Monday, October 19, 2020

No Overlaps - The source of the Urban Heat Island

Yesterday, Eli showed that surface warming depends on the average distance that an IR photon will travel before being absorbed by a greenhouse gas molecule. Today, some riffs.

Looking at a high resolution IR spectrum looking down from more than 30 km

The large bite into the blackbody emission at ground level (320 K) in the region of the CO2 bend at 640 cm⁻¹ is obvious, but the question is how much of that comes from CO2  and how much from H2O, in other words how much overlap. Well, let's look at the spectra from a 1m path in water vapor and  CO2 at atmospheric pressure and concentrations using high resolution

and it is clear that there is little overlap and most of the concentration is from  CO2. If you add up the absorbance between 620 and 720 cm⁻¹ the total from  COwould be 30 times that from H2O.  

If you asked how much of the IR would be absorbed in a 10 km path (the troposphere) by water vapor in this spectral region the answer would be pretty much all of it. That's what you get from the usual figure that is shown in such discussions elsewhere, or at least that there is a great amount of overlap

But that's the wrong question. The right one is, of the IR emitted from the surface, how much of it is absorbed by CO2 and how much by H2O, and there, the answer is CO2 absorbs essentially all of it in the first few meters so very little is left for water vapor to absorb.

Bunnies can ask, well, what is the distance at which the absorbance is 1 (~2/3 of the IR, or to be more exact 63%, is absorbed). At high resolution this is pretty confusing to look at

but it does look like very little gets more than 30 m or so. By averaging over 1 cm⁻¹ the picture becomes clearer   

So, remembering yesterday's lesson, in this region of the spectrum, essentially all of the surface warming is controlled by CO2 back radiation and none of it by water vapor (there is, of course some convection and condensation, but radiative emission and absorption are the primary effect.

By implication this means that a significant part of the urban heat effect is driven by CO2  backradiation.

Perhaps tomorrow the rest of the IR spectrum and the minor greenhouse gases.


William M. Connolley said...

> By implication this means that a significant part of the urban heat effect is driven by CO2 backradiation.

You left out some important steps there. I for one doubt your implication.


Too bad the Mad Hatter sent his theory to the White House before the White Rabbit could:

Chris A. said...

What I'm missing is the effect of the "dense matter" urban islands have compared to free landscapes. Do the many walls of stone that buildings provide not radiate heat away that would not be radiatet without the buildings?

I mean: Many buildings -> much matter with some heat capacity -> many radiatively active surfaces -> much heat radiation.

Just because of that I would say: "Sure there is more heat within urban areas"...

I might be wrong.

EliRabett said...

Stone radiates. In the absence of back radiation the cooling would be enourmous. A good guide to this is how quickly deserts cool after the sun goes down.

Canman said...

With all this hare-splitting, are you implying that urban areas would not be warmer than rural areas without the 100 ppm or so industrial increase in CO2 concentration?

EliRabett said...

Can, it's more than 100, urban areas can have very high ghg concentrations capping the surface


The Rabbet's right , Can- one reason Keeling set up his CO2 gear way out of town in Hawaii is that measurements in places like Paris and London often ran past 500ppm .

Canman said...

But aren't there other things in urban areas besides more CO2 that make them warmer than rural areas like asphalt?