Humidity and Houseplants Humidity and Houseplants Grant Wood Humidity is a measure of the amount of water that air will hold. The water is usually in the form of invisible droplets. At 100 percent humidity the air cannot hold any more water.
Fog occurs when the air is saturated and the invisible water now becomes visible.
Remember that the furnace is taking dry outside air and warming it. If no water is added to this outside air, then it will still be dry. You can increase the humidity inside to a certain extent by adding water to the air. Warm, moist air is always being lost from the house, and cold dry air is always being brought in, so high humidity in the entire house is not possible. When warm, moist air comes in contact with a cold, dry surface, the water in the air condenses.