in terms of physics farber, the steam from the gaseous phase of boiling water is invisible .... but is still evaporation [vaporization], which can happen at any temperature .... so mixing your metaphors doesn't increase your accuracy
if folks really watch their systems it'd be obvious the coolant loss is as brokenhead mentions, a bit of expansion loss at hot shutdown because the rad was overfilled ... there will be no 'evaporation' loss from the properly functioning system, unless you're taking the cap off when hot - vapor rises, doesn't descend
Bill
