On Fri, 11 Feb 2005 09:26:18 -0500, "Morgans" wrote:

"Stefan" wrote in message
...
Peter wrote:

The temperature of a surface that's radiating heat to a clear night sky
can drop considerably below the ambient air temperature.

Err... no.

Err... yes.

Let me guess what's going on here.

"Ambient air temperature" means " the current local temperature of the air."
As someone said in another post, this is a fairly imprecise term and
depends on where the measurement is made at an airport.

Heat can be transferred by conduction (two masses in contact), convection
(circulation of gases or liquids), and radiation (infrared rays carry heat
away from the warm mass elsewhere).

Two masses in contact with each other (airplane skin and the air
that contacts it) have got to reach thermal equilibrium, all things
being equal and given sufficent time. Stefan seems to be focused
on this fact--the skin and the layer of air near it have to be at
the same temperature. JSM says, "But that layer of air may
be cooled more than the ambient air because the surface
loses heat not only to the ambient air but also by means of
infrared radiation."

The contrary situation certainly seems to be true: some surfaces
can be way hotter than the ambient air temperature because they
gain heat by "soaking up the sun's rays" (both infrared and visible,
I imagine). The air in contact with the hot surfaces must be in
equilibrium with the hot surface, though the air temperature would
decline to ambient air temperature as you move further away
from the surface.

Or so it seems to me.

Marty