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The Sun's warmth

  1. May 21, 2006 #1
    On a sunny day, notably in Winter, your hand can feel warmth from the Sun, but as soon as you step out of the Sun your hand feels cold. This suggests that the effect is radiant energy from the Sun. That's all rather trite.

    But is the radiant energy that makes you feel warm actually mostly due to absorption of visible light and UV (ultraviolet) and less due to IR (infrared)?

    Referring to a spectrum of incident solar radiation at ground, such as here in fig 2.5 http://www-paoc.mit.edu/labweb/notes/chap2.pdf . As the intensity of solar radiation peaks in the visible, and the lower part of the IR band is heavily attenuated. It seems to me that the warming is not IR direct from the Sun, but visible and UV being absobed and causing heating.


  2. jcsd
  3. May 21, 2006 #2
    Look at figure 2.5c - that's the relevant one, the fraction of radiation actually absorbed between the troposphere and ground level. Don't look at transmittance past 3 microns, there's negligible solar radiation there. "Most" of the UVA and visible/IR gets through pretty easily. Since 95% of of the radiative energy is between 250nm and 2500nm (fig. 2.2 caption) (i.e., UVB, UVA, visible, and near infrared), that's where the heating comes from. One more factor, how much of incident radiation on the skin is absorbed vs. reflected? IR has an advantage here.

    The same atmospheric transmittance graph is also on wikipedia, only more readable because it "fills in" the area under the curve.
    Last edited: May 21, 2006
  4. May 21, 2006 #3
    Look what I found - Solar Irradiance at sea level!

    Always pay very close attention to the scales. This graph is linear in wavelength, and the y-axis has units of watts per wavelength, so just integrating the area gives you a measure of power! This graph only goes up to 3um (the relevant energies); other graphs above went up to 15um (mostly irrelevant stuff).
    Last edited: May 21, 2006
  5. May 21, 2006 #4

    Dr Transport

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    Up to 15 microns is irrelevant??? There is quite a bit of work done in industry for radiation out to 15 microns. We are not interseted until we hit 2 microns and the interest builds as we get to longer wavelengths.
  6. May 22, 2006 #5
    Thanks for the graph you link to, which effectively takes the imagination out of combining the sun's spectrum with the absorption spectra. One small point, I think you mean graph b, in the doc I linked to, whose caption is: " The fraction of radiation absorbed while passing from the surface to the top of the atmosphere as a function of wavelength. " graph c is from top of atmosphere to tropopause.

    I get what you say about integrating. You'd be integrating the area above the curve up to the 100% level, as the graph (b) is of absorption, not transmission.

    And yes I'd not mentioned absorption vs reflection because I was using the hand merely as an example. This all comes from a discussion I've been having about spectral saturation of CO2 IR absorption. And it struck me that whilst I normally have a lay-persons understanding that IR is radiated heat. According to the boltzman equation even UV, for example, can be emitted if an object is hot enough.

    So what I'm trying to understand is whether the visible components of the sun's light will at least contribute to the warming. Or if as most IR is blocked by the atmosphere, and the Sun mainly radiates above IR, what we feel as directly radiated warmth is mainly due to absorption of visible/UV.

    No need to reply, I know that my undertsanding is a bit woolly at present but I've got more to consider now.

    Once again thanks for the wikipaedia link, very useful.


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