tisdag 20 juli 2010

Conduction vs Radiation

In heat transfer by conduction particles interact locally, with more energetic particles feeding less energetic neighbors in a local spatial averaging process. Heat conduction is the result of local interaction by averaging. This process has a direction in time given by decreasing differences or smoothing, or quick damping of high frequencies.

Radiative heat transfer is carried by electromagnetic waves at the speed of light over distance.
This process  may be viewed as a form of action at distance, but it appears to be more fruitful to view it (partly) as a local process where an incoming Planck spectrum is transformed into an outgoing spectrum of lower temperature by spectral cut-off of high frequencies, at the surface of absorption/emission. This process has a direction in time by the high frequency cut-off: What has been cut-off is gone and cannot be retrieved.

We conclude that conduction and radiation share the property of high frequency damping/cut-off, while their underlying physics is different.  

This post connects to earlier posts arguing that back-radiation is as unphysical as back-conduction or time moving backwards. See The Clock and the Arrow: A Brief Theory of Time and Computational Blackbody Radiation.

10 kommentarer:

  1. A very simplistic observation - a cloudless atmosphere at night quickly becomes rather chilly at surface of earth. A cloudy atmosphere stays relatively warmer at night - both events ought to share the same CO2 levels where the CO2 backradiation seems to be missing clear nigths - could this be an argument re backradiation to exist or not, or do I misunderstand something here?

    SvaraRadera
  2. Far too simplistic an observation, Christer. You can easily detect the EM radiation being emitted by the night sky on a clear night.

    SvaraRadera
  3. Clouds act as a retailer between surface and space thereby inflating the temp/price at the surface. But the retailer does not sens goods back to the surface/producer.

    SvaraRadera
  4. Your analogy does not make sense, Claes.

    SvaraRadera
  5. Roger - Of course the sky is full of EM radiation - from Big bang echo's, broadcasting etc to moonlight and certainly some from the atmosphere - the observation is more on scale - the backradiation from all heat leaving the earths surface on a clear night is not having much of an effect on my skin

    SvaraRadera
  6. Your statement "could this be an argument re backradiation to exist or not" implied that you believed it did not exist. It's good to see that you don't think this. The radiation reaching you from the atmosphere actually has quite a major effect on your skin. It's not clear to me why you think it hasn't.

    SvaraRadera
  7. Sorry Roger - I do need an extra sweater clear nights even if there is a lot of EM radiating back, and likely up quickly again, but not when humidity and clouds are plentiful, then I can do without the sweater - this tells me without the maths that there is an order of magnitude difference in ability of CO2 versus H2O? Or do you have some good math/fundamental physics to show me why my need for cover is different?

    SvaraRadera
  8. H2O concentrations and cloud cover vary dramatically; CO2 concentrations hardly vary at all. Your simplistic observations cannot tell you anything about the radiative effect of CO2 compared to H2O because CO2 is constant in all of them. Your observations do show that a varying concentration of a greenhouse gas can have a major effect on the thermal balance of the atmosphere, but for reasons I can't understand, your original post attempted to conclude exactly the opposite.

    SvaraRadera
  9. Christer, try doing the math instead. Roughly how much energy does a human body emit as blackbody radiation? How much energy do we produce to keep warm? How do these numbers compare?

    SvaraRadera