CarnivourKid:
I read that "how it works" thing, and at first was a bit incredulous about the "adiabatic" humidification that ultrasonic is supposed to do (adiabatic means no energy is transferred as heat). Then later in the article it makes it clear that by "adiabatic" it's referring to the action on the incoming air, not the water. I'm 99% sure there has to be a certain amount of energy lost as heat in the process of nebulizing (or however the process of putting the droplets into the air is called), because there's no such thing as a perfect cycle or work without heat (2nd law of thermodynamics). In this case, what that means is that it's not possible to do the work on the droplets needed to push them into the air, and then have them recombine with the bulk of the water, with no net energy expenditure (ie without the water absorbing some heat in the process).
In other words, some heating of the water HAS TO take place, just because work is being done on it. That means that even the small
misters with just a piezo element and transformer (it seems there are two basic devices offered, the smaller of which is probably more than enough for any terrarium. The cheapest i could find that one for is
here, if the shipping is not too much, and it comes with a replacement ceramic element) will heat the water somewhat.
I wish i could explain it more quantitatively, but my thermodynamics is much more rusty than it should be considering i have qualifying exams in a month.
PS OT: Are you a scoutmaster, BCK? Everytime i see your sig, that's what comes to mind - a troop of 12-year-olds running rampant across the countryside with their leader just realizing they've gone. LOL