RE: [APML] dew formation. Was: Film Creep

["Greg Hartke" <ghartke@adelphia.net>]

Hi, Steve,

As you would expect, there's an enormous amount of complexity to atmospheric
energetics and I tried to limit the original exposition to what seemed to be
salient facts only. Then again, your question is an excellent one that I
should have foreseen. I'll offer a limited answer because you can get into
an infinite regression of "what if" cases because the parameter space is so
large.

As the atmosphere cools to the dew point, of course the water vapor
condenses out. This condensation process gives up heat (the heat of
vaporization that went into evaporating the water into the atmosphere in the
first place) which warms the atmosphere and acts to keep the temperature
pretty stable. Obviously given sufficient time the temperature of the
saturated atmosphere would decrease as the dew point decreases with the
reduced water vapor content (some of it is leaving the atmosphere and
condensing on the surface, after all) but it happens slowly enough that for
practical purposes the temperature stays pretty steady in the little time
available before sunrise. In some situations a new equilibrium is reached
between the saturated atmosphere and the available surface water and this
keeps the atmosphere at a constant temperature for a while.

"Other factors" refers to changes in cloud cover, insolation (i.e., solar
heating due to sunrise), front passage, winds, and similar environmental
factors that cause rapid changes to the system. The general description
refers to a system it might be fair to call quasi-stationary.

I spent part of my career developing advanced transport models for the lower
atmospheric boundary layer with applications to climate models. I still
wouldn't say I was an expert on the subject because I *know* some experts
but of course I know more than most on the subject. In developing transport
models for climate models, you have to be extremely careful to make sure
your model is robust. A climate model that might run for decades or
centuries of model time will explore every corner of the available parameter
space and if you have a weakness in your model, a numerical climate
experiment will find it. Take it from someone who knows. ;) Bottom line is
that there's a seemingly infinite number of what-if cases and caveats in
modeling, then understanding, atmospheric dynamics so I hope you'll forgive
me for sticking to what might be called a baseline description that I hope
will still be useful.

Regards,

Greg

> -----Original Message-----
> From: astro-photo-bounces@seds.org
> [mailto:astro-photo-bounces@seds.org]On Behalf Of Steve Walters
> Sent: Wednesday, October 01, 2003 7:06 PM
> To: Discussion of Film Astrophotography
> Subject: Re: [APML] dew formation. Was: Film Creep
>
>
> Hi Greg,
>
> This is an excellent exposition but I have a question. You said:
>
> > The atmosphere will not get cooler than the dew point. Once the
> temperature
> > of the atmosphere reaches the dew point, fog will form (a relative
> humidity
> > of 100%, of course) and the temperature remains pretty stable
> until other
> > factors cause changes.
>
> Can you explain why this is true? And what are the other factors that will
> cause changes?
>
> Thanks!
>
> Steve...


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