`` NEWTON: Greenhouse Gas and Heat Transfer
 
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Name: Robert
Status: teacher
Grade: 9-12
Location: AK
Country: USA
Date: Summer 2013


Question:
It would appear from a superficial reading that heat flows out of a greenhouse gas more slowly than heat flows into the same gas. This has to be an incorrect interpretation. It seems more likely that molecules with high heat capacities resist heat transfer-both into and out of such a molecular system. At a molecular level how does heat move out of a hot greenhouse gas? I have seen plots of Cv vs Tempt which indicates that heat moves from translational modes of motion-into rotational modes and finally into modes of vibration. The energy spacing of vibrations is generally grater that rotation which are greater than translation. Could it be that it is this quantization of the energy levels and the difference in energy between such quantum states that is the source of the resistance to heat flow or transfer?

Replies:
I think you're missing the point of a greenhouse gas. It is not at all complicated. The point is that visible light passes through the gas without being absorbed; the visible light is then absorbed by the ground, roofs, roads, etc.; they get warm and radiate infrared light. The infrared light /is/ absorbed by the greenhouse gas, so it does not just pass through back out into space, but instead heats the gas.

It is not about heat flowing in at a different rate than it flows out, and it is not about how heat is shared among different degrees of freedom. It is simply about greenhouse gases being transparent to visible light, and not transparent to infrared light. The reason for this is that greenhouse gas molecules resonate at infrared frequencies, and they do not resonate at visible light frequencies.

Tim Mooney


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