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Heat (radiation) without direct air transfer - greenhouse effect?

  1. Jul 4, 2013 #1
    Hi everyone

    I realised that I had a mistaken belief about the meaning of the term greenhouse effect. I thought that it refers to the phenomenon whereby there is an exchange of heat due to light but without there being a direct air contact between the body being heated and the surrounding environment; specifically, the effect that you get when you are indoors on a sunny but cold day, and the air inside is getting heated due to the Sun's rays coming in through the window, despite the room becoming colder if any windows were actually opened to let air in.

    So I thought that the term refers to the phenomenon both at a planetary scale (as in the greenhouse effect leading to rises in temperature on Earth) as well as at a smaller scale (rooms, houses etc). I now read that the greenhouse effect actually has to do specifically with greenhouse gases...

    So what is the name of the effect that I described earlier (if there is even a name for it)?
    Many thanks in advance :)
     
  2. jcsd
  3. Jul 4, 2013 #2

    mfb

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    Greenhouse gases are the "window", and there is no (significant) exchange of air with the environment (=space).

    Thermal radiation?
     
  4. Jul 4, 2013 #3
    So sun shining through the closed windows is still a greenhouse effect (+heat transfer due to thermal radiation)? And the same phenomenon happens, at a planetary scale, with the sun rays as they encounter the greenhouses gases (as opposed to the closed window)?

    The first effect seems to make sense to be called a greenhouse effect, as greenhouses are indeed places that only get heat from the sun through thermal radiation and not through air - but how does this analogy extend to the Sun+Earth? Is it the case that the ozone layer blocks out most of the air contact, leaving just radiation to get through to the Earth?
     
  5. Jul 4, 2013 #4

    mfb

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    No. Air contact does not exist due to gravity - the atmosphere has a finite height. Carbon dioxide, water, ozone and some other molecules reduce the infrared radiation going to space, similar to the glass window.
     
    Last edited: Jul 4, 2013
  6. Jul 4, 2013 #5
    Sorry, I'm still not clear: so the window blocks out air, not radiation. If the ozone layer reduces radiation (IR, as you say, rather than UV i.e. sun rays) rather than masses of air, then how is this effect similar to the "closed window" effect, so as to warrant both of them the same name, i.e. "greenhouse effect"?
     
  7. Jul 4, 2013 #6

    SteamKing

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    The ozone layer, which is located high in the atmosphere, is important because it blocks most ultraviolet radiation coming from the sun. Some UV reaches the ground and causes folks to tan. If there were no ozone, lethal amounts of UV coming from the sun would make it very difficult for terrestrial life to exist on earth.

    When the sun shines during the day, it heats up objects on the earth's surface. When this heat re-radiates, such as at night, when the atmosphere is cooler, instead of it escaping through the atmosphere back into space, it is absorbed by greenhouse gases in the atmosphere and becomes 'trapped'. It is this gradual accumulation of heat, which would otherwise radiate back into space, which is thought to raise global temperatures.
     
  8. Jul 4, 2013 #7

    mfb

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    The combination of greenhouse gases (ozone is just one of many) and the finite height of the atmosphere gives a greenhouse effect.
     
  9. Jul 5, 2013 #8
    Thanks for your answers guys!
     
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