If air is heated, how does relative humidity change?

When air is heated, its capacity to hold moisture increases. Relative humidity is defined as the amount of water vapor present in the air compared to the maximum amount of moisture the air can hold at that temperature, usually expressed as a percentage.

As temperature rises, the maximum amount of water vapor the air can contain also increases, but if no additional moisture is added to the air while it is heated, the actual amount of water vapor present remains constant. Since relative humidity is a ratio of the current moisture content to the maximum moisture capacity, the relative humidity will decrease as the temperature rises—because the air can now hold more moisture than it did at the lower temperature. Thus, it becomes a smaller percentage of the total capacity, leading to lower relative humidity values.