August 13th 10, 12:46 PM
posted to alt.global-warming,sci.environment,sci.geo.meteorology,sci.skeptic
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First recorded activity by Weather-Banter: Mar 2008
Posts: 10,601
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Stephen Hawking and Carl Sagan on the Greenhouse Effect
On Aug 13, 11:20*am, Giga2 wrote:
On 13 Aug, 04:31, "
wrote:
On Aug 11, 8:26*pm, Roger Coppock wrote:
On Aug 11, 8:17*pm, Roger Coppock wrote:
Please see:
http://www.youtube.com/watch?v=mu1PicT0TMU
Below are lecture notes from a college course:
The greenhouse effect of Venus
--- Venus' perpendicular solar flux is ~2600 W/m2
From geometry, we can calculate the average solar flux over the
surface of Venus. It is approximately 661 W/m2.
Venus is very reflective of sunshine. In fact, it has a reflectivity
(or albedo) of 0.8, so the planet absorbs approximately 661 X 0.2 =
132 W/m2.
By assuming that the incoming radiation equals the outgoing radiation
(energy balance), we can convert this into an effective radiating
temperature by invoking the Stefan-Boltzmann law (total energy = _
T4). We find that T=220K.
But Venus’ surface has a temperature of 730K!!!
The explanation for this huge discrepancy is the planet’s greenhouse
effect.
The greenhouse effect of Earth
--- Earth's perpendicular solar flux is ~1350 W/m2
From geometry, we can calculate the average solar flux over the
surface of Earth. It is approximately 343 W/m2.
The earth has a much lower albedo than Venus (0.3), so the planet
absorbs approximately 343 X 0.7 = 240 W/m2.
By assuming that the incoming radiation equals the outgoing radiation,
we can convert this into an effective radiating temperature by
invoking the Stefan-Boltzmann law (total energy = _ T4). We find that
T=255K.
Earth’s surface has a temperature of 288K
While much smaller than Venus’ greenhouse effect, earth’s is crucial
for the planet’s habitability. Without the greenhouse effect, the
temperature today in Los Angeles would be about 0 degrees Fahrenheit.
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