Here is a "real world" observed phenomenon. Now to see who can figure out what's going on. :-)
Enjoy...
In Laramie, Wyoming, a friend when he lived there discovered his garden hose (black or dark
grey in color) contained ice one morning early, even though it was summertime and the low for
the night was no colder than 35 degrees (F).
We all know that water freezes at 32 degrees F, not 35. So how did it freeze. There were no
clouds, if that matters. Please explain.
We'll see where this goes, then I'll present the answer if someone else doesn't beat me to it.
Clarke
At ground level, like on the ground is cooler than higher up.
Pete & Jean
Fantasy
I've already had my glass of milk for the night so without much energy or thought, I'll guess a combination between radiant (since the dark color and clear sky) and altitude ( i recall that the lower atmospheric pressure lowers boiling temp but i dont recal if it raises or lowers freezing temp) ::)
I think the color of the hose is the key. I think it's called black body radiation phenomena. The heat from the water radiates into outer space? It's really cold out there.
It is a really strong hose and is under 1500+ psi of pressure, which would sufficiently elevate the freezing point of the water ;D
But seriously, I'll subscribe to the black body radiation theory.
WEC4104
Well we all remember from high school physics that water freezes at 32 degrees at sea level. The water as it turns to ice expands but I don't remember the volume change but I believe it is 10 percent expansion. The air pressure below sea level will cause the ice not to form at 32 degrees because it cannot over come the air pressure and expand until the temperature is even lower than 32 degrees. Now we have moved to Laramie WY which would have a higher than sea level altitude and therefore less air pressure allowing the ice to form at a higher temp than the fore mentioned 32 degrees. I don't believe that the change in altitude alone can account for the ice formation. You would also have to introduce the black body radiation that has already been mentioned. It happens particularly when the humidity is low as in the desert and as near as I understand it ( THIS COULD BE TOTALLY WRONG ) ( but the effect is real ) the temp of an object will begin to drop when the sun goes down and will continue to drop ( actually the object is giving up it's heat ) even to the point where the temp of the body is below the air temp -- kind of like a going too fast and missing the exit ramp -- I also don't remember how far the temp can drop but I remember being impressed by the amount and the person who described this to me worked for search and rescue here in the desert southwest.
Just my guess
Melbo
It depends on the amount of humidity in the air.
If the air temp F' is in the 30's and the wetbulb temp is below freezing, ice or snow will form.
Cliff
Clarke, radiated heat into space will cause this effect. If the humidity is low, horizontal surfaces can get the better part of 10 degrees cooler that the air does. Frost can form easily at temperatures above freezing.
When we were near Quartzsite and the afternoon temperature was around 80 degrees, I aimed our IR gun straight up into a cloudless sky and got a reading of -51 F.
For what it's worth.
Tom Caffrey
I'll try to clean up my answer/ guess. Laramie is at about 7200 feet above sea level. The earths atmosphere is an insulating layer that helps to keep heat from escaping. The atmosphere is relatively thin in Laramie. Clouds also keep the night warmer. Heat travels in various ways, through conduction, convection and radiation. The night that the water froze in the hose was a clear night. Heat always moves toward cold. The temperature in outer space is absolutely cold or as cold as can be I suppose because the heat sources are far away and far apart. Also there is little there to absorb and retain heat, like the earth. So the heat from the water in the hose radiated toward outer space enough to freeze.
This guess comes from an old brain that doesn't read much and never took a physics class.
Clarke's keyboard is busted right now, so it might be a while before he is able to post the answer ....
Quote from: Nick Badame Refrig. Co. on January 15, 2007, 09:31:02 AM
Quote from: Brian Diehl on January 15, 2007, 09:29:38 AM
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
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? :)