustic shadows, areas where sound is not heard
25 Atmospheric Acoustics ThunderSharp clap or rumbling following lightning dischargeLightning heats the air along conducting pathRapid rise in temperature is accompanied by a tremendous increase in air pressure, which generates a shock waveShock wave propagates outward, producing a sound waveStorm cells that are more than 20 km (12 mi) away are too distant for thunder to be audible, but lightning is still observedKnown as heat lightningLight travels about a million times faster than soundReason for seeing lightning before hearing thunderThis flash-to-bang method can be used to estimate the distance to a thunderstorm
26 Atmospheric Acoustics Sonic boomCaused by aircraft traveling at speeds that exceed the speed of sound (Mach 1)Noise propagates in all directions faster then speed of the aircraftA narrow, conical zone of compressed air in the form of a shock wave is producedAeolian soundsProduced by winds blowing over obstacles, creating humming, singing, or whistling soundsTurbulent eddies responsible for sounds