Unformatted text preview: the actual surface area is 4R2 , so the average \insolation" (as it is called)
is 1/4 the solar constant
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 eect." 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:
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
(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!
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- Fall '09
- Atmosphere, Greenhouse gas