Lect17 -- Solar

T energy emitted per unit area hot object total

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Unformatted text preview: ation curves λ max = const./T Energy emitted per unit area Hot Object Total energy emitted = kT4 k = 5.67 x 10- 8 J s- 1 m- 2 K- 4 Cold Object Wavelength Sun = hot -> visible radiation Earth = cold -> IR radiation Solar radiation reaching Earth, and its fate How much energy comes from the sun? We need to know the Sun s temperature! Wien s Law à༎ wavelength of emitted radiation is shorter for bodies at higher temperature Measure sunlight wavelengths ~ 500 nm So sun is about 5500-6000 K How much energy comes from the sun? Sun is about 5500-6000 K We know E = kT4 And we know k: k = 5.67 x 10-8 J s-1 m-2 K-4 Esun = 5.67 x 10-8 J s-1 m-2 K-4 * (5780 K)4 Esun = 63 x 106 J s-1 m-2 Whoa!!!!! How does this then give Esunlight ~ 340 W/m2 How much energy actually reaches the Earth? Energy from Sun arriving at Earth = (Solar output)(4πr2)/(4πR2) = (Solar output)(r2/R2) = 1360 W/m2 NOT ACCOUNTING FOR REFLECTION r = Sun’s radius = 6.96 x 105 m R = Earth sun dist. = 1.50 x 1011 m Solar radiation reaching Earth, and its fate Solar radiation reaching Earth watts/m2 1365  Arriving top of atmosphere, equator, noon (“solar constant”) 435 Average over day at equator (divide by π) 341 Average over Earth (divide by 4) 164 52% reflected or absorbed by clouds, atmosphere (depends on latitude, season, atmospheric conditions; latitud...
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