Of course the level gradually drops (despite what some claim) - the cooling system is considered closed, but only to a point. The pressure cap releases at 1 bar, or slightly above, into the overflow bottle or the system would blow out a hose or waterpump seal. So, there will inevitably be some water evaporation, since coolant, once out of the sealed system, is being shuttled in and out of the overflow bottle at normal atmospheric pressure, but if the system is operating correctly, the loss will be negligible to the point of only needing top-up during temperature extremes, and at that, maybe after several months in those conditions. I changed out the factory coolant in my Sonata last September 3. It's now down in the overflow bottle when cold about 8 oz. from where it was when I refilled nearly a year ago in spite of a rather warm So-Cal summer this year. Where you are, you could reasonably expect a bit more loss.
Addendum: it just occurred to me that while my car's overflow bottle is not pressurized, many domestic, European, and some Japanese vehicles use a seperate pressurized expansion tank in liew of a removable pressure cap on a radiator with an integrated expansion tank. Still, the cap used will allow venting to the atmosphere when the internal pressure exceeds the nominal pressure rating of the cap. Any water vapor present in the pressurized expansion tank will be subject to partial expulsion when venting occurs. So, though these vehicles have a somewhat different system, there will nevertheless be some gradual water content loss.
[ August 31, 2005, 01:14 AM: Message edited by: Ray H ]