Super-breath is easily one of Superman's lamest powers in the modern age. Unlike other ridiculous abilities, from super-kissing to super-ventriloquism to super-weaving, which have mostly been mercifully forgotten, super-breath is pretty much constant.
And you know, it makes a good deal of sense. Someone with Superman's strength and his increased lung capacity should be able to blow things with a great deal of force (no snickering!). And I won't dispute that. It's a lame power, but it's also really only a corollary power; it's not a primary ability like "super-strength" or "flight," it's an ability that results naturally from a combination of primary abilities. It's a side-effect power. And I don't really have a problem with that--in fact, I think comic writers ought to be more aware of side-effect powers. If I ever end up writing a Barry Allen Flash book, there would be a lot of exploration of that sort of thing--but that's a "Comics I'd Write for Free" post that I haven't written yet.
Getting back to the point, my problem is less with the "moving air with great force" aspect of super-breath and more with the "flash-freezing things by blowing at them" aspect of super-breath.
See, on the surface it makes sense. When you have a hot cup of coffee, or a hot spoonful of soup, you blow on it to cool it off. Similarly, when you're hot and lack air conditioning, you turn on a fan, which blows air at you, which cools you off. Therefore, blowing on things cools them off. And if you have someone who can blow really hard, then they ought to be able to make things really cold, right?
Yeah, not quite. There's a reason you don't hear about towns being frozen solid after a tornado plows through. See, when something (soup, coffee, skin) is hot--specifically, hotter than the air around it--it transfers heat to the surrounding air. This cools the hot object and heats the air. Of course, now the air is warmer, so the temperature differential between the hot body and the air is smaller, so there's less heat transfer.
The solution: move the warm air out and replace it with cool air, which will then absorb more heat from the body. Repeat. It's called convection, and it's the principle behind blowing on food and buying fans in the summer.
There's a limit, though: you won't be able to cool the hot object any lower than the temperature of the cool air you're blowing at it. I imagine we've all been in the situation where you've got the fan on, but it just feels like it's blowing hot air around. If the air's too hot to allow any significant heat transfer from your body, then the fan won't do much good.
So, if I'm recalling my thermodynamics correctly, Superman's ability to freeze people is somewhat limited by the temperature of the air around his target. Unless it's a really, really cold day, he probably won't be able to make the person more than a little chilly. Even if it were twenty below zero, he'd be blowing for quite awhile before they froze to the degree of solidity that Superman routinely accomplishes.
Please, feel free to correct my science if I've made a mistake.
I can think of some technobabbley ways to get around this--some sort of biological internal super-cooling mechanism (I think The Science of Superman suggests that he has a superconducting nervous system, which would likely require just such a mechanism) that cools the air close to boiling point before it leaves his lips--but it seems a bit unlikely, desperate, and rife with undesirable side-effects (what's Superman's normal body temperature? Can he control the temperature of his breath? Does he have some really hot organ processes to balance this out so he feels normal to the touch?). Which will make it kind of odd when I say that even this--Superman freezing villains in their tracks--doesn't particularly bother me. If I'm reading a comic and he freezes someone/thing by blowing at them, I rarely even bat an eye at the implausibility.
No, what I started this post to talk about, what really grates on me, is when Superman's using his freeze-breath on some hapless foe, and somehow they end up encased in a block of ice.
The exact situation varies; sometimes it's a jagged ice sculpture in the rough shape of the villain's body, other times it's a full-fledged ice cube, a solid rectangular block straight out of a (Pup Named) Scooby-Doo cartoon.
I can suspend my disbelief for villains flash-frozen solid by convection alone. I can suspend my disbelief that someone could somehow survive that sort of treatment. I can't quite see some crook frozen into the middle of an ice cube without wondering where the heck did all the ice come from? Is it normal ice, frozen water? If so, where the heck did that much water spontaneously appear? There's certainly not enough water vapor in the surrounding air to create that much ice; are Superman's lungs filled with vast amounts of fluid for just this purpose? Or is he somehow freezing the gases in the air through convection alone? Besides this seeming absurd on its face, why isn't there a solid stream of frozen gas leading from his lips to far beyond the trapped criminal? Nitrogen (which makes up 70-some percent of the air) has a freezing point just above 63K--63 degrees above absolute zero. Is Superman really able to bring atomic motion to a near-standstill by exhaling?
These are the kind of thoughts that keep me up at night.