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UCD ATM 10 - Severe and Unusual Weather

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ATM 10 Severe and Unusual Weather Prof Richard Grotjahn http atm ucdavis edu grotjahn course atm10 index html Lecture topics Optics Scattering Optics refraction Sky Colors and Rays Mirages and Refraction Rainbows Sun Dogs and Halos Optics diffraction reflection coronas halos Scattering Sky Colors and Rays Optical phenomena caused by SCATTERING Why is the sky Blue Why are clouds White Nitrogen oxygen argon water vapor carbon dioxide and most other gases are colorless BUT they cause light to scatter Figure 1 3 Clouds are composed of cloud droplets that do not have color either but are translucent Chapter 4 Atmospheric Optics Scattering Scattering is a deflection of light due to reflection into different directions For example a cloud Figure 4 2 Rayleigh Scattering makes Blue Skies Zion NP R Grotjahn Figure 4 4 Air molecules are only large enough to reflect the smallest wavelength light which is blue Rayleigh scattering takes a beam of sunlight and selectively creates blue skies A yellow sun is evidence of this removed blue light Dark Blue above white near horizon Whiter near horizon because light from that direction has come many different distances through atmosphere Encounters more dust Encounters more air Longer paths scatter reds too Colorado R Grotjahn Colorful Sun Clouds Figure 4 8 Low on the horizon the sun s light passes through nearly 12 times more atmospheric gas and aerosol which scatters most short wavelengths Longer oranges and reds then comprise the sunlight which reflects from clouds and water Arches NP R Grotjahn Geometric Scattering White Dark Clouds Cloud water droplets are poor absorbers of light but large enough to reflect all wavelengths as geometric scatterers By scattering all visible wavelengths equally the cloud does not change the color of the light Figure 4 2 Thick clouds however diminish the passage of light and appear dark Reflected Transmitted Absorbed Figure 4 3 scattered As a wave of light passes through a cloud only a small amount is absorbed and the rest is either reflected or transmitted as a function of cloud thickness Other large particles Tasmania R Grotjahn Raindrops Death Valley R Grotjahn Large dust particles Mie scattering Smog Looking towards sun bright Looking away from sun dark Ground based smog is visible because those reactants of nitrogen and ozone scatter absorb light different than clean air These particles are smaller than droplets but much bigger than molecules Mie scattering Boulder CO photos R Grotjahn Geometric Scattering Crepuscular Rays Grand Tetons R Grotjahn Figure 4 7 Dust and salts are large enough to cause geometric scattering and change blue skies into hazy white skies Concentrated dust or salts beneath clouds can create white crepuscular rays of sunlight Refraction Mirages Rainbows Sundogs and Halos Refraction 2 principles 1 Refraction occurs when light encounters a substance of different density at an angle 2 A refracted beam bends towards the denser substance Towards cooler air Recall ideal gas law P R T Refraction Inferior mirages Two underlying principles 1 2 Light passing through air of different densities is refracted Light is bent towards cooler air Recall ideal gas law P R T Figure 4 16 Refraction Superior Mirage Figure 4 17 A superior mirage occurs when warm air rests above cold air causing the line of sight to head upward and the object appear at a higher altitude polar regions Superior mirage Refraction Rainbows Red on the outside Blue on the inside Is that all Capitol Reef NP R Grotjahn Rainbow Colors Prism White light is composed of many colors When white light passes through something of different density it refracts Different colors refract different amounts Red refracted less than blue Primary Reflection Refraction Angle incidence angle of reflection Refraction when go from air to water Figure 4 27a Figure 4 27b Primary Rainbow Figure 4 28 Red bends least and leaves the raindrop below the blue and violet colors An observer however sees a rainbow by capturing one color per drop where red and its small bend is now required to reside at the rainbow top Rainbows Rainfall Rainbows and red skies have been used to foretell the weather Rainbows are only seen when the viewer has their back to the sun and raindrops before them Figure 4 26 Because weather is predominantly from the west a rainbow at sunset means the storm has already passed Double Reflection for Secondary Bow Figure 4 29 Figure 4 30 Sunlight entering a raindrop may reflect twice before exiting which reverses the color sequence and diminishes the rainbow intensity A few more things Navajo Pk Colorado R Grotjahn Supernumerary arcs Dark band between Primary and Secondary rainbows Boulder Colorado R Grotjahn Delta Utah R Grotjahn Refraction Sun Dogs and Halos Sundog in high elevation ice cloud Sundogs in ice crystals blowing in air near the ground note hillside Refraction Sun Dogs Ice plate on left side Figure 4 22 Ice crystals selectively refract bend sunlight like a prism The angle change is again greater for blue than for red Hexagonal plates fall like feathers flat side up Ice Crystal Halo Figure 4 18 Figure 4 19 Cirriform cloud ice crystals randomly oriented to the ground refract light at an angle of 22 to create an arc Less common are the 46 halo which require more regular column type crystals Reflection Sun Pillar Sun pillars are formed by reflection of light off not refraction of light through ice crystals falling in still air Figure 4 24 Halo Tangent Arc An arc of light forms tangent to the halo when large hexagonal crystals fall with their long axis horizontal to the ground Figure 4 20 So just what is this Here s a hint Summary of Ice Crystal Phenomena Figure 4 25 Can you see The 22 degree halo The sundogs The sun pillar AND The upper tangent arc Diffraction Coronas and Glories Diffraction Defined Mainly due to diffraction of light What is Diffraction Bending of light around an obstacle The obstacles are small ice particles or liquid water droplets Diffraction is like the waves bending at the point of a beach Morro Bay CA Diffraction Coronas Figure 4 31 Figure 4 32 Solar and lunar coronas are caused by diffraction or bending of light as it passes around not through ice crystals or pollutants Diffraction causes circles of increased illumination where a wave of light reinforces other waves of light Diffraction Colors Figure 4 33 Bending white light by non uniform cloud droplets causes distorted separation of the different wavelengths as seen


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