ay45c3-page29 - the actual surface area is 4R2 so the...

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Unformatted text preview: the actual surface area is 4R2 , so the average \insolation" (as it is called)  is 1/4 the solar constant 1 F ins = 4  1400 w=m2  3:5  105 erg s 1 cm 2 : The outgoing infrared ux at temperature T is T 4, so:  5:67  10 5 erg cm 2 s 1 K ) 4  T 4 = 3:5  105 erg s 1 cm 2 T = 280 K = 7 C = 44 F chilly! The reason the Earth is actually warmer than this is because of the \greenhouse e ect." Atmospheric gasses absorb the IR emitted from the ground and re-radiate it to space. You might think that this is a \wash," but not so, as the following gure shows: I greenhouse gases H2O, CO 2 IR I 2I IR visible IR I You see that the ground must radiate twice the incident ux I to stay in equilibrium. Thus, 4 (5:67  10 5 )TK = 7:0  105 ) TK = 333 K = 60 C = 140 F hot! The reason it is not this hot is that the greenhouse is not totally absorbing (note the partial \windows" between 8 and 20 in the gure in Section 3.5.5). You can see why there are environmental worries about increasing the greenhouse gasses, however! 53 ...
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