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

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

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.

The bottom is subject to radiation heating from the pavement.

Jose
--
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"Jose" wrote in message
. net...
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.

The bottom is subject to radiation heating from the pavement.

Jose

Zactly - radiation depends on what the surface is exposed to. I park my
mini-van next to a big pine tree in the driveway - the tree side is clear in
the morning- it "see's" the tree. The other windows are frosted over (even
if the overnight low is still a bit above freezing) - they are exposed to
the sky. It's a real problem with telescopes too. Since they are pointed
directly at the sky (on clear nights) the objective lens cools off rapidly
and will fog up. Look up "telescope heater".

The earth surfaces cool first through radiation, which, in turn, cools the
air (which is reasonably "transparent" to a lot of wavelengths). That's what
makes clear nights colder than cloudy nights - more heat loss to space via
radiation. And having the air cooled by the surface of the earth is what
causes surface winds to often die down at sundown - you get a layer of cold,
heavy, air that just sits there under the warmer air that is moving around
the high and low pressure areas.

During the daytime, of course, radiation from the sun warms the surface,
which then warms the air. The nice thing about that is that it is the
mechinism that triggers the thermals that keep glider pilots aloft (dragging
it back to aviation content, eh?).

But to get back to the orignial post - just ask the guy in the next hanger,
he/she ought to know if they have been in the area a few years, right?

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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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You'll notice some have claimed to observe frost forming as I had
suggested it might, When theory and observations differ, it's the
theory that should change.



On Dec 8, 7:47 pm, Mxsmanic wrote:
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...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.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Tony wrote:
You'll notice some have claimed to observe frost forming as I had
suggested it might,

Agreed. It's quite common for frost to form on glass and metal
surfaces that are exposed to a clear night sky even when the local air
temperature never drops below 35F at any time.

When theory and observations differ, it's the
theory that should change.

But in this case the observations are in good agreement with the theory
of radiant heat transfer to a clear night sky.

The effect is well known to amateur astronomers since it causes the
optics of their telescopes to cool down below the dew point and have
water condense on them even though the air temperature remains well
above the dew point. Various solutions are used such as slight heating
of the optical elements or installing tubes (dew caps) that extend well
past the end of the telescope and therefore don't allow as much heat
radiation to the cold, clear night sky.

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.

--
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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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