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



Also, if you check times at 4:00 AM at the site you linked us to, one finds that when the air is clear (CLR) a low Dew Point temperature (the real measure of the amount of water vapor in the air) is correlated with a low temperature and a higher Dew Point is correlates with a warmer temperature.

DP 9 deg F ------- Temp = 30 deg F

DP 23 deg F -------- Temp = 46 deg F

While this is a single anecdote, it can be seen over and over on station reports throughout the country.

Bob at PC


________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] on behalf of John Denker [jsd@av8n.com]
Sent: Wednesday, April 27, 2011 6:23 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Greenhouse effect / 2nd law

On 04/26/2011 02:46 PM, Dan Schroeder wrote:
Although the greenhouse effect plays a role here, I think the more
important factor is condensation. Once the temperature drops to the
dew point, further heat loss merely causes more water vapor to
condense, with little effect on air temperature.

You are of course free to define your own scenario...

but you are not free to redefine my scenario.

It may be that in your scenario, condensation is
"more important" ...

but the scenario I described behaves as I
described. The desert is so hot and so dry
that a layer of clouds aloft at night will
keep the temperature from dropping so much
... and even an increase in humidity at all
levels (including ground level) will create
a perceptible greenhouse effect in the absence
of condensation.

Here is some actual factual data. This is the latest data from
http://www.srh.noaa.gov/data/obhistory/KSOW.html

Date Time Wind Vis. Weather Sky Temp Dewpt RelHum Pressure
mst mph mi Condition ºF ºF InHg

27 03:15 NE 6 10.00 Fair CLR 28 7 40% 30.10

26 14:35 W 23 G 32 10.00 Fair and Breezy CLR 66 7 9% 29.85

That's a 38 ºF temperature swing. At the low end, there is still a 21 ºF
spread between the temperature and the dewpoint.

As I said before, there is a range of possibilities ... but the
scenario I have described does sometimes occur and does serve
to answer the original question. The atmosphere is colder than
you, but it is warmer than outer space, and in selected situations
this is very easily observable.
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