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UA ATMO 170A1 - Halos and Rainbows

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ATMO 170A1 1st Edition Lecture 8Outline of Last Lecture I. Why The Earth Has Seasonsa. Maximum RadiationII. The Fate of Solar Radiationa. Absorptionb. Scattering (Reflection)c. Scattering (Diffuse) Outline of Current Lecture II. Why is the sky blue? Why are sunsets orange/red/yellow?III. TransmissionIV. Halos and Rainbowsa. RefractionCurrent Lecture- Oxygen and Nitrogen are selective scatterers. ◦ Mountains that are farther away will look blue (sunlight is scattered by air molecules between you and the mountain). - Why is the sky blue? Air molecules preferentially scatter shorter wavelengths (blue).- Larger molecules scatter all wavelengths and look white.- Why are sunsets orange/red/yellow? Only warm colors make it to our eyes.◦ In clear skies, sunsets will be more yellow. If you have particles, sunsets will be red.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.Transmission- Transmission is the movement of radiation without modification to its direction, intensity, or wavelength. - There is no change as it goes through the atmosphere. - On a very clear day, 80% of radiation is transmitted. When there is an overcast sky, 20-30% of radiation is transmitted. Halos and Rainbows- Refraction: Deviation from the original path because of passage through a different density medium◦ When a light passes a denser substance, it slows down, and if it enters at an angle, it will bend.▪ Depends on angle▪ Depends on density- Halos are caused by ice crystal refraction of sunlight. The result of such refraction by a large number of crystals is a circle around the sun with the angular radius of the inner edge equal to 22°.- Rainbows are caused by refraction of sunlight with rain drops.◦ To see a rainbow, the observer must have the sun behind them, and the rain falling the samedistance in front of them. ◦ The observer ends up seeing a rainbow with violet at the bottom to red at the top.◦ When light passes through a droplet of water:▪ Refracted▪ Each wavelength is bent differently (red light bends less, violet light bends most)◦ There is only one ray per drop. If you move, you will see light from a different drop (each person sees their unique rainbow).◦ The secondary rainbow, with its upside down colors, is due to light rays that reflect twice within the raindrop before exiting. It exits at an angle of about


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