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Atmospheric Optics - I First Midterm Exam Is This Friday!

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Atmospheric Optics - I First Midterm Exam Is This Friday! Atmospheric Optics - I First midterm exam is this Friday! The exam will be in-class, during our regular lecture • on Friday September 28 at 9:30am The exam will be CLOSED BOOK • ♦ No textbooks ♦ No calculators ♦ No cheat-sheets Alternate seating • The grades will be posted on WebCT • The exam covers Chapters 1,2,3,4,5, and 19. • The sections which are not covered are listed on the • Class Notes web page. Recap Condensation above the Earth surface • produces clouds. Clouds are divided into 4 main groups: • ♦ High ♦ Middle ♦ Low ♦ Clouds with vertical development an astronaut’s Satellites give a bird’s eye view of • clouds. ♦ Geostationary satellites ♦ Polar-orbiting satellites Infrared images can provide a three- • dimensional view of the clouds: ♦ Height of the cloud ♦ Temperature of the cloud Color The human eye is sensitive to the visible part of the electromagnetic spectrum • (λ=0.4-0.7 µm): ♦ Intensity - light or dark; ♦ Wavelength – color. ♦ We see the objects because they emit and/or interact with light (reflection, transmission, absorption, refraction, scattering, diffraction) Perception of color: • ♦ Each color corresponds to a particular wavelength. ♦ White: all wavelengths are present with equal intensity; ♦ Black: no light is emitted and/or reflected from the object. Color of emitted light and temperature: • ♦ The sun appears white. λ λ ♦ Colder stars look redder ( max is longer than max of the sun). λ λ ♦ Hotter stars look bluer ( max is shorter than max of the sun). Scattering of light The scattering of light in the atmosphere depends on the size of • the scattering particles, R, and on the wavelength, λ, of the scattered light. Geometric scattering: R>>λ λ • ♦ Rain drops (R~10-100 µm) ♦ All wavelengths equally scattered R ♦ Optical effects: white clouds Mie scattering: R~λ λ • ♦ Aerosols (R~0.01-1 µm) R ♦ Red scattered better than blue ♦ Blue moon, blue sun Rayleigh scattering: R<<λ λ • ♦ Air molecules (R~0.0001-0.001 mm) R ♦ Blue scattered better than red ♦ Blue sky, blue mountains, red sunsets White Clouds White clouds Description: the clouds appear white or grey. • Physical process: geometric scattering by rain drops. • Explanation: • ♦ Visible light at all λ is scattered in all directions. ♦ Clouds are optically thick with respect to light scattering but they do not absorb light well. ♦ The thicker the cloud is the more light is scattered backwards and less solar light reaches the bottom of the cloud. Therefore thicker clouds appear darker. ♦ At the bottom of very thick clouds the raindrops are even larger and absorb sun light better. These clouds look even darker. Conclusion: By the appearance of the cloud one can judge • whether it will rain and how strong the storm will be. Blue Sky Description: on clear days the sky is blue and on hazy days the sky is • white. • Physical process: Rayleigh scattering by air molecules. Explanation: shorter • λ (violet, blue) are scattered more efficiently by air molecules than longer λ (red). Conclusion: Clear • sky, distant objects have a bluish hue. Blue Mountains Crepuscular Rays Light scattered underneath low clouds • ♦ “the sun is drawing up water”, “Jacob’s ladder” Recap: blue sky and white clouds White Sky Description: on hazy days the sky is white. • Physical process: geometric scattering by dust and • salt particles. Explanation: all λ are scattered equally well. • Conclusion: By the color of the sky we can judge the • quality of the air, the dust load or the humidity of the air. Extraterrestrial skies Moon sky Mars at noon Mars at sunset Venus sky Titan The Color of the Sun The Color of the Sun The Color of The Sun Description: at sunrise and sunset the sun is yellow, orange • or red Physical process: Rayleigh scattering by air molecules and • fine dust particles. Explanation: on clear days only the blue light is scattered • away, on hazy days the yellow and the orange wavelengths are also scattered and only the red remains in the direct solar light. Conclusion: Red sunsets suggest that there is dust in the • air (pollution, haze over the ocean, volcanic activity, dust storms). Blue Moon Description: the moon appears blue. • Physical process: Mie scattering by dust particles. • Explanation: When the size of the dust particles is • approximately equal to the visible wavelengths the red light is scattered better than the blue light. Conclusion: one can guess what is the size of the • particles in the air. Reflection and Refraction of Light The speed of light in vacuum • is c=300,000 km/s Snell’s law: The angle of • incidence is equal to the angle of reflection. Light that enters a more- • dense medium slows down and bends toward the normal. Light that exits a more- • dense medium speeds up and bends away from the normal. True and apparent position of objects Due to the refraction of • light the objects on the sky appear higher than they actually are. ♦ Star location and scintillations; ♦ Timing of the sunset and the sunrise; ♦ The sun on the horizon looks flattened; ♦ Twilight. The Timing of the Sunset & Sunrise We see the sun before it actually rises above the horizon and after it sets below the horizon. Twilight Flattening of the Sun’s Disk at Sunset Green flash Flattening of the Moon Refraction by the Earth’s atmosphere (image from ISS) http://www.sundog.clara.co.uk/atoptics/phenom.htm White light --mixture of all visible wavelengths of electromagnetic radiation Color of objects—absorption and reflection Basics of Raleigh, Mie, and Geometric scattering Color of clouds – white to black Why is the sky blue? Why is the sun yellow or red? Why is haze blue? What is refraction of light-why does it happen? Why does sun/moon flatten as it sets? Why do stars “twinkle”?.
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