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Re: [Phys-L] Solar Constant Puzzle.

Don't forget the 'greenhouse effect' (ie 'backradiation') in the calculations. There is a handy online calculator called MODTRAN ( for atmospheric IR radiation.

In particular, for 'Midlatitude Summer' looking up from Altitude=0, there is 310 W/m^2 of incident IR. If you block the sunlight with some sheet of material, that would be akin to adding a layer of clouds to the model. Low clouds (0.33 km) increase the backradiation to 393 W/m^2, or an extra 83 W/m^2. Direct noon sun is ~1000 W/m^2, so this correction is relatively small, but certainly not trivial (especially as the sun gets away from the zenith). Other details (like geometry, emissivity, temperature of the blocking sheet) would all make a difference.


(PS This is more of a general comment, not specifically addressing the most recent riddle).

From: Phys-l [] on behalf of []
Sent: Tuesday, May 27, 2014 11:22 AM
Subject: Re: [Phys-L] Solar Constant Puzzle.

Sure, I'll bite.

I see two factors: heat flux and heat capacity.

Given identical exteriors in steady state, the heat flux is constant.
Thus the temperatures are constant.

The heat capacities of the solid cylinders is greater than the cans,
so the tin cans will reach steady state faster. The cans are more
responsive to changing solar irradiation.

Dr. Roy Jensen
Lecturer, Chemistry
E5-33F, University of Alberta

On Mon, 26 May 2014 09:57:56 -0500, you wrote:

Today's physics puzzle:

Let's suppose you wanted to do a determination of the Solar Constant
using a two cylinders of aluminum blackened with candle soot, one of
them shaded from direct sunlight and provided with an electrical heater.

And suppose I wanted to replicate your determination of the Solar
Constant using two empty tin cans of the same size and surface area.

Which cylinders would develop the higher surface temperature in the
steady state: the solid aluminum ones, or the empty blackened tin-cans?

Brian Whatcott Altus OK Indian Territory.
Forum for Physics Educators
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