Relative humidity is a relative measure of how much water vapor is in the air. The term “at water saturation”—for example a fog, is where the relative humidity is 100%. The term “arid”—for example, over a desert, is where the amount of water vapor in the air is very low, therefore a low relative humidity. If the amount of water vapor in the air increases, the relative humidity increases, and vice versa. ... If the water vapor content stays the same and the temperature drops, the relative humidity increases. If the water vapor content stays the same and the temperature rises, the relative humidity decreases.