Dry air and water vapor combine to form atmospheric air. The pressure of the dry air and water vapor pressure together make up the total pressure of the air mixture. Specific humidity is the ratio of mass of water vapor to total mass of air, while relative humidity is the ratio of actual water vapor pressure to saturation water vapor pressure. The enthalpy of atmospheric air depends on the enthalpies of dry air and water vapor as well as their relative quantities based on specific and relative humidity.