Well, it boils down to simple physics. Water holds onto heat more efficiently than air. Hot water holds less oxygen than cold water. A pan of water, left in the sun will absorb more heat than a pot left exposed to air. It can take longer to heat up, but retains that heat longer (it also takes longer to cool down). That means a string of hot days in the summer can result in hot water, even if you exchange it every so often. And FWIW, BC roots in nature aren't sitting in hot stagnant swamp water. They are buried under a few feet of mud that keeps them a lot cooler than they will be in a standing tub of water.
http://www.fofweb.com/Electronic_Images/Onfiles/FOFSE-041.pdf
http://www.freedrinkingwater.com/wa...r-absorb-heat-better-than-most-substances.htm
Another thing to think about--hotter, or warmer, water hold less oxygen than water that is colder.
http://www.ask.com/science/cold-water-hold-oxygen-warm-water-ebea012c9d2e9ac7
That may not make a huge difference at first, but again, over time in a hot summer, it can.
I stopped submerging my BC a decade ago after being told all of this by BC growers. Ask any of the "name" BC people if they're still growing in water filled tubs, or have stopped in favor of more water retentive soil mixes. If you have to leave them all day without being able to water, submerge only the bottom couple of inches of the container...not the entire thing.
Wow! Didn't really want to get in a pissing match on thermodynamics and water or debate whether it is good to submerge a tree or not; just wanted to show off my trees. I did not say you were wrong, but just that I just disagreed with what you said. Anyhow, I still disagree. The first two references do explain that water can hold heat longer, especially if it is compared to potting soil in a styrofoam cup, but it also explains why it doesn't heat up quickly in a tub that you have your cypress in, it takes a lot of energy just to raise the temperature of water. So half a day in the sun is not gonna get hot enough to cause any damage. I know a lot about chemistry and heat transfer, I work in a lab. Some of the analysis that we perform involve heat transfer. But anyway, it does not matter, submerging in a tub of water works for me, your's and other's milage may vary.
"A pan of water, left in the sun will absorb more heat than a pot left exposed to air." LOL, really? I hypothesize that most anything left in the sun will absorb more heat than if just exposed to the air. Maybe in your case it is because a pan is shallower than most pots, I don't know.
I also mentioned above that the water gets no hotter than a pot that is sitting if full sun. In fact, it is probably cooler. A dark colered pot (which contains the roots of your tree) in full sun will be quite hot after just an hour, not so for the volume of water I have in the tub.
For what it's worth, the tree is not completely under water, the water level is only up to an inch or so below the rim. As for the oxygen-stagnant thing, I did say that I flushed the tubs with fresh water to keep it from getting skunky. Also, in my area there are places where BC area growing, quite well, in "hot" (hot is a subjective term) water. And if the water is only a couple of feet deep and hot, the mud below it is pretty warm, too. Spent many a day in my youth roaming around creeks and swamps.
I don't really know what it takes to be a "named" BC person nor do I care to be one, I just want to grow some bonsai. I guess if I was really into the you tube thing and blogging I could make a bunch of videos, write a bunch of articles, get a following; maybe then I would be "named." ------Not intended to be a jab at you, johng, YOU ROCK! I have learned a lot from your offerings.
Thanks for the lesson on the thermal properties of water and an idea for my tenth grader's science project.
John