On 28-Jan-2005, (m Ransley) wrote:

it is that you warm air the air expands
but the moisture is constant thereby lowering humidity.

Not exactly. The air in winter is cold, hence less able to
hold humidity. The relative humidity can be high (i.e. the
amount of water that can be absorbed by cold air is near max),
but when you heat it, the relative humidity is then low - since
warm air could absorb a lot more. The absolute humidity is the
same. A humidifier can then be used to add more moisture to
the warmed air. It isn't because the air expands, it's because
of the capacity of the air to hold moisture.

Mike