Re: Physics on a sunny day

On Mon, 16 Jul 2006, Ken S. Tucker wrote:

Edward Green wrote:
I bought a new patio table with a sun umbrella. I bought a new garden
bench. I sat on the bench under the peach tree in the hot sun, and it
was good -- a mild breeze blew, and it was pleasant. I moved to under
my new sun umbrella, and it was not good -- a radiant heat as from an
oven came off the underside, and it was not pleasant.

The umbrella, it seems, is not a good design. Not as good as the peach
tree, which wasn't even designed for our benefit, so far as we know.
The dark green leaves evidently reflect a large portion of the sun's
radiation, so the tree as a whole does not heat up significantly beyond
ambient temperture, though exposed to non-equilibrium radiation. Or is
the tree actively cooling itself by evaporation? Whatever it is the
tree is doing, the umbrella is not. The umbrella, though apparently
lighter in color than the tree, is absorbing light and reradiating it
in the IR.

Moral: the works of man suck. Shady trees rule.

Yup...
I'm wondering if aluminized mylar (IIRC), you
know the stuff they make shiny balloons from,
might make a better canopy, or use it to fit
over a tarp or canopy. Al seems to be a good
reflector of IR as I've used around hot wood-
stoves and it doesn't even get warm.

It would make a good layer underneath the umbrella canopy, as would Al foil or aluminised paper. Polished Al has a very low emissivity in thermal IR, lower even that in the visible. Thus, tends to get quite hot in the sun. It still won't re-radiate as much downwards as the umbrella, but might get hot enough to cause problems. Solution: use the umbrella to stop too much visible light reaching the Al.

--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html

.

Relevant Pages

  • Re: Physics on a sunny day
    ... I sat on the bench under the peach tree in the hot sun, ... The umbrella, it seems, is not a good design. ... tree, which wasn't even designed for our benefit, so far as we know. ... over a tarp or canopy. ...
    (sci.physics)
  • Re: Physics on a sunny day
    ... I sat on the bench under the peach tree in the hot sun, ... The umbrella, it seems, is not a good design. ... tree, which wasn't even designed for our benefit, so far as we know. ... though exposed to non-equilibrium radiation. ...
    (sci.physics)
  • Re: Physics on a sunny day
    ... I sat on the bench under the peach tree in the hot sun, ... and it was not pleasant. ... The umbrella, it seems, is not a good design. ... tree, which wasn't even designed for our benefit, so far as we know. ...
    (sci.physics)
  • Physics on a sunny day
    ... I bought a new patio table with a sun umbrella. ... I sat on the bench under the peach tree in the hot sun, ... tree, which wasn't even designed for our benefit, so far as we know. ...
    (sci.physics)
  • Re: Physics on a sunny day
    ... I sat on the bench under the peach tree in the hot sun, ... The umbrella, it seems, is not a good design. ... tree, which wasn't even designed for our benefit, so far as we know. ...
    (sci.physics)