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

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.

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.

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 12:23 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Greenhouse effect / 2nd law

On 04/27/2011 04:49 PM, LaMontagne, Bob wrote:
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.

Well, maybe such a trend "can be seen over and over on station reports
throughout the country" ... but it cannot be seen in the data I cited
for Show Low, AZ.

Here is a scatter plot of temperature versus dewpoint.
http://www.av8n.com/physics/img48/temp-dewpoint-ksow.png

I would not be in a big hurry to describe this data in terms of
temperature and dewpoint being "correlated".

In other locales where there is more moisture and more lush vegetation
the story will be different.

============

Rule of thumb: Everything having to do with fluid dynamics is much
more complicated and much trickier than most people think it is.

This includes meteorology.
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