I have been watching and enjoying the BBC’s Frozen Planet series on Wednesdays. This Wednesday they showed this amazing footage of a brinicle forming

http://www.youtube.com/watch?v=LMhBuSBemRk

worth the licence fee alone. The amazing thing is that this is salty water being frozen by even saltier water.

I guess it is obvious that if you took say a fish finger from a freezer at -10C and put in water at say 0C, that the cold fish finger could cause the water in contact with it to freeze. The fish finger at -10C is below the freezing point of water at 0C and so it is cold enough to cause it to freeze.

Brinicle‘s form by basically the same process except that it is salty ocean water, with around 3% salt being frozen by colder saltier water. The point is that the saltier water is, the lower is the temperature at which it freezes. This is why salt is used in winter to de-ice roads. At 3% salt it freezes at around -2C but if it is has 10% salt in it it freezes at-6C.

Thus if you have a jet of water with say 10% salt in it, then the jet can be as cold as -6C and still not freeze but if this jet of salty water is flowing through ocean water than the surrounding less salty water will freeze – creating a sheath of ice around the jet of very salty water – this is a brinicle, a hollow pipe of ice that forms around the jet of very cold very salty water.

Cool, and caught on camera for the first time by the BBC cameramen Hugh Miller and Doug Anderson. They are just really talented heros, I wince at the idea of swimming in water under + 20C, they were in water at – 2C. That’s cold. Even with fancy warm wetsuits, that’s got to be cold.

Brinicles only form in still water – I guess that flows mix the very cold very salty water with the warmer less salty water and this mixing would prevent ice formation. To get ice you need heat to move from the less salty warmer water to the colder saltier water faster than salt moves in the opposite direction. This heat flow then cools the less salty ocean water faster than the salt reduces the freezing temperature, and ice forms.