How fast does the skin of the airplane cool to surrounding temperatures?

Mxsmanic opined

writes:

On a clear night the skin temperature can go BELOW the air
temperature due to the radiation losses into space.

The temperature of the aircraft won't drop below the temperature of
the ambient air, as long as the aircraft is dry.

Actually, it can. I have seen frost on metal surfaces (like wings) when the air
temp is above freezing. Taxiing will sometimes melt frost.

A clear night sky is /cold./


-ash
Cthulhu in 2005!
Why wait for nature?

Ash Wyllie writes:

Actually, it can. I have seen frost on metal surfaces (like wings) when the air
temp is above freezing.

What was the temperature profile of the air during the preceding
hours, and at the exact time of the observation?

If the air is warming up and moist, metal surfaces might well be below
that temperature and below the dew point.

A clear night sky is /cold./

Yes, because large masses of water vapor have a moderating effect on
temperature, making warm days cooler and cold days warmer.

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On 8 Dec 2006 18:39:30 -0500, "Ash Wyllie" wrote:

Mxsmanic opined

writes:

On a clear night the skin temperature can go BELOW the air
temperature due to the radiation losses into space.

The temperature of the aircraft won't drop below the temperature of
the ambient air, as long as the aircraft is dry.

Actually, it can. I have seen frost on metal surfaces (like wings) when the air
temp is above freezing. Taxiing will sometimes melt frost.

As usual, mxsmanic is both right and wrong (as in 'a little knowledge can be
a dfangerous thing"). When the skin radiates, it cools down, and will cool
the air in contact with its surface. This creates a thin inversion layer,
which (in the absence of any appreciable wind) is stable. The air
temperateure a few millimeters above the skin will be significantly warmer.


A clear night sky is /cold./


-ash
Cthulhu in 2005!
Why wait for nature?

Dan, it's not an important point, but from the physics/theromdynamics
side of the issue, the top surface of the wing is really at risk of
radiational cooling. The wing's leading edge's shape would allow
convective warming, as the warmer air in contact with the surface would
cool and flow downward.

writes:
On a clear night the skin temperature can go BELOW the air
temperature due to the radiation losses into space.


Tony wrote:
Dan, it's not an important point, but from the physics/theromdynamics
side of the issue, the top surface of the wing is really at risk of
radiational cooling. The wing's leading edge's shape would allow
convective warming, as the warmer air in contact with the surface would
cool and flow downward.

And that's where we see frost forming: on top of the wings,
fuselage and stabilizer.


Mxmanic wrote:
The temperature of the aircraft won't drop below the temperature of
the ambient air, as long as the aircraft is dry.

You really should read the textbooks. Try this website:
http://www.islandnet.com/~see/weathe...nts/frost1.htm

I quote from this article from the above site:

"Frost forms first on rock, glass, or metal surfaces that lose heat
more rapidly through radiative cooling than the surrounding air. This
is why car windshields frost over before frost forms on surrounding
vegetation. If the surface on which it forms has a temperature below
the frost point, frost may even appear when the officially reported air
temperature is above freezing."

The aviation weather textbooks teach the same thing. Another website,
this time by NASA:

http://aircrafticing.grc.nasa.gov/co...4_2_1a_RI.html

Now try to tell people the dry surface temp won't drop below the
air temp.

Dan

wrote:
writes:
On a clear night the skin temperature can go BELOW the air
temperature due to the radiation losses into space.


Tony wrote:
Dan, it's not an important point, but from the physics/theromdynamics
side of the issue, the top surface of the wing is really at risk of
radiational cooling. The wing's leading edge's shape would allow
convective warming, as the warmer air in contact with the surface would
cool and flow downward.

And that's where we see frost forming: on top of the wings,
fuselage and stabilizer.

Yea, same here. Often we'll get lots of frost on the top of the plane,
almost never on the bottom. I wasn't sure if the difference was temp or
moisture.

-Robert

writes:

You really should read the textbooks.

I have.

Try this website:
http://www.islandnet.com/~see/weathe...nts/frost1.htm

I quote from this article from the above site:

The above site is not a textbook.

"Frost forms first on rock, glass, or metal surfaces that lose heat
more rapidly through radiative cooling than the surrounding air. This
is why car windshields frost over before frost forms on surrounding
vegetation. If the surface on which it forms has a temperature below
the frost point, frost may even appear when the officially reported air
temperature is above freezing."

Surfaces such as metal and glass cool more during the night because of
radiative cooling. However, they emit in the infrared, and water
vapor, CO2, and other gases in the atmosphere absorb it, so it doesn't
get very far.

Early in the morning, as temperatures rise, the air may be filled with
moisture but slightly warmer than surfaces that have cooled greatly
during the night and conduct heat well (such as metal). The moisture
in the air may condense (and even freeze) on such surfaces. But there
isn't much radiation loss at ground level unless the air temperature
is lower than surface temperatures. Remember that not only does the
atmosphere absorb infrared, but it also emits it.

The aviation weather textbooks teach the same thing. Another website,
this time by NASA:

http://aircrafticing.grc.nasa.gov/co...4_2_1a_RI.html

This site requires Flash, so I can't read it.

Now try to tell people the dry surface temp won't drop below the
air temp.

I already have.

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Mxsmanic wrote:
Peter R. writes:

A question for those of you more adept at chemistry/physics than I: How
fast does the aluminum skin of the standard single engine GA aircraft take
to cool to surrounding air temperatures? For example, how long would it
take for the skin to cool from a heated hangar at 65 degrees F to outside
air at 20 degrees F?

Nor more than a couple of minutes. Aluminum is an excellent conductor
of heat.


How well aluminum conducts heat has nothing to do with it. It is all
matter of thermal mass and surface area to mass ratio. The good heat
conductivity or aluminum means that once the outside of the plane has
cooled off, the inside will cool off fast too.

--
Chris W
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Chris W writes:

How well aluminum conducts heat has nothing to do with it.

It has a great deal to do with it, because it determines how much heat
energy must be removed or added in order to reach a given temperature
at a given point. Since aluminum conducts heat well, as soon as the
skin of the plane cools, heat from within will flow into the skin and
further cooling will occur, and just about everything made of aluminum
will become cold very quickly.

If the skin were Styrofoam, it would cool almost instantly at the very
surface, but the remaining mass of foam and whatever was behind it
would stay warm much longer.

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Mxsmanic wrote:
Chris W writes:

How well aluminum conducts heat has nothing to do with it.

It has a great deal to do with it, because it determines how much heat
energy must be removed or added in order to reach a given temperature
at a given point. Since aluminum conducts heat well, as soon as the
skin of the plane cools, heat from within will flow into the skin and
further cooling will occur, and just about everything made of aluminum
will become cold very quickly.

If the skin were Styrofoam, it would cool almost instantly at the very
surface, but the remaining mass of foam and whatever was behind it
would stay warm much longer.

Sure. But it's the surface that ice forms upon, not the warmer
interior. A composite skin might cool off faster since it's insulated
from the rest of the structure.

Dan