Temperature inversions

[Rick Murchison]

Hey, I said "until it freezes", so the density can decrease all it likes after it freezes...

Ok... let me rephrase my question...
If "fresh" water has a 4 degree C split between max density and freezing, and "salt" water doesn't, at what salinity does water continue to increase density right down to the point where it becomes ice? Taking your statement at face value you must accept that the reduction in density when sea water freezes (and the concurrent increase in volume) would happen with explosive suddenness. Well, it doesn't, and salt water ice does float in water of the same salinity. Want to deconflict all that for us?
I like graphs...
Thanks,
Rick

 

[ianr33]

When sea water freezes the ice formed is pure water,not salt water. That may affect the current discussion.

I remember a great PBS? show about Antarctica,icebergs would calve off the ice cap,this ice is much colder than the surrounding sea,causing freshwater ice to form on the underside of the bergs. That results in a greater salinity underneath the icebergs and guess what? A complete ecosysyem has developed that only lives in the super saline seawater beneath icebergs. Amazing !

 

[RIOceanographer]

A curiosity question: At what salinity does the density of water fail to reach a minimum with temperature?

Oops, I am glad you pointed that out because I got a bit careless with what I said above. It wasn't completely accurate.

I said "saltwater" doesn't reach a maximum density before freezing, when really I should have said something like "seawater". If the salinity is low enough you do get a temperature of maximum density before freezing.

As you add salt the temp of maximum density AND the freezing point both decrease but eventually the temp of maximum density intersects the freezing point. Seawater is normally high enough in salinity that this happens.

I am not positive of the exact salinity where the two intersect but I believe it is in the ballpark of 20 psu. Oceanic salinity averages about 35 psu so it is normally well above that except near to major sources of freshwater.

I can probably look up the salinity where this happens in one of my chemical oceanography books when I get back to my office if anyone really wants to know the exact number.

 

[RIOceanographer]

Taking your statement at face value you must accept that the reduction in density when sea water freezes (and the concurrent increase in volume) would happen with explosive suddenness. Well, it doesn't, and salt water ice does float in water of the same salinity. Want to deconflict all that for us?
I like graphs...
Thanks,
Rick

Water is a weird molecule. With some molecules, the solid phase is more dense than the liquid phase. This is not true with water because as it forms ice the molecules arrange themselves into a crystalline structure that forces them further apart. So yes the density does decrease very suddenly as water freezes. I googled up an image that shows this hexagonal structure:

http://www.its.caltech.edu/~atomic/snowcrystals/ice/ice.htm

A simple way to see this expansion is if you fill a glass bottle with water cap it and then freeze the water. When the water freezes it expands, and this will usually cause the glass bottle to crack. I am sure most of us have accidently frozen some water in a glass container and cracked it at some point.

Also as ianr33 correctly said, when the ice crystalizes it excludes the salts. The neat thing about this is that as sea ice forms it increases the salinity of the water right below the ice because that salt has to go somewhere.

 

[Green_Manelishi]

I've heard of lakes where the thermocline reverses in winter. Do we have that happen around here? Just wondering if I might encounter the phenomenon in an ice dive or right after ice out in the spring.

From what I've seen watching buoy temperatures and depths the ocean is well enough churned to prevent this from happening.

Pete

Pete,

The temp-inversion of most importance is the air will probably be colder than the water. It's passing through that barrier in either direction, especially diving under ice, that causes most of the problems.

G_M

 

[RIOceanographer]

A curiosity question: At what salinity does the density of water fail to reach a minimum with temperature?

Okay.... I looked it up in my old Chemical Oceanography book. The salinity where the temp of maximum density and freezing point intersect is 24.628 psu (freezing point=-1.338 degrees C).