Dear Philip, Jonathan and Christiane, > > > >> In addition, the mass fraction of water vapor in dry air is not > zero, > >> this is possible and used in models. > > > > What does it mean, then? I assume "dry air" means "air containing no > water". > > Can it have a non-zero mass fraction of water vapour? > > > > Hi Jonathan, > > In this case I would consider 'dry air' to just be the denominator. > This > has the advantage that concentrations of species won't change > indirectly > due to moist processes affecting the denominator. Hence, 'the mass > fraction of water vapor in dry air' is a perfectly reasonable quantity > to > use in a model, and can be non-zero. Of course, this may not be the > appropriate quantity for some calculations, but in that case 'the mass > fraction of water vapor in dry air' is just what is needed to convert > the > mass fractions of all species in dry air to actual mass fractions :-). > > Best wishes, > > Philip >
Philip is exactly right, I meant 'dry_air' to be the denominator of the fraction. So if A is the mass of water vapor present in a sample of air and B is the mass of the whole sample, mass_fraction_of_water_vapor_in_dry_air means A/(B-A). Its definition would be exactly the one currently in the table for humidity_mixing_ratio, i.e. " the ratio of the mass of water vapor to the mass of dry air". Is that OK? We already have mole_fraction_of_water_vapor_in_air in the table and we could certainly also introduce mass_fraction_of_water_vapor_in_air which, using the same definitions of A and B as before, would mean simply A/B. If we use 'ambient air' instead of just 'air' in this case then we ought really to change all the mass|mole_fraction_of_X_in_air names to be consistent. That would mean creating 104 aliases. Personally, I'm not convinced of the need for this. The AMS definition of 'air' is very clear: "air- Mixture of gases forming the earth's atmosphere, consisting of nitrogen (~78 percent), oxygen (~21 percent), water vapor, and other trace gases such as carbon dioxide, helium, argon, ozone, or various pollutants." So I would vote for continuing to use 'air' to mean moist/ambient air and 'dry_air' to mean air excluding water vapor. Best wishes, Alison ------ J Alison Pamment Tel: +44 1235 778065 NCAS/British Atmospheric Data Centre Fax: +44 1235 446314 Rutherford Appleton Laboratory Email: [EMAIL PROTECTED] Chilton, Didcot, OX11 0QX, U.K. -- Scanned by iCritical. _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
