Ch02-Radiat - Copyright 2015 by Roland Stull Practical Meteorology An Algebra-based Survey of Atmospheric Science 2 Solar Infrared Radiation Contents

Ch02-Radiat - Copyright 2015 by Roland Stull Practical...

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27 Copyright © 2015 by Roland Stull. Practical Meteorology: An Algebra-based Survey of Atmospheric Science 2 SOLAR & INFRARED RADIATION Solar energy powers the atmosphere. This ener- gy warms the air and drives the air motion you feel as winds. The seasonal distribution of this energy depends on the orbital characteristics of the Earth around the sun. The Earth’s rotation about its axis causes a daily cycle of sunrise, increasing solar radiation until so- lar noon, then decreasing solar radiation, and finally sunset. Some of this solar radiation is absorbed at the Earth’s surface, and provides the energy for pho- tosynthesis and life. Downward infrared (IR) radiation from the at- mosphere to the Earth is usually slightly less than upward IR radiation from the Earth, causing net cooling at the Earth’s surface both day and night. The combination of daytime solar heating and con- tinuous IR cooling yields a diurnal (daily) cycle of net radiation . Orbital FactOrs Planetary Orbits Johannes Kepler, the 17 th century astronomer, discovered that planets in the solar system have el- liptical orbits around the sun. For most planets in the solar system, the eccentricity (deviation from circular) is relatively small, meaning the orbits are nearly circular. For circular orbits, he also found that the time period Y of each orbit is related to the distance R of the planet from the sun by: Y a R = 1 3 2 · / (2.1) Parameter a 1 ≈ 0.1996 d·(Gm) –3/2 , where d is Earth days, and Gm is gigameters = 10 6 km. Figs. 2.1a & b show the orbital periods vs. dis- tances for the planets in our solar system. These figures show the duration of a year for each planet, which affect the seasons experienced on the planet. Orbit of the Earth The Earth and the moon rotate with a sidereal (relative to the stars) period of 27.32 days around their common center of gravity, called the Earth- moon barycenter . (Relative to the moving Earth, Contents Orbital Factors 27 Planetary Orbits 27 Orbit of the Earth 27 Seasonal Effects 30 Daily Effects 32 Sunrise, Sunset & Twilight 33 Flux 34 Radiation principles 36 Propagation 36 Emission 36 Distribution 39 Average Daily Insolation 40 Absorption, Reflection & Transmission 41 Beer’s Law 43 Surface Radiation Budget 44 Solar 44 Longwave (IR) 45 Net Radiation 45 Actinometers 45 Review 47 Homework Exercises 47 Broaden Knowledge & Comprehension 47 Apply 48 Evaluate & Analyze 50 Synthesize 51 “Practical Meteorology: An Algebra-based Survey of Atmospheric Science” by Roland Stull is licensed under a Creative Commons Attribution-NonCom- mercial-ShareAlike 4.0 International License. View this license at . This work is available at .
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28 CHAPTER 2 SOLAR & INFRARED RADIATION the time between new moons is 29.5 days.) Because the mass of the moon (7.35x10 22 kg) is only 1.23% of the mass of the Earth (Earth mass is 5.9726x10 24 kg), the barycenter is much closer to the center of the Earth than to the center of the moon. This barycen- ter is 4671 km from the center of the Earth, which is below the Earth’s surface (Earth radius is 6371 km).
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