Why is Venus's atmosphere so hot?

[Drakkith]

Let's take this to an extreme and assume that ALL radiation below the visible part of the spectrum is unable to penetrate the atmosphere. So that radiation is now absorbed by the atmosphere, heating it up just like normal while the visible light makes it through. When the atmosphere and surface emit radiation back out it just gets reabsorbed again! (One key thing is to remember that gases can transmit heat directly by contact, so they don't need to release radiation to heat up their surroundings) Only near the very top of the atmosphere where the air is very very thin can any infrared radiation make it back out, resulting in about 95% of the outgoing radiation simply being absorbed again.

The result of all this is that the atmosphere and the surface just keep heating up until the temperature is high enough to make its black body spectrum emit enough energy in the visible range (since that is the only part that can make it out) to equal the incoming energy from the Sun. That's about...2,000 k?

 

[chazzone]

Or not listening to their professors when they have already explained the part you think is missing.

The Sun's light, much of which does indeed penetrate the Earth's atmosphere without being absorbed, strikes the Earth's surface itself. Some of it is absorbed and re-emitted as infrared light, which we feel as heat. It then travels from the surface of the Earth upward, encountering various greenhouse particles as it goes.

See my previous post, a few above this, for the more complete explanation. I would feel silly quoting my own post for you.

I read your post, and I completely understand the process.

 

[chazzone]

Let's take this to an extreme and assume that ALL radiation below the visible part of the spectrum is unable to penetrate the atmosphere. So that radiation is now absorbed by the atmosphere, heating it up just like normal while the visible light makes it through. When the atmosphere and surface emit radiation back out it just gets reabsorbed again! (One key thing is to remember that gases can transmit heat directly by contact, so they don't need to release radiation to heat up their surroundings) Only near the very top of the atmosphere where the air is very very thin can any infrared radiation make it back out, resulting in about 95% of the outgoing radiation simply being absorbed again.

The result of all this is that the atmosphere and the surface just keep heating up until the temperature is high enough to make its black body spectrum emit enough energy in the visible range (since that is the only part that can make it out) to equal the incoming energy from the Sun. That's about...2,000 k?

I'm not sure of why you are using an example that doesn't exist in reality.

100% of all IR is eventually re-radiated back into space, and most of it within a very short time frame.

The IR from one molecule does not just bounce around and heat up another, since it is always re-emitted at a lower energy/longer wavelenght, and gasses only resonate in a very narrow frequency band.
This is why the idea of CO2 as a major greenhouse gas on Earth, is so ridiculous. Water already covers the same wavelength as CO2, and is a thousand time more prevalent in Earth's atmosphere. So, it relegates CO2 to an irrellevant status. It's like heating a BB to red hot, and throwing it into an olympic sized swimming pool. The effect is so negligible as to be nonexistant.

I'm willing to accept that it has a greater role to play on Venus, but Venus' albedo is .9 (where Earth's is between .3 and .36. There simply isn't enough light getting to Venus' surface to create the kind of heat observed.

 

[Drakkith]

I'm not sure of why you are using an example that doesn't exist in reality.

The same reason we simplify things in basic physics to perfect machines and frictionless surfaces, to help people understand the basics.

100% of all IR is eventually re-radiated back into space, and most of it within a very short time frame.

Eventually sure, not within a very short time frame though.

The IR from one molecule does not just bounce around and heat up another, since it is always re-emitted at a lower energy/longer wavelenght, and gasses only resonate in a very narrow frequency band.

You are forgetting that gases bounce around and transfer energy and are able to emit thermal radiation in a broad band because of this.

This is why the idea of CO2 as a major greenhouse gas on Earth, is so ridiculous. Water already covers the same wavelength as CO2, and is a thousand time more prevalent in Earth's atmosphere. So, it relegates CO2 to an irrellevant status. It's like heating a BB to red hot, and throwing it into an olympic sized swimming pool. The effect is so negligible as to be nonexistant.

You must be looking at a different chart than I am. CO2 absorbs wavelengths from 15-20 μm at nearly 100%. Water vapor fall off in the 15-18 μm range. In addition it absorbs wavelengths of 4-5 μm where water vapor absorbs almost none of that.

I'm willing to accept that it has a greater role to play on Venus, but Venus' albedo is .9 (where Earth's is between .3 and .36. There simply isn't enough light getting to Venus' surface to create the kind of heat observed.

The greenhouse effect on Venus is substantially stronger than here on Earth and Venus receives double the irradiance that Earth does. That's how it is able to accumulate so much heat even though it's albedo is so high. Note that Venus relies strongly on the sulfur dioxide, water vapor, and sulfuric acid in it's clouds for it's greenhouse effect. (Ref: http://www.imcce.fr/vt2004/en/fiches/fiche_n13_eng.html )

 

[D H]

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