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

["LaMontagne, Bob" <RLAMONT@providence.edu>]

Slight quibling with the wording here. The dew point does not actually go up in an adiabatic rise of a parcel of air. It decreases at at a rate of about 2 deg Celsius per kilometer (1.2 deg Farenheit per 1000 feet). It is the relative humidity that increases with altitude (reaching 100 percent at cloud base) - not the dew point. Yes, that does allow estimating cloud base for fair weather cumulus - the temperature lapse rate for well mixed air is 10 deg C per km and the dew point lapse rate is 2 deg C per km, so if you divide the difference in temperature and dew point near the surface by 8, you will find where (in kilometers) the two temperatures converge to form condensation. 

Bob at PC

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
> Sent: Thursday, April 28, 2011 4:21 PM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] Greenhouse effect / 2nd law
>
>
> 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.
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