Re: [Phys-l] Greenhouse effect / 2nd law

[John Denker <jsd@av8n.com>]

On 04/28/2011 09:47 AM, LaMontagne, Bob wrote:
 
> I was refering to something much narrower and more in answer to the
> original posting. In the predawn hours on a night with clear skies,
> one sees warmer temperatures at reporting stations when the dewpoint
> is higher. The water vapor returns more IR than the cold sky. I saw
> this often when I lived in Reno and was working for the Desert
> Research Institue. In the summer the radiational cooling through the
> night was quite severe and the predawn temperatures could often swing
> more than 60 degrees below the daytime temperatures. Nights where the
> relative humidity was down around 15 percent gave the most severe
> cooling.

We agree.  If we look at temperature and dewpoint as a 
function of air mass, we will see this sort of correlation.

> The correlation I was pointing out would not be expected over data
> taken throughout the day, except for the fact that warm air masses
> generally are more moist because of their tropical origin and cold
> air masses would tend to be drier because they are propably
> continental air. Your data show two clusters which probalby indicate
> two distinct air masses.

Yes, one needs to keep track of which air mass is which.

This is very hard to do with surface observations at a
single station.  One could do better using the "convective
derivative" i.e. by following a single air mass as it flows
from station to station.  This is what I was hinting at when
I mentioned "fluid dynamics" in a previous post.

=========

If we do in fact look at temperature and dewpoint within a 
single air mass, we find anti-correlation, i.e. the opposite
of the correlation mentioned above.

In particular, consider the vertical profile of a single air
mass.  Suppose that the air mass is well stirred, so that the
profile is very nearly adiabatic.  That is, looking here and
there in the air column is very nearly the same as lifting
a parcel from here to there adiabatically;  it does not exchange
enthalpy with its surroundings, and (!) does not exchange H2O
with its surroundings.  Then, as we go up in the air column,
the temperature goes down but the dewpoint goes up.  Anti-
correlation.  The physics here is simple:  the mole fraction
of H2O remains the same, i.e. the same number of moles of
water per mole of air.  However, the air parcel cools and
/shrinks/ as we go up the air column, so the molar /volume/
goes down i.e. the dewpoint goes up.

This has an immediate practical application:  If you see
fair-weather cumulus clouds, you can estimate the height
of the cloud bases in terms of the ground-level spread
between temperature and dewpoint.  The rule of thumb is
4.5 degrees F per thousand feet.

   To get this right you need to account for not just
   the lapse in temperature but also the increase in
   dewpoint.