I know it sounds like there’s a trick to this question, but I was just wondering: At what temp does water freeze at a high altitude? Does lower air pressure change the freezing point at all? - Jinx
Changing the pressure does change the freezing point, but it is not as volatile as the boiling point of water is. This site:
http://www.lsbu.ac.uk/water/phase.html
has a phase diagram (look for the big phase diagram) that includes the change in freezing point. Denver (assuming it really is the “mile high city”) has a pressure of roughly 63,000 Pa. The change in the freezing point occurs at roughly 1000 Pa, or (if I am doing the math correctly) 121,800 ft above sea level; about 22 miles up.
I’m really only familiar with this going the other direction; that is, when calculating the change in boiling temperature due to changes in pressure. I found this cite helpful as well:
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html
A real chemist of physicist should be along shortly to correct my misunderstandings.
Sorry, I did that wrong. One of my sites gives changes in mmHg, the other gives it in Pa’s. The correct conversion is 1 mmHg = 133 Pa, not 1 = 100 like I thought. This gives the point at which the freezing point of water changes (according to the phase diagram) at closer to 7.5 mmHg, or about 130,000 ft, or call it close to 25 miles up. Note that at that point, water doesn’t have a “liquid” state; your choices are frozen or vapor.
Again, check my math before moving to Denver.
According to my Hausman and Slack Physics book, at temperatures found in the atmosphere\surface interface a pressure increase of 100 atmospheres lowers the freezing point less than 1 Celcius degree.
This is one of the reasons the melting point of water (0 °C) is no longer used as a defining/fixed point in the temperature scale. It was replaced by the triple point of water (0.01 °C) in 1948.
Um, so (at) what temperature **does ** water freeze in Denver?
Moving directly from solid to gaseous state is sublimation, and occurs even at normal lowland air pressure with water in dry, cool weather – the morning sun on a thin icy coating, with a very low humidity, will cause it not to melt but to turn directly to water vapor. It’s why liquid carbon dioxide occurs on earth only in high-pressure laboratories – the “liquefaction pressure” is something like 20 atmospheres.
I’d have to have one of our resident planetologists discuss this in more detail, but as I recall, the only known place off Earth where liquid water can exist under “natural” conditions (though owing to lack of atmospheric water vapor it won’t) is the Hellas basin on Mars, where the air pressure is – barely – high enough and the temperature at local summer noon is sufficient, for liquid water to be present. (But the effectively 0% humidity of the Martian atmosphere would mean it would quickly vaporize.)
Looks like we’ll never know. 
I’m too lazy to work out the math, plus I’m not too good at math to begin with.
Wow, I actually left that part out. 0.01C. Under all of the normally-occuring pressures on the surface of the planet, the freezing point of water is roughly 0.01C. At REALLY extreme ranges (e.g. 25 miles above sea level) it changes.
So, for those who aren’t into metrics and still use U.S. Standards and as f = 9/5 x C + 32, then it freezes at about 32.018 degrees F, I think.
It may be possible there; I can’t speak to that. But it’s not the only place. There’s almost certainly liquid water beneath the ice on Jupiter’s moon Europa, and I wouldn’t be surprised to find it in/on any of the other gas giant moons, either. It might even be possible to have liquid water deep inside the gas giants themselves.
What bashere means is that water freezes at 0.01C ~= 32.018F at sea level, down in Death Valley, and up at Denver, and on Mount Everest, and even up to 120,000+ feet above sea level.
Only above there, as the air pressure starts to fall of towards that of a decent vauum, do you begin to see even minimal changes in the freezing temperature of water.
And maybe down in the oceanic abyss where pressures can reach 1000 atmospheres.
Only the boiling point of water is affected by a degree or two. But the freezing point remains the same 0 degrees Celcius.
A lot more than a degree or two. At the top of Everest, the boiling point is 31 K lower than at sea level.
True – and I knew that as I was writing. Somewhere in my statement there was a phrase about “on the planetary surface” that got deleted as I revised my draft post. :o
There’s no escape. If you don’t preview there are mistakes. If you do preview you leave something out. God is frowning at us all - with both eyebrows.
People are saying the freezing point doesn’t change no matter what the altitude or atmospheric pressure is and although that may be scientifically and mathematically true, I can assure you that the deviation of freezing and boiling in Denver is about 3 to 4° below normal. Standing water freezes at about 28-29°F (instead of ~32°) yet it can snow at 36°. And although it’s said water boils at about 203-204°F (8 to 9° lower, instead of at a normal ~212°) I have only measured the same difference in freezing, boiling at 209° even with a ton of salt added. The locals have always known the phase points to be sightly lower than average and not by belief alone, but tried and tested experimentation.
It’s probably a combination of many factors; mountain to valley air pressure, general high altitude, dry dessert air, unusual geographic location (the Denver “bowl”), and undoubtedly many others, but there is quite a variation depending upon where you live. I can also confirm that in upstate NY, when humidity is high and pressure drops quickly, standing water can easily freeze at 36°.
Geography & the local environment, wind currents & air pressure, moisture patterns & weather anomalies, density of contaminates & human caused air-pollution, countless factors known & others otherwise insignificant (like the density of local vegitation & native flora or the amount of urban sprawl & paved development) all skew measurements. Individually they’re to slight to calculate, but combined they do have a measurable effect; exactly what all the variables are, how exactly they change things, to what degree, when and why, we may never know.
If there’s any detectable difference from 0 C / 32 F, it’s due entirely to either impurities, or inaccuracies in your temperature readings, either of which are pretty much equally likely in any city, not specific to Denver.