OT?: Brain Teaser

[Brian Diehl]

Clarke's keyboard is busted right now, so it might be a while before he is able to post the answer ....

[Nick Badame Refrig/ACC]

Clarke's keyboard is busted right now, so it might be a while before he is able to post the answer ....

I'll Bet it "Froze" up on him......LOL

Nick-

[Clarke Echols]

Clarke's keyboard is busted right now, so it might be a while before he is able to post the answer ....

I'll Bet it "Froze" up on him......LOL

Nick-

Actually, one key started acting up, so I gave it a "bath". :-)  Unfortunately, some water got down into
the keys and it took some time to get them dried out enough to get it working (mostly) again.  There
are still a few keys that are a bit erratic, but it's usable.

Several got it right or close to right.

From basic thermodynamics, heat always travels from warm/hot to cold, by conduction, convection,
or radiation.  At high altitudes the air density is low and there is less air between the earth and the
depths of space.  Space, due to the distance between radiating bodies (stars) is extremely cold
(my "v" and "c" keys still acting up here), so anything on earth tends to lose heat by radiation into
space.  If there is cloud cover, some of that heat is reflected back to earth, thus reducing the loss
into space.

"Black bodies", such as a dark-colored hose or other black or near black object, radiate more
efficiently -- particularly infra-red (lower in frequency than visible light that goes from red at the
low end to violet at the high-frequency end of the visible-light spectrum and ultra-violet above
that) which is where the most efficient heat transfer occurs.  That is why infra-red radiant heaters
are so effective in shop heaters -- especially the overhead tube-type heaters.  It is also the principle
of radiant heating systems in floors because infra-red light an pass through very opaque materials.
However, highly reflective films such as bright aluminum help minimize that transfer.  Thus an
aluminum bus roof will tend to be somewhat ccoler in the hot summer sun than a steel roof, even
when both are under a painted surface.

This started from when my friend, a former Laramie, WY college professor and world-renowned
asphalt chemist was talking with a friend across town and the friend mentioned the temperature
from his thermometer.  My friend looked at his and it was 10 degrees warmer!  That was not
possible, he concluded, but then he realized his was under the roof over the back porch.  When
he moved the thermometer into the back yard where it was exposed directly to the night-time
sky, the temperature dropped dramatically by about 10 degrees.  That led to his realizing why
his black garden hose sometimes had ice in it in early mornings when the temperature all night
had been well above freezing!

Addressing some of the suggested answers:

The phenomenon has little to do with humidity, other than any reflection of radiated infra-red
light by water molecules in the air, and it was summer time so there was no snow on the ground.
Ground temperature would still be above freezing because it is less efficient at radiating (because
it is not a full black color.

Hot ice?  Now that is a very interesting answer. :-)  I had never heard of it before, but I also
will admit that 10 million volts per meter is a rather intense electric field, and definitely one you would
never want to encounter if you plan to stay alive.  That's 20 megavolts from head to toe for a
six-feet-tall person!  One could be critically killed! :-)

Early morning hailstorms occur rarely or never.  They require a lot of thermal energy in large cumulus
clouds, and that happens always (from my experience) in the afternoon, usually after 2 or 3 PM; never
in the morning.   Dew on the hose wouldn't be an issue because it evaporates only after the temperature
rises above the dew point, and on that morning there may have been no dew if the relative humidity was
low.  Aliens is an interesting twist, but I am unware of any sightings of UFOs in Laramie by the UFO freaks
in our society. :-)  [No insult intended if any UFO nuts are on this board -- after all, we're "bus nuts", right?]

The freezing temperature of water is not related to pressure because it is an uncompressible liquid, and it
is not related to altitude.  The lowering of the boiling point of water as you go up in altitude is due to the
vapor pressure of the water exceeding the surrounding air pressure which, of course, drops dramatically
as you reach high altitudes such as the Colorado Rockies or the heights of Wyoming.

So there you have a reasonable summary of the principles involved.  That reminds me of another
problem presented in the text book we used when I took Heat and Thermodynamics back in the
ancient times of 1967 or so, but that's a topic for another thread. :-)

I thought this one would stir up some brain cells. :-)  If you ever teach physics, this is a good example
of how you can make it interesting to kids who think it's dull.  Don't do like the physics teacher when
my oldest daughter was a senior in high school and took physics.  He  started them off with electric
field theory!  I have an extended major in physics in my undergrad degree, and I'd NEVER do that!!!!!
Start with useful stuff like kinematics, then relate that to safe driving on ice, for example...

Clarke

[gumpy]

So, explain why it isn't colder under the black shingled porch roof than out in the open yard. If black surfaces radiate more efficiently, then wouldn't the porch roof radiate all the warmth from under it more efficiently than the green grass? 

[FloridaCliff]

Hey,

Where was the porch thing in the problem???

Hmmmm, I want this graded on a curve now!

Cliff