I was driving by Drayton Harbor yesterday, which is at the 49th Parallel. Drayton Harbor has a wide tidal flat, and the tide was out. The exposed mud was covered in ice a couple of inches thick, as were the rocks on the shore. I assume that the higher ice is made when spray freezes. Of course, I’ve seen that crabbing show on TV where the ships are coated in salt ice.
The high temperatures here have gotten up to 30ºF or so, and the lows have been in the low-20s. The tidal flats have a creek running into them, so the water is brackish (less salty than ocean water). What is the maximum temperature at which salt ice can form in such an environment?
Well, as the salt concentration in water goes up, the freezing point goes down. Minimally salty water would freeze just under 32 degrees F, while maximally salty water would freeze at -6 F.
Now that’s for water saturated with NaCl. Seawater has a lot of other salts in it too, so that would probably affect results. Cursory research shows me seawater salts are only 85% NaCL, with the rest being mostly sulfate, magnesium, calcium and potassium.
So depending on how brackish the water is, I’d declare it’d freeze anywhere from just under 32 to -6 degrees, possibly lower, but not higher.
The SO mentioned that it’s breezy there, so perhaps the wind is carrying away some heat, resulting in a cooler surface temperature. (I can’t remember the term. I’ll have to dig out my meteorology book sometime.)