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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