Iced up Cirrus crashes

"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.

Stefan

Err... yes.
--
Jim in NC

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

In article ,
"Martin X. Moleski, SJ" wrote:

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.

Not quite. They have to reach thermal equilibrium if there is no heat
flowing in or out of the system. But, as you correctly note, heat can
(and doe) flow in and out via radiation. Surfaces can "soak up" the
cold of the night sky (actually, they radiate their heat into the night
sky) and become colder than the surrounding air, just as they can "soak
up" the heat of the sun and become warmer than the surrounding air.
Eventually some of the cold/heat does get transferred to the air. This
is why clear nights tend to be colder than cloudy ones (and why clear
days tend to be warmer, all else being equal).

rg

In article ,
Ron Garret wrote:

In article ,
"Martin X. Moleski, SJ" wrote:

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.

Not quite. They have to reach thermal equilibrium if there is no heat
flowing in or out of the system. But, as you correctly note, heat can
(and doe) flow in and out via radiation. Surfaces can "soak up" the
cold of the night sky (actually, they radiate their heat into the night
sky) and become colder than the surrounding air, just as they can "soak
up" the heat of the sun and become warmer than the surrounding air.
Eventually some of the cold/heat does get transferred to the air. This
is why clear nights tend to be colder than cloudy ones (and why clear
days tend to be warmer, all else being equal).


It is worth noting also that dark surfaces absorb and radiate more
readily than light ones, and so they get hotter during the day and
colder at night. Cirri are all painted white in order to take advantage
of this phenomenon and keep the skin from getting too hot in the sun.
(You'll never see a non-white Cirrus. It's part of the certification
conditions to paint the white.) Accordingly, Cirri are less prone to
radiation-induced cooling and icing than a dark-colored plane would be,
all else being equal.

FWIW,
rg

("Ron Garret" wrote)

It is worth noting also that dark surfaces absorb and radiate more
readily than light ones, and so they get hotter during the day and
colder at night. Cirri are all painted white in order to take advantage
of this phenomenon and keep the skin from getting too hot in the sun.
(You'll never see a non-white Cirrus. It's part of the certification
conditions to paint the white.) Accordingly, Cirri are less prone to
radiation-induced cooling and icing than a dark-colored plane would be,
all else being equal.


I get the sun heating darker surfaces up (many degrees!) more than an
identical white surface.

What I don't get is: Two wings of identical design and an identical starting
temp, both sitting out on a cold February night (no sun). How does the dark
wing get colder than the white wing?

If it's a microscopic temperature difference because of star twinkle and
ambient light pollution from the surrounding city, I can see that. However,
a number of degrees between the white wing and the dark wing at night? Nope,
I still don't get it.


Montblack

What I don't get is: Two wings of identical design and an identical
starting
temp, both sitting out on a cold February night (no sun). How does the
dark
wing get colder than the white wing?

Visible light and infrared radiation (heat) are both forms of
electromagnetic radiation, they just have different values of
frequency/wavelength. Objects that are absorptive or reflective of
radiation in the visible spectrum can (but don't necessarily) also
exhibit the same or similar properties of absorption or reflection of
radiation in the infrared spectrum. I think.

-R

This is a classic heat transfer problem from college engineering. The
heat transfered by radiation is proportional to the emissivity of the surface.
Most paint emissivities range from .98 for flat black to as low as .8 for some
very shiny paints. Oxidized aluminum, like my plane, would run .25, while a
highly polished aluminum surface could be as low as .04.

The other part of the heat transfer equation is that the transfer is
proportional to the ratio of the temperatures to the fourth power. That is
why something as far away as the sun is such a great heat source. It's
temperature is very high.

Finally, the radiant heat transfer is effected by the "view" of one surface to
the other. This part is very complicated to calculate, depending on the
geometry. I never was worth a crap at this part of the calculations.

Have fun,
tom pettit

radiation-induced cooling and icing than a dark-colored plane would be,
all else being equal.


I get the sun heating darker surfaces up (many degrees!) more than an
identical white surface.

What I don't get is: Two wings of identical design and an identical starting
temp, both sitting out on a cold February night (no sun). How does the dark
wing get colder than the white wing?

("Ron Garret" wrote)
Not quite. They have to reach thermal equilibrium if there is no heat
flowing in or out of the system. But, as you correctly note, heat can
(and doe) flow in and out via radiation. Surfaces can "soak up" the
cold of the night sky (actually, they radiate their heat into the night
sky) and become colder than the surrounding air, just as they can "soak
up" the heat of the sun and become warmer than the surrounding air.
Eventually some of the cold/heat does get transferred to the air. This
is why clear nights tend to be colder than cloudy ones (and why clear
days tend to be warmer, all else being equal).


So on the 41F night in question, and having imaginary temp probes build into
the composite wing surface, we might see overnight wing temperature readings
of say 29F or 30F?

Is there a way to (WAG), in advance, what different surface temps will be on
the night in question? (41F overnight and 40F at 8:15 am)

Knowing air temp, humidity, cloud cover, wind, etc - could someone predict
that the composite wing will be in the 25F - 31F range overnight, whereas
the aluminum wing might only get briefly down to say 35F? Aluminum being
willing to give up its heat to the air more readily than the composites?


Montblack