From the Thermodynamics Textbook

Greenhouse effect:
The university undergraduate textbook, Thermodynamics: An Engineering Approach, states the following:
Glass allows the solar radiation to enter freely but blocks the infrared radiation emitted by the interior surfaces.
This causes a rise in the interior temperature as a result of the thermal energy buildup in a space (i.e., car). [1]

This explanation is from what is generally a very good Thermodynamics textbook – But, most people will agree that this is wrong!
It has been demonstrated experimentally [2] that a “greenhouse” with a cover of rock salt (which is transparent to infra red) heats up an enclosure similarly to one with a glass cover.

Greenhouses work primarily by preventing convective cooling

Cengel [1] continues as follows

The surface of the earth, which warms up during the day as a result of the absorption of solar energy, cools down at night by radiating part of its energy into deep space as infrared radiation.
Carbon dioxide (CO2), water vapor, and trace amounts of some other gases such as methane and nitrogen oxides act like a blanket [a] and keep the earth warm at night by blocking the heat radiated from the earth. The result is global warming.
These gases are called “greenhouse gases,” with CO2 being the primary component [b].

[a]: Blankets restrict heat transfer by inhibiting heat conduction and convection – not by absorbing radiation. The vacuum of space surrounding the Earth is an excellent blanket.

[b]. CO2 makes up about 350ppm or 0.035% of the atmosphere, H2O is about 4% of the atmosphere and is a much stronger IR absorber.  (ie  there are approx. 100 water molecules for each CO2 molecule.)

The question is – how did this explanation get into a Thermodynamics textbook? What is the source of this information, did anyone check?

[1] Yunus A. Çengel and Michael A. Boles Thermodynamics: An Engineering Approach, 7th edition McGraw-Hill 2011

[2] Wood, R.W. (1909). “Note on the Theory of the Greenhouse”. Philosophical Magazine 17: 319–320. http://www.wmconnolley.org.uk/sci/wood_rw.1909.html

2 Responses to From the Thermodynamics Textbook

  1. Brian says:

    “The vacuum of space surrounding the Earth is an excellent blanket.”

    You’ve got to be kidding! Without the Sun, exposure to deep space will have you down to 3K in so time, so you won’t feel a thing!

    • wstannard says:

      It is excellent at doing what a blanket does – preventing heat flow by conduction and convection..

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: