How is water vapour a greenhouse gas? how is it the most imporant and lethal one?
Water vapor is a greenhouse gas!!!
What do you mean by most important and lethal???
And what do you mean by "how"? Do you know what it means to be a greenhouse gas? Is this homework?
THis is not Hwk, it is school notes. What I mean is why is it a greenhouse gas what causes it to be one
Basically, a water molecule has many vibration modes that allow it to absorb infrared radiation; which is what it means to be a "greenhouse gas". It's linked to the structure of the molecule as a dipole.
I think this thread probably belongs in Earth science.
Cheers -- sylas
I'm not sure the at what level of detail you need to get into for your work/notes or personal curiosity. The reasons why H20 (or CO2, CH4, etc) are greenhouse gases involves quantum physics and technicalities which you might not be interested in. If you're doing this just for a high school or lower level undergraduate question then it may suffice to say that water vapor is a greenhouse gas because it strongly absorbs infrared radiation emitted by the Earth. Note that at Earthlike conditions the diatomic molecules (O2, N2, etc) cannot act as greenhouse gases but the triatomic molecules do. To behave as a greenhouse gas, either the molecule must possess a dipole moment or some of its vibrational transitions must produce a temporary dipole moment (this explanation breaks down in other planetary cases, for instance when you have a very dense atmosphere, but is adequate for Earth where oxygen, nitrogen, and argon do not absorb infrared light).
For an explanation which is somewhat detailed but doesn't require a strong mathematical or physics background, I suggest Chapter 3 of David Archer's book describing some of the chemistry of greenhouse gases. This chapter is accessible online at http://geodoc.uchicago.edu/archer.ch4.greenhouse_gases.pdf
Your last question is very ill-posed. I'm not sure what you mean by "lethal." We obviously need water on Earth and the gaseous phase of water which contributes to the greenhouse effect is very important in keeping the temperatures of the Earth above freezing and also in contributing the the hydrologic cycle. I am personally not even in agreement that it is "the most important" greenhouse gas (I think it's sloppy terminology and requires several caveats to make it meaningful to the person learning the subject), although my reasons for saying this are probably more of an academic matter than anything really physically meaningful to the discussion. "Most important" or not, water vapor does however contribute the most to the infrared opacity of the atmosphere when compared to clouds or other greenhouse gases. Because it is a bent triatomic molecule, it has a permanent dipole moment, and thus pure rotation bands in addition to vibration-rotation bands which makes it efficient at absorbing infrared.
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