Relative Humidity (RH)

Relative humidity (RH) is a measure of the amount of water air can hold at a certain temperature. Air temperature (dry-bulb) is important because warmer air can hold more moisture than cold air. As a rule of thumb, the maximum amount of water that the air can hold doubles for every 20°F increase in temperature.

Lines of constant relative humidity are represented by the curved lines running from the bottom left and sweeping up through to the top right of the chart. The line for 100 percent relative humidity, or saturation, is the upper, left boundary of the chart.

The 100 % RH (saturation) line corresponds to the wet bulb and the dew-point temperature scale line and the line for zero percent RH falls along the dry-bulb temperature scale line.

Note that air holds more water vapor at increasing temperatures. Air at 60 percent relative humidity contains 60 percent of the water it could possibly hold (at that temperature). It could pick up 40 percent more water to reach saturation. Because RH varies significantly with dry bulb temperature, it is important to state dry bulb temperature and relative humidity together, such as 70°F and 50% RH.

RH should not be confused with absolute humidity which is defined as the actual amount of moisture in the air and is measured in pounds of moisture per pound of dry air (lb/lb of d.a). Relative humidity (RH) is a measure of how much moisture is present compared to how much moisture the air could hold at that temperature. RH is expressed as a percent.