Relative Humidity refers to the amount of moisture in the air relative to how much water the air can hold at a given temperature. It is expressed as a percentage and is calculated by subtracting the actual atmospheric pressure from the saturation vapor pressure, then dividing that difference by the saturation vapor pressure. In other words, it measures the amount of moisture in the air compared to how much water the air can hold at a given temperature. When relative humidity is high, there's more moisture in the air and when it's low, there's less moisture in the air.