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When does water freeze?
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Mar 5, 2007 at 9:44 pm #1222222
O.K., I'm not dumb, water freezes at 32 degrees. But are there localized conditions that cause water to freeze above 32 degrees? For example, when I wake up and have frost on my sleeping bag or tarp, can I assume that the temperature reached "freezing" the night before? Or is the bag frost created by some combination of moisture and wind that only freezes the surface of the bag even though the ambient temp is higher?
Mar 5, 2007 at 10:04 pm #1381220It is my understanding that wind chill affects people and animals but to freeze water you need a temperature lower than 32f.In other words the air temperature in you case was below 32f.
Expect a 700 word version of this from PJ.
FrancoMar 5, 2007 at 10:54 pm #1381223pj must be doing something less productive than monitoring BPL, such as sleeping or eating.. He really should set up an RSS/XML feed to automatically email his cellphone and trigger a ring when "pj" appears on this website…
Anyway, I'll go first..Ice is the result of a liquid water becoming solid ice by an event called nucleation. A water droplet must first reach its nucleation temperature to freeze. As a water droplet cools, heat energy is released into the atmosphere at a rate of one calorie per gram of water. As it freezes into an ice crystal, the water droplet will release additional energy at a rate of 80 calories per gram of water. this quick release of energy RAISES the water droplet temperature to 32 F, where it will remain while freezing continues. To be precise, the water will continue to freeze as long as it remains at or below 32 F, but only after it has first cooled to its nucleation temperature [less than 32' F].
Edited from here:
http://www.snowathome.com/snowmaking_science.phpMar 6, 2007 at 1:43 am #1381228> when I wake up and have frost on my sleeping bag or tarp, can I assume that the temperature reached "freezing" the night before?
Yep.
Mar 6, 2007 at 1:55 am #1381230Roger you win the concise reply contest. Next time I'll copy and paste the question and add Y or !
FrancoMar 6, 2007 at 2:33 am #1381234Brett, funny guy! like your sense of humor. And, you're absolutely right about water needing to have its temp lowered below 32F before it will begin to freeze (you get an A+), and for the reason you stated (you get a Gold Star and a Smiley face too). That's why it TRUE to say the ice MELTS at 32F, but it's not so true to say that ice forms at 32F, but perhaps i'm waxing too theoretical here.
Franco, what can i say. I have an ethnic gift of the gab (Jewish on my mother's side). LOVE your sense of humor. BTW, i think some people don't realize that the Aarn front packs are really two packs, one on each pack strap(at least the pics that i've seen), and NOT one continuous pack acroos the entire chest – which could obscure downward vision to the feet. I have some hunter's lumbar packs that accept so-called "Buddy-Lok" attachments (1/2 to just a bit smaller than the Aarn front packs, or 20-24oz water bottle attachments) – one on each pack strap; also, no prob. w/downward vision.
Roger, you sure have the gift of the gab! Great reply.
Also, I'm wondering if Christian was perhaps thinking that suppose the air temp was, let's say, 33F or 34F, for example, and a wind comes up that passes over some condensation on the underside of some fabric (tarp, or single-wall tent, for instance), and causes some of the water to evaporate. Maybe he's asking, under those conditions when the general ambient temp inside or under the shelter is just a tad above freezing, could the evaporation effect cause the water to freeze? Don't know if that's his question, however, just guessing as to why someone would ask this question, when the answer seems obvious.For example, what Christian may(???) have been thinking of and trying to apply to a slightly different problem is that evaporation is just one of several factors that explains why snowflakes are present and falling when the air nearer the ground is slightly above the freezing point, i.e. evaporation helps to cool the snowflake and slow down the warming effect or heat transfer from the warmer ambient air into the snowflake. The airflow over the snowflake as it falls can actually cool the snowflake just a bit. Of course, as the temp rises, the rate of heat loss due to evaporation (which gets successively slower as temps get cooler) becomes less than the rate of heat transfer into the snowflake from the ambient air, and so under those condition, the snowflake may never make it to the ground. Now, of course at some point the temperature was cold enough previously to cause the snowflake to form. There is also the issue of a boundary layer of cooler air forming around the snowflake, but i'll defer to an Aero "guy" (or "gal") on this point. i know just enough about Aero to be usually WRONG!!! Magical stuff, low Reynolds numbers!
In fact, i once read of an ice forming experiment where ice formed at room temperature in response to atomic-scale/nano- friction producing extremely localized low temps. This was done by dragging a wire (made of W, IIRC – but best check on that) across some type of carbon or graphite plate. Then there's the Peletier effect, but i don't know if one can get it cold enough to freeze water, i've only read of it used to chill air? Of course, neither of these are found under a tarp.
Perhaps i contributed to Christian's thought processes, by posting about how much water weighs depends upon the temperature of the water? Maybe i make things too complicated and muddy the waters by waxing theoretical at times? Wouldn't surprise me in the least!
Mar 6, 2007 at 3:01 am #1381235526 words left PJ….
The point is that some think that when they get the weather report and it is” 40f with a wind chill of 30f” ( I hate farkenheights) that it is 30F. That is not the case. We feel the wind chill because we produce heat and we loose it at an increasing rate in stronger wind. That does not apply to an object. I am still waiting for a scientific polysyllabic version of this from a native English speaker. Australians need not to apply, the same goes for any successful (therefore Australian) New Zealander.
FrancoMar 6, 2007 at 3:21 am #1381237Franco, you didn't acutally count the words in my original Post did you? Or, are you a Savant, of sorts, like the true multi-faceted, super-genius Thomas Young?
BTW, i edited my prev. Post. If you haven't done so already, please re-read the "Franco" paragraph. Just a thought?
[Note: Thomas Young = philosopher, physicist, linguist [Champollian prob. never would have suceeded in deciphering Egyptian hieroglyphics w/o Young's work on the cartouche], etc., etc., etc. – sometimes called the "Last Man to Know Everything". He memorized the Bible and KNEW how many times each word appeared in the King James translation of the Bible and in which verses they occurred – hence, he wrote his Analytical Concordance to the Bible, detailing this info.]
Mar 6, 2007 at 5:04 am #1381243Word word counted your original words.
Weird your comment about savant. I have been recently diagnosed as semi idiot savant , commonly known as autistic. I am missing the savant bit.
As it happens I am very partial to Jewish humour and if your mame is Jewish that is good enough for me.
Here is another , to me fascinating fact. Rain can fall at below freezing point, around 30f and freeze on landing.
FrancoMar 6, 2007 at 5:32 am #1381244Franco, we get that type of rain more often than we would like where i live. It's called "freezing rain" in contra-distinction from "sleet" which freezes on the way down and pelts off of everything. Hail is yet diff., but most people know what hail is. BTW, Freezing rain makes for very nasty driving conditions, as well as hiking up rock faces – VERY slippery, to say the least!
BTW, wind chill affects living things BECAUSE of WATER, or more precisely, because of evaporation (well,…and, of course, convective heat loss).
Mar 6, 2007 at 7:20 am #1381253Franco, absolutely right, Ive seen falling supercooled rain freeze instantly as it hits the windshield of an airplane. Evidently as it falls it is cooled below the temperature at which nucleation would cause freezing, but not below the temperature at which it would freeze in a free air stream; so when the nucleation substrate is provided (surface of the windshield, some dust, etc..) it freezes. This is bad for the leading edge of wings as it changes the wing camber to something less.. uplifting?
There was a plane crash caused years ago when the pilot was back grab-assing (talking really) with a stewardess, while supercooled ice built up on the wings.. Co-pilot didn't turn on the de-ice in time; plane crashed. Tragedy. Moral is don't grab-ass at work when freezing rain is falling.
Mar 6, 2007 at 11:22 am #1381292In junior high (=middle school) they told us we could put a tub of water in the garage if freezing temps were expected overnight. The freezing of the water would release enough energy to keep the rest of the garage from freezing.
I've always wondered if this could work in a tent. Anyone tried it?
Mar 6, 2007 at 11:35 am #1381296Sorry, Brian, i've never tried it.
Mar 6, 2007 at 7:07 pm #1381354Brian, that an interesting concept; a modern garage would be insulated well enough that some of the 80cal/g of heat would linger. In a non-insulated tent, the heat would escape as quickly as it is produced; and I think you'd have a block of ice in your tent the next morning?
Mar 6, 2007 at 7:59 pm #1381359Thank you for your responses to my seemingly inane question. PJ was right about my thought process. I wondered if there was some sort of localized evaporative cooling process that could create a patina of frost even though the surrounding temperature, say at waist level as opposed to ground level, was above 32. This question ultimately relates to gear. Sometimes I wake up, see a bit of frost and wonder "how cold did it get?", and then "Gee, I was in a 40 degree bag but wasn't cold, did it really reach freezing?"
I often bring a thermometer but sometimes not. I would rather be able to mystically determine the temperature using my own senses. Based upon the responses I would say that that I've reached the first step, frost = freezing.
Now for my next ponderous question. If I can see the moon does that mean that the moon is out?
Mar 6, 2007 at 10:13 pm #1381382Goodness, I was waiting for Ben to come out, never realised that the Moon is like that !
Franco
Mar 8, 2007 at 2:40 am #1381555> I wondered if there was some sort of localized evaporative cooling process that could create a patina of frost even though the surrounding temperature, say at waist level as opposed to ground level, was above 32. This question ultimately relates to gear. Sometimes I wake up, see a bit of frost and wonder "how cold did it get?", and then "Gee, I was in a 40 degree bag but wasn't cold, did it really reach freezing?"
OK, I have only used up one word out of my allowance, so can I reply some more maybe? :-)
The question was does frost on the SB mean it got down to 32 F (freezing), and the answer is yes. In fact, given that the sleeper inside the bag was pushing out both heat and moisture, the air was probably a degree or two lower.
I suspct the sleeper may have been camped in a slight valley? We call that a frost hollow. The coldest air settles at the bottom. Don't camp by the creek in the snow: get up the hillside a bit, into the trees. Much warmer.
Can a wind cool off water to freezing? Yes. The Arabs used to use this to make ice in the desert: they set up a low wall in the shadow with a pool of water behind it. The expansion of the cold night air blowing up over the pool chills the air enough that it cools the water enough to make sheet ice on the surface. Yeah, serious.
But, and this is a big but: I don't think hoar frost on an SB would be formed this way. Sheet ice maybe, but hoar frost implies the vapour coming out of the bag (or from the sleepers mouth) was chilled fast enough to make little crystals. Happens on the inside of my tent in the snow sometimes – and when I sit up in the morning my head brushes against the frost and it falls down the back of my neck. YUK!
Cheers
Mar 8, 2007 at 10:55 am #1381599Roger, you're right about the SB. i had tunnel vision and focused on the "tarp" that Christian mentioned. Thanks for taking the time to point out the frost on the SB aspect.
Didn't know about the Arabs and ice – thanks for sharing that info. Wonder at what temp range they make this scheme work? Do you know?
Mar 19, 2007 at 10:04 am #1382816Frost can form when the measured temperature is greater than 32F. If there is a clear sky, radiant heat loss can drop the temperature enough on surfaces facing the open sky (upward) to cause the temperature to slip below the freezing point. Camping under a tree will prevent some of this heat loss.
Think of the reverse situation when you are around a camp fire on a cold night, and someone steps between you and the fire (infared heat source). The change is instantaneous, and is not mainly from conductive or convection heat transfer.
Mar 20, 2007 at 12:57 pm #1382926Interesting thread, guys. I had no idea how many factors could affect the freezing of water. Personally, when I wake up on a cold morning with frost on my bag or bivy, I am 100% sure that it is, indeed, frosty cold.
Mar 20, 2007 at 1:24 pm #1382927Hi pj
> Didn't know about the Arabs and ice – thanks for sharing that info. Wonder at what temp range they make this scheme work? Do you know?
I don't know the exact temps, but it would have to be near freezing of course. As Thomas said, a clear sky at night can get awful cold!Mar 21, 2007 at 11:05 am #1383058Pressure changes can affect the freezing point. Place a glass of water in front of you in a sealed room with an ability to regulate temperate at, say 25C (77F), and start increasing the pressure. At some point the 25C water will freeze. Of course, you will not have survived long enough to see it.
Under the normal atmospheric pressures at which we recreate, a decrease in pressure results in a decrease in temperature. This explains, in part, the ice-in-the-desert phenomenon. It also explains why aircraft icing forms where it does–wings, tail assemblies, propellors, engine inlets, pitot tubes, and carburetors. Carburetor icing can occur under humid conditions at ambient temperatures as high as 25C. Of course, at the point of ice accumulation, the microclimate temperature is no higher than 0C (32F, 273.15K).
Wind blowing across a tarp can serve to create a microclimate with freezing temperatures when the ambient temperature is above freezing, but the presence of ice is unquestionable proof that the temperature at the point of ice formation is no greater than 0C.
Bill
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