Give Your Plants The Humidity They Need – Cold air requires only a small amount of water vapor before it becomes saturated, and so on an average winter day the air is moist. When you turn on a radiator to warm up this cold air, its capacity to hold water vapor is greatly increased. As the room becomes comfortable the amount of water vapor in the air is no longer enough to keep it moist. The air becomes ‘dry’; in technical terms the relative humidity has fallen.
Central heating in the depths of winter can produce air with the relative humidity of the Sahara desert.