Blankets, Greenhouses and CO2

[BaalChatzaf]

This is an essay on analogies.  Analogies are useful in that they convey a likeness and describe something in terms of what he already know  and understand.  Analogies  take the form  A is like B  with respect to properties P1, P2,....   This is one of the steps  we take to understand B.  Children learn by analogy.  Even grownup learn by analogy but grownups soon realize that analogies must not be pushed to far.  Taking an analogy literally is equivocating because essential difference between A and B are ignored and they should not be.

Now  analogy 1  the Greenhouse.  Real honest to god  greenhouses  work primarily by blocking convection.   Consider a hot object near the ground.  The object radiates heat and also transmits heat by conduction (lots of juggling molecules bumping into other molecules).  The air will be heated by radiation and conduction. Then the air rises if it is not impeded.  This is convection. The rising air takes the heat it acquired and becomes less dense and so rises.  When it rises high enough it radiates it heat out in the cooler are around it and eventually regains its density and begins to fall again.  This is convection. 

Literal Greenhouses/Hothouses work because they thwart convection.

Anology 2, blankets:  What about blankets.  You are under granny's eiderdown quilt on a cold winter's night.  Locally speaking your are a raging hot heat source.  At 98.6 F or 37.5 C  you are radiating heat into the surrounding air.  If you we not covered up the air would move upward and away by convection,  but granny's eiderdown quilt blocks convection. Eventually the quilt heats up it starts to radiate the heat mostly out to the cooler air above  it.  But that takes some time.  In effect granny's eiderdown quilt has slowed your rate of heat loss.  It is slow enough that the fats and sugars stored in your body can metabolize and replace the heat that was lost to the thin lay of air around you, and lost to granny's eiderdown quilt.  

By the way what would happen if you covered yourself with a damp sheet.  Answer: you would die of hypothermia.  Water has a very high heat capacity. It absorbs a lot of heat with very little temperature change.  So your damp sheet is sopping up heat which causes evaporation.  When water evaporate is sucks in lots, and lots  of heat to produce water gas (vapor)  and there is no change in temperature.  This is the so called latent heat  of evaporation,  a phase change.  So even more heat is sucked out of  your poor shivering body at a rate much faster than your metabolism can balance.  You quickly metabolize your ready reserves of glycogen and you are on you way to dying of hypothermia.  At no time do you freeze.  You just use up for matabolates and die.   This is how the poor suffering passengers on Titanic died. Even though they had life vests to keep their heads out of the water the cold Atlantic which was at 40 degrees F that April  night sucked the heat out of their bodies.  The bodies of these victims soon was a 40 degrees, equilibrium temperature at that time and that place. 

Now lets get to CO2.  The transfer energy to the CO2 from the Sun itself and the ground is by radiation primarily, not conduction or convection.  The CO2 is a non-dense blob of gas  spread out (mixed) with the rest of the atmosphere.   The CO2 blob of gas absorbs heat by radiation (mostly) and by convection (some) and conduction (but poorly. gasses are poor conductors).   CO2 prefers to  absorb  electromagnetic energy mostly in the IR bands. CO2 masses are not blackbodies that absorb radiation in all frequencies equally;  rather they prefer IR radiation.  We call such bodies grey bodies.  Even so when they are hot enough they radiate out the heat that they absorb until they are in thermal equilibrium with their surroundings.   Unlike Venus which has  a CO2 density of 960,000 ppm,  we here on Earth currently have CO2 density near 400 ppm.  Think about that.  The next time an AGW panic monkey warns us that we are turning into Venus  you are permitted to scoff. 

The CO2 radiates its energy mostly to the atmosphere around it and way high up  radiates out to space which is very cold and does not radiate much energy back.  The hard vacuum of space is around 3 Kelvin which is very near absolute zero. However some of the IR is radiated back to the ground  so there is a gradual warming effect. But even this warming is limited. The temperature over the ages cannot tend to infinity.  as the temperature  of the surface increases it will throw out (radiate) its energy according to the Stefan-Boltzmann law.  The rate of radiation is (hold on now!)  proportional to the 4 th power of the absolute temperature of the radiating body (that is because space for all practical purpose can be thought of as a zero Kelvin heat sink).   So even Venus whose atmosphere is 96 percent CO2  does not get as hot as the Sun.  It reaches thermal equilibrium  at around 950 deg F  which is the melting point of lead.  Venus is not only a shitty place to visit,  you would not want to live there. 950 F is the average surface temperature of Venus.  The heat  is spread through the atmosphere of Venus so even though Venus rotates rapidly (its day is nearly as long as ours)  and half the planet faces away from the Sun at any time it retains its heat.   That is a matter of radiative heat/energy exchange not convection or conduction.

By the way human could live on Venus if they live in cloud cities suspended in the upper  atmosphere of Venus kept aloft by balloons.  The upper atmosphere of Venus is between 90 and 100 F temperature.  A bit warm  but livable.  Unfortunately Venus does not have much free O2 for the earthly visitor to breath. Like I say, you wouldn't want to live there.

Here are a couple  of articles which disambiguate the overlap between radiative cooling and convective cooling.  

Please see:  http://www.washington.edu/news/2014/11/10/global-warming-not-just-a-blanket-in-the-long-run-its-more-like-tanning-oil/

and http://scienceblogs.com/startswithabang/2011/08/30/if-you-believe-that-blankets-k/

The take away here is that analogies should not be pushed passed the point of correctness and usefulness. 

[Brant Gaede]

Analogies are like clothes that suggest the body but are not the body. More clothes are not too helpful. You can't use them to power the logical constructions of sequential reasoning. They can be excellent for illustrative purposes.

--Brant

[BaalChatzaf]

1 hour ago, Brant Gaede said:

Analogies are like clothes that suggest the body but are not the body. More clothes are not too helpful. You can't use them to power the logical constructions of sequential reasoning. They can be excellent for illustrative purposes.

--Brant

analogy is the main tool of learning for the young.  It is not exactly Aristotelian Logic, so Ayn Rand would likely disapprove.  But what does Rand know about children anyway?  She was barren or chose to be barren and she was no one's favorite aunt.  

[Brant Gaede]

33 minutes ago, BaalChatzaf said:

analogy is the main tool of learning for the young.  It is not exactly Aristotelian Logic, so Ayn Rand would disapprove.  But what does Rand know about children anyway?  She was barren or chose to be barren and she was no one's favorite aunt.  

Know about children and their growth and development? Not much. But I don't think she'd be down on a good analogy. I once sent a rhetorical question containing a humorous analogy in writing up to Henry Holzer after one of his law lectures. Rand was in the audience and when she heard my question read by him--he was annoyed by it--Rand laughed and applauded as did the rest of the audience. I wish I could reconstruct it, but too much is now missing.

--Brant