Discussion: Science Question

As I drive over the Mississippi River in the winter I notice that even when the air temperature is well below freezing, the river is mostly free of ice. I get that the movement of the water is what allows this, but here's my question to which I've been unable to find an answer:

Does the movement lower the actual freezing point of the water or does that movement heat the water and thus keep it just above freezing?

I know this isn't an orienteering question, unless one is considering a route across moving water in the winter (unadvisable), but I know there are smart people on this site, so I thought I would give it a try.

A few things at work here, not just the movement of the water:

- it takes longer for water to cool than air
- river water may well freeze below 32F/0C because it's not 'pure' water. the presense of minerals can lower the freezing point (think salt on ice)
- in order for water to freeze, it needs to 'stick' to something. the faster it's moving, the harder it is to get seeded and freeze.

So, the movement of the water itself isn't changing the freezing point, it's just making it harder to freeze...add the fact that the water's probably not as cold as the air, and that the freezing point is lower, and it's going to take a bit of work from Mother Nature to freeze a rushing rapid.

The water is heaviest, when the temperature is +4 C. In lakes + 4C water is in the bottom and the coldest 0C water rise up near the surface. This is why Mother Nature is able to freeze (the surface) lakes ans seas without having to cool all the water to 0C. Running water gets mixed all the time, and it makes it harder to freeeze. The faster it's moving, the better it gets mixed.

Also, there's a LOT more energy in liquid water at 0C than frozen water at 0C. When the water is still, you only have to remove that energy from the water that actually freezes. When it's getting mixed, that energy loss gets spread throughout the liquid water as well so much more needs to be removed to form the same amount of ice (assuming, of course, that only a small portion of the water on the surface ever freezes).

To add to the above, there's more to getting water to freeze than just getting the temperature to 0 C. Although air at 1 C is happy to drop to -1 C without much struggle, water has to relinquish the "latent heat of fusion" in order to make the state change before it will make the same drop. That adds a lag to the willingness of the water to freeze. And at the bottom of the river is the ground, which is probably well above freezing. Around where I live, the ground at a depth of something like 4 feet essentially never gets below freezing, which means that you can safely bury water pipes at that depth. So as the water in the river gets churned around, the cold air at the top tries to freeze it, and the warm ground below tries to keep it liquid. The ground is more massive than the air, so it can win, provided the motion of the water gives it a chance to get a chance to warm it all. In the still water of a lake, some water is staying at the top and it gets frozen, but the air doesn't have enough oopmh to make the ice layer very thick.

I doubt that river water has a high enough molality to affect the freezing point much. If you stick a thermometer in the river, I suspect you'll find that the temperature is right around the freezing point.

I'm not a scientist, and I've long since forgotten whatever bits of science I once learned, so I'll restrict myself to reporting the following observations --

1. When I broke through the ice on today's run, the water (maybe a foot deep) underneath seemed like it might well have been rather close to 0 C.
2. If you pull your foot out fast enough you don't really get wet.
3. I was not tempted at all to test how much warmer the water might have been close to the ground.

Have just defended my master's thesis on alloy crystallization, I feel obligated to chime in. But we've got a pretty well educated bunch here and it looks like the main points have been hit!

I would note that some systems can be cooled below their equilibrium freezing temperature without freezing, but these situations are rare and river water is certainly not one of them!

PS. I am not contradicting those who mentioned freezing point depression by dissolved solids. This is legitament way to lower the freezing point, and it makes polar ice a really interesting subject. Anybody know what the solute concentration of the mississippi is?

As a hydrologist in a department with much cold region hydrology/climatology expertise I'll add a line or two...

Frazil ice can form in turbulent and supercooled rivers at about -1.5C. The ice forms from below by attaching to rocks/river bed and builds upwards as anchor ice. Also regional groundwater has a temp about the same as the mean annual temp of the region and can offset cooling/energy loss early in the freeze-up period in some streams/rivers.

When O'ing in areas with open water streams in winter wear SealSkinz socks.

I bought some SealSkinz socks for the PTrot. Luckily, I didn't need them because I couldn't fit my foot in my spikes with the Skinz on. Do you wear your Skinz with O shoes or something else?

I love this thread. It is so abstruse and I barely understand any of it.

I am usually wearing running shoes when I wear the seal skinz as it is hard to fit seal skinz and active ankles into O shoes. I put yak trax ont he running shoes for grip.

Two January's ago, I was placing controls in Sharon Woods. It was cooling off, and I kept comparing the rain and the very still lake. I was expecting the rain to change to snow and the lake to slowly freeze from the banks to the center. That's not what happened. First there were a few flakes of snow in the rain, and the lake was completely liquid. The mixed rain-snow went on for over an hour. The change from mostly rain to all snow was very abrupt (within say a minute). I saw the lake about a minute later, and it was all ice (sure it was thin, but no visible water). The whole lake went from no ice to completely iced over in probably two or three minutes. I was surprised.

You would have been even more surprised if an attack badger had snucked up on you and launched an attack while you were admiring the ice. Watch a song bird sometime--they are always twisting their head around and looking up from what they are doing every few seconds. They know.

The salt concentration is unlikely to have much effect. Ocean water (35 psu) freezes at about -2 C. The Mississippi is quite fresh in comparison (especially near the source in beautiful Minnesota), so the depression of the freezing point is probably pretty minimal.

The main effect (already mentioned) is: You have to cool the entire water column to 0 C before you start freezing anything. In comparison, in lakes you only need to cool the upper part of the water column before you can start making ice at the surface. Whether lake, river, or ocean, you need to remove the same amount of energy to make the same amount of ice (i.e., the latent heat of fusion is roughly the same for all systems).

I thought polar ice was mostly accumulated precipitation. Is it really frozen seawater?

I don't know much about polar ice, but at least some of it is frozen seawater. Here's a cool page that shows examples.
Link.
The frazil ice dendrite picture is pretty cool. For those familiar with solidfication, dendrites are generally associated with supercooling. So my earlier comment was maybe not totally accurate.

Anybody know what the solute concentration of the mississippi is?

Even at its mouth, the Mississippi RIver is, for practical purposes, not a solution. It's a suspension. ("It's too thick to navigate, but it's too thin to plow...")

If you pull your foot out fast enough you don't really get wet.

If your foot is coated with ice, is that considered "wet"?

Even if you were slow to get your foot out and it did get wet, it wouldn't freeze because it would be moving, and water has a hard time freezing when it's moving. Isn't that where this thread started? And what all these smart folks are saying? :-)

My pa always told me, "if you're not part of the solution, you're part of the precitipate." :-)

Your pa was a nerd.

Hmmm, I wondered when someone was going to mention the Nerd word in association with this thread.

The same could be said for all threads though.

Thanks for the information. Being a high school social studies teacher, some of it was well over my head, but I think I get the main ideas.
As a teacher I am also quite comfortable with the word "nerd." It's not the worst thing a student has called me.

blegg--Thanks for the link. I will pass your pa's wisdom onto our science teacher. It will match his sense of humor.

This all sounds far too complex to be explained by hydrology. The lack of ice on the Mississippi River would seem to prove the existense of a higher power. I don't understand what the dogmatic hydrologists are afraid of....but then again, I'm from Kansas.

When they keep boats in the water over the winter they put these water swirlers around them.
http://www.iboats.com/mall/index.cgi?func=special&...
I guess it proves the heavier and warmer water from the bottom theory...
Fooling around along the Sawmill River (NY) when I was a kid I broke through some ice and went in over my boot tops. Panicked that my feet would freeze solid I started running for home (about a mile away). Halfway home I realized my boot now contained quite warm water............

your dad (sorry, pa) is a charlatan... i'll precipitate on him

I started running for home (about a mile away). Halfway home I realized my boot now contained quite warm water............

By the time you had run half a mile in a panic, your foot had warmed up the water.

TimGood wrote: I thought polar ice was mostly accumulated precipitation. Is it really frozen seawater?

Polar ice is frozen sea water. A few things ...
* Arctic Ocean surface water is pretty fresh by oceanic standards (30 parts per thousand compared with normal ocean water of 35 parts per thousand) but is certainly still quite salty compared with river/lake water (<1 parts per thousand).
* When sea water freezes, it rejects quite a bit of the salt (this is called brine rejection), so the salinity of the ice is something like 6 parts per thousand.
* Sea ice is quite a bit more flexible than lake ice, i.e., if you walk on thin sea ice (not recommended), it will flex underneath you quite a bit before it breaks.

Tom - When wearing your skinz with O shoes, try taking the inner sole out to give more room.

I use neopreme booties from Performance or Nashbar (have pair of both) that work fine. I also frequently just use plastic bags that our local paper gets delivered in. That's much thinner and works just as well and perhaps better as there are no seems like the booties. The main point is to create a vapor barrier layer (VBL) to keep the frazzle out (?) or the badgers away from the piggies.