What happens to the relative humidity of outside air heated by a steam heating coil in an air handling unit?

When outside air is heated by a steam heating coil in an air handling unit, the relative humidity decreases. Relative humidity is a measure of the current amount of water vapor in the air compared to the maximum amount of water vapor the air can hold at a given temperature. As the air is heated, its capacity to hold moisture increases since warmer air can contain more water vapor than cooler air.

For example, if you have outside air at a certain temperature and humidity level and the air is passed over a steam coil, it will gain heat. While the actual amount of moisture in the air remains the same (in terms of absolute humidity), the increased temperature results in a lower relative humidity because the air can now hold more moisture. This change effectively reduces the percentage of water vapor in the air compared to its saturation point at that new temperature. Thus, heating the air leads to a decrease in relative humidity.