Relative humidity expresses how close the air is to saturation. It is the ratio of the actual water-vapor content to the maximum the air could hold at that temperature, expressed as a percentage.

When relative humidity reaches 100 percent, the air is saturated and condensation can occur as dew, fog, or cloud droplets. Relative humidity changes with temperature even if the actual moisture content stays constant: cooling raises relative humidity; warming lowers it.

High relative humidity on warm days makes the air feel oppressive because sweat evaporates slowly. Low relative humidity can dry skin and increase static electricity. Weather reports often include relative humidity alongside temperature, yet dew point is sometimes a clearer indicator of moisture for forecasting purposes.

• Percentage of saturation
• Depends on both moisture and temperature
• 100 percent means saturation
• Affects comfort and condensation