Neither I had considered this definition of VMR. But would it not make sense to follow it? Then a statement that the atmosphere contains 20.95% oxygen makes more sense. You yourself pointed at that it would make sense to scale N2 and O2 for low humid altitudes, where the amount of water can be several %. In code preparing data for ARTS I normally do this adjustment. Should be more correct!?

A problem is to define what is the wet species when we go to other planets. Or maybe there are even planets with several wet species?

That is, I would be in favour to define VMR with respect to dry air, if we can find a manner to handle other planets.



On 2021-09-15 18:27, Stefan Buehler wrote:
Dear all,

Eli Mlawer brought up an interesting point in some other context:

we recently had a LBLRTM user get confused on our vmr, which is amount_of_gas / 
amount_of_dry_air. They weren’t sure that dry air was the denominator instead 
of total air.  I’m too lazy to look at the link above that @Robert Pincus 
provided, but I hope it is has dry air in the denominator.  So much easier to 
simply specify evenly mixed gases, such as 400 ppm CO2 (and, 20 years from now, 
500 ppm CO2).

I’ve never considered that one could define it this way. Perhaps this 
convention explains, why VMRs in climatologies like FASCOD add up so poorly to 

I’m not suggesting that we change our behaviour, but want to make you aware 
that this convention is in use. (Or perhaps you already were, and just I missed 

All the best,


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