Iced up Cirrus crashes

Stefan wrote:

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. The Romans used to make ice in North Africa by taking advantage of
this phenomena.

George Patterson
He who would distinguish what is true from what is false must have an
adequate understanding of truth and falsehood.

That's pretty cool (Pardon the pun ) Where can I read up on that George?

Patrick


"George Patterson" wrote in message
...


Stefan wrote:

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. The Romans used to make ice in North Africa by taking
advantage of
this phenomena.

George Patterson
He who would distinguish what is true from what is false must have an
adequate understanding of truth and falsehood.

W P Dixon wrote:

That's pretty cool (Pardon the pun ) Where can I read up on that George?

I read about it in the 70s and don't remember where; probably a Science Fact
article in Analog or Popular Mechanics. As I recall, the technique is to dig a
hole large enough to keep your water container completely below ground. Cover it
during the day and insulate it (the Romans used straw). Leave it open to the
night sky. It will freeze in a few days. The article said it only works in areas
where the night sky is usually perfectly clear (ie. the desert).

George Patterson
He who would distinguish what is true from what is false must have an
adequate understanding of truth and falsehood.

On Fri, 11 Feb 2005 15:12:27 GMT, George Patterson wrote:

W P Dixon wrote:

That's pretty cool (Pardon the pun ) Where can I read up on that George?

I read about it in the 70s and don't remember where; probably a Science Fact
article in Analog or Popular Mechanics. As I recall, the technique is to dig a
hole large enough to keep your water container completely below ground. Cover it
during the day and insulate it (the Romans used straw). Leave it open to the
night sky. It will freeze in a few days. The article said it only works in areas
where the night sky is usually perfectly clear (ie. the desert).

This site gives a reference:

http://www.madsci.org/posts/archives/nov99/941723540.Sh.r.html

Marty

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

On Fri, 11 Feb 2005 15:12:27 GMT, George Patterson
wrote:

W P Dixon wrote:

That's pretty cool (Pardon the pun ) Where can I read up on that George?

I read about it in the 70s and don't remember where; probably a Science Fact
article in Analog or Popular Mechanics. As I recall, the technique is to dig
a
hole large enough to keep your water container completely below ground.
Cover it
during the day and insulate it (the Romans used straw). Leave it open to the
night sky. It will freeze in a few days. The article said it only works in
areas
where the night sky is usually perfectly clear (ie. the desert).

This site gives a reference:

http://www.madsci.org/posts/archives/nov99/941723540.Sh.r.html

Marty

Except that particular reference isn't very clear about the relative
roles of heat transfer to the air (and into the surrounding ground) and
radiative transfer to the sky -- e.g., it says

". . . at night, the pit would be uncovered so that it could lose
heat to the desert air."

You might take that to imply that the air temp in those African and
Palestinian deserts goes below 32 F at night? (Anyone know?)

You guys are into some interesting physics here. Do the plane wings
frost even when the surrounding air is above 32 F because their net heat
transfer to the surrounding above-freezing air is not as strong as their
net radiative heat transfer to the cold sky? (which should also imply
equally strong net heat connectivity to the ground below, is that not
so?)

Or do they acquire and retain frost because some colder and therefore
heavier below-freezing air just fell down on them at some earlier time?

I'll wait for an authoritative answer, but bet on the latter explanation.

AES wrote:

You guys are into some interesting physics here. Do the plane wings
frost even when the surrounding air is above 32 F because their net heat
transfer to the surrounding above-freezing air is not as strong as their
net radiative heat transfer to the cold sky?

No, it's not necessary for the heat transfer to the night sky to be
"stronger", just for it to be a significant effect. According to:
http://www.efunda.com/formulae/heat_..._enclosure.cfm
the radiative temperature of the clear night sky is about -150 F, so
if the wing surface got no heating from its surroundings and reached
thermal equilibrium with the night sky then it would cool to -150 F.
On the other hand, if it didn't lose any radiational heat to the sky
but was in complete equilibrium with the ambient air temperature that
dropped to a low of 40F, then that would have been its lowest
temperature. In reality, both effects occur. It gets some heating
from the surrounding air and also conduction from other parts of the
wing and plane, but it also loses some heat through radiation to the
cold night sky. The resulting temperature is therefore somewhere
between +40 F and -150 F and in practice probably around 25F - 30F on
a perfectly clear night and very close to 40F on a cloudy night.

(which should also imply
equally strong net heat connectivity to the ground below, is that not
so?)

Yes, for the bottom surface of the wing. I wouldn't expect ice to form
on that surface on nights where neither the air nor ground temperatures
drop below freezing.

Or do they acquire and retain frost because some colder and therefore
heavier below-freezing air just fell down on them at some earlier time?

In that case a thermometer registering the ambient air temperature
should also record this below-freezing temperature sometime during
the night. But the lowest temperature recorded on the night in
question was reported to be +40F.

I'll wait for an authoritative answer, but bet on the latter explanation.

How much?

"George Patterson" wrote in message
...


Stefan wrote:

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. The Romans used to make ice in North Africa by taking
advantage of
this phenomena.

You were there?


--
Matt
---------------------
Matthew W. Barrow
Site-Fill Homes, LLC.
Montrose, CO.