onsdag 30 mars 2011

A note on Thermal Diffusivity

I would like to bring to your attention a thermodynamic property which I havn't given very much attention until recently. It is the thermal diffusivity, that is the rate at which a temperature change will spread in the medium. Quoting from the G&T paper:



In particular you should pay notice to the density term occuring in the expression for the thermal diffusivity. In many cases the density appears to be a trecherous quantity. The reduction in density with height explains why the equilibrium state may very well be isothermal. Ironically however, if we adopt the non-equilibrium approach this non-uniformity might very well explain why the lapse rate is steeper closer to the surface than higher up. Maybe it could even explain why the stratosphere is stratisfied: Since the density is so low the thermal diffusivity is so high that any temperature gradient will quickly disappear. At least it is worth to think about.

2 kommentarer:

  1. Hi Anders,

    you write: "Since the density is so low the thermal diffusivity is so high that any temperature gradient will quickly disappear. At least it is worth to think about. "

    I'm afraid it's not really worth thinking about, though, because convection effects more or less completely overwhelm conduction effects in atmospheric heat transfer.

    SvaraRadera
  2. When convection does occur it completely overwhelms conduction, you are right. But convection only occurs above a certain critical lapse rate.

    Below that lapse rate only conduction (and radiation) participate. That is why i invoke thermal diffusivity at the altitudes where the lapse rate is very small or almost non-existing.

    But I realise that convection must be taken into account in the troposhere.

    SvaraRadera