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: alison.pamment at stfc.ac.uk
Chilton, Didcot, OX11 0QX, U.K.
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Received on Thu Nov 06 2008 - 05:33:36 GMT
