Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is infrared rays belonging to solar energy?

  1. Sep 7, 2004 #1
    my teacher said that all greenhouse gases is good at absorbing heat energy,but not solar energy. I think it was wrong because i think that solar enery should consist of heat energy and light energy. And also, for an example ,carbon dioxide is absorb infrared rays and then give out infrared rays(heat) . So.. i am very confused now!!!
  2. jcsd
  3. Sep 7, 2004 #2
    The type of infrared radiation that is absorbed by Co2 is the radiation reflecting from the earth.

    From the sun the radiation passes through the atmosphere.
    Because of its wavelength it is not absorbed "on the way in"
    After the radiation hits earth it warms the surface.
    The infrared radiation that is emitted by the "warm" earth has a longer wavelength than the radiation comming from the sun.

    This Ir with a longer wavelength does get absorbed by the Co2 in the atmosphere (at least a part of it).

    Both the radiation from the sun and from the earth are in the Ir wavelenght range.
    The Ir in sunlight is in the upper ranges, while the Ir from the earth is in the lower (longer wavelength) ranges.
    The last is also called Thermal Ir.

    (note i haven't put it very sientific, but if i'm not mistaken the theory is right. feel free to rephrase it in a sientific sentence)
  4. Sep 7, 2004 #3
    Excellent abstract Marijn.

    However again CO2 gets a lot more attention than it may deserve. Take this for instance.

    Now, let's get that old enveloppe out again.

    Note in the right hand upper graph of fig 10-1 that the absorption characterics of CO2 show only two shallow spikes whereas water vapor shows much higher absorption along almost the whole range. Compare the contribution of one unit CO2 and one unit of water vapor to the total atmospheric absorption.

    Also in the lower part of 10-1 we see that the absorption of the incoming radiation at about rougly 10% is mainly caused by water vapor.

    Then we should realize that standard amosphere air can hold about http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/atmospheric_moisture/concept_of_saturation.htm [Broken] of water vapor pressure at 15C in a standard atmosphere of 1013 milibar at saturation or 12,000 ppm. That's still 6000 ppm at 50% relative humitity. The same air holds about 370 ppm of CO2.

    At 18,000 feet where atmospheric pressure is half, and temperatures at some -5 degrees this would decrease to 2mB relative to 506 mB or about 4000 ppm at 50% relative humidity, still 2000 ppm.

    Now correct me if I'm wrong but even if the contribution of CO2 and water vapor was equal in characteristics, the relative abundance about 370 versus average 4000 ppm would give a relative contribution of no more than 10% of the total greenhouse gas absorption to CO2.

    But the IPCC estimate is 25%. So what am I doing wrong? Can anybody explain me why?
    Last edited by a moderator: May 1, 2017
  5. Sep 7, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That was a good question, no idea, and you already have some good answers.

    Just to add a small thing - it's not only CO2 that 'traps' IR, methane and water vapour do so too (and maybe those horrible CFCs eating the polar ozone layers too?).
  6. Sep 7, 2004 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, but to a much lesser extent than CO2, water vapor, etc.
  7. Sep 7, 2004 #6
    True andre.
    Yet he was talking about Co2 so i kept it a that.

    Ofcourse there are for starters several other gasses that absorb thermal Ir.
    Then we have watervapour, dust, ashes etc......
  8. Oct 12, 2004 #7

    Don't forget the increased cloud formations, which are a strong reflector of IR. Less of the radiated heat from the surfce is able to get back out, heating up the atmosphere.

    To pull out the important detail from Andre, CO2 absorbes around 10.6um (Ozone), which is in the middle of the long wave IR spectrum.

    And when you talk about dust and ash, you are really talking about carbon, which is a strong blackbody absorber (think BBQ grill paint).

  9. Oct 12, 2004 #8


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You are not nearly so confused as your teacher --- probably more diplomatic if you don't pass that along. The sun and the earth can both be treated as "black bodies." Black bodies radiate at all spectral wavelengths. The intensity of radiation at any given wavelength is function of the temperature of the black body and of wavelength; peak intensity for lower temperature black bodies (the earth ~ 290 K) is at longer wavelengths, around 10 microns, and for higher temperature black bodies (the sun 6000 K ?) at shorter wave lengths, visible light (0.3 - 0.6 microns). In a nutshell, "Yes, you are correct. The sun does radiate in the infrared, and that radiation is absorbed by the earth's atmosphere." What your teacher was trying to say is that the radiation absorbed by earth at other wavelengths keeps the earth's surface at an average of 290 K at which the peak intensity of radiation from the earth is retarded somewhat by the atmosphere from escaping back to space.

    Uh, Red Fox, care to enlighten us as to what particular absorption band of CO2 occurs at 10.6 microns? Caution, this is a trap --- you have posted incorrect information, and there's nothing personal intended --- every hundredth outrageous remark on PF provokes me into stepping all over the individual responsible. The active vibrational bands of CO2 in this region occur at 5 microns (assymmetric stretch) and at 15 microns (bending) --- there ain't a 10 micron absorption.
  10. Oct 13, 2004 #9
    that link andre posted states the lifetimes of various ghgs, two years for CO2. what does CO2 break into? and CH4, too, for that matter?
  11. Oct 13, 2004 #10


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    "Residence time" of a component of a reservoir is defined as the total amount of that component in the reservoir divided by the absolute value of either the flux of all sources contributing to, or of the flux of all sources removing the component from the reservoir.

    There is 1.5 trillion tons of CO2 at any given moment; biological activity removes 250-500 billion tons per year (and contributes about the same); exchange between the ocean and atmosphere is another large flux (from Andre's link, of about the same size). Residence time is then 1 1/2-3 years. There isn't necessarily a breakdown into other compounds, though that can be a significant sink in some cases.

    Methane is oxidized very quickly in a high oxygen activity atmosphere as is found on earth.
  12. Oct 14, 2004 #11
    oh i think i got the concept now . The greenhouse gases is good at absorb longwave radiation but not good at shortwave radiation(solar radiation). Therefore greenhouse effect can keep the earth warm
  13. Oct 18, 2004 #12


    User Avatar

    Staff: Mentor

    Exactly - and the reason the sun's radiation is shorter wavelength (higher energy) is that the wavelength is based on temperature.
  14. Oct 20, 2004 #13
    so which one's infra rays wavelength are longer, sun's or human body's? I ask this question because i think infra rays have consist of many different waves
  15. Oct 21, 2004 #14


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    The infrared rays are the same RANGE of wave lengths whether they come from the sun or from a human body. The distribution, the proportion of photons with each given wave length is different.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook