Re: [Phys-L] Earth as a "heat engine"

["Folkerts, Timothy J" <FolkertsT@bartonccc.edu>]

Francois says:
> For the atmosphere there is a net input of radiative energy in the tropics and a net loss at high latitudes.  
My conceptual objection here is that this is simply convection.  If there is a hot object at one side of a room (eg a radiator or wood stove) and a cool object at the other side (eg a window) then I will set up exactly the same sort of convective circulation, with a net source of thermal energy in one place and a net sink of thermal energy in another place.  Is this empty room a "heat engine"?  

John says:
> However, in a very practical sense, you can stick a windmill into the atmosphere ... 
I would say you have made a fundamental change here.  
* If water just flows down a river, it is not an 'engine'.  If I add a waterwheel to extract energy, then I have an 'engine'.
* If I have a hot object touching a cold object, that is not an 'engine'.  If I add a thermopile between the objects to extract energy, then it is a 'heat engine'.
* The atmosphere is not an engine.  When I add a wind turbine to extract energy, THEN it is an engine.  

John says:
> but it is not entirely crazy.
I agree.  People who know about thermodynamics can navigate these ideas easily and get to the core of the issue.  (Much like people who understand thermodynamics can deal with people using "heat" when they are discussing "internal energy".)

I have also heard people call the atmosphere a "heat pump" because it pumps heat from the tropics toward the pole.  But the atmosphere is certainly now a 'heat pump" in the sense HVAC people/engineers/physicists would understand. 

It is probably just another case of letting go in colloquial discussion and educating in non-colloquial discussion.