  relative humidity (abbreviated rh) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at the same temperature. relative humidity depends on temperature and the pressure of the system of interest. == definition == the relative humidity \left(\phi\right) of an air-water mixture is defined as the ratio of the partial pressure of water vapor (h2o) \left({e_w}\right) in the mixture to the equilibrium vapor pressure of water \left({{e^*}_w}\right) at a given temperature. relative humidity is normally expressed as a percentage and is calculated by using the following equation:perry, r.h. and green, d.w, perry's chemical engineers' handbook (7th edition), mcgraw-hill, isbn 0-07-049841-5 , eqn 12-7 :: \phi = {{e_w} \over {{e^*}_w}} == significance == ===climate control=== climate control refers to the control of temperature and relative humidity for human comfort, health, and safety; for the technical requirements of machines and processes; and in buildings, vehicles, and other enclosed spaces. ===comfort=== humans are sensitive to humidity because the human body uses evaporative cooling, enabled by perspiration, as the primary mechanism to rid itself of waste heat. perspiration evaporates from the skin more slowly under humid conditions than under arid conditions. because humans perceive a low rate of heat transfer from the body to be equivalent to a higher air temperature,. the body experiences greater distress of waste heat burden at high humidity than at lower humidity, given equal temperatures. for example, if the air temperature is 24 Â°c (75 Â°f) and the relative humidity is zero percent, then the air temperature feels like 21 Â°c (69 Â°f).. if the relative humidity is 100 percent at the same air temperature, then it feels like 27 Â°c (80 Â°f). in other words, if the air is 24 Â°c (75 Â°f) and contains saturated water vapor, then the human body cools itself at the same rate as it would if it were 27 Â°c (80 Â°f) and dry. the heat index and the humidex are indices that reflect the combined effect of temperature and humidity on the cooling effect of the atmosphere on the human body. humans can be comfortable within a wide range of humidities depending on the temperature â from thirty to seventy percent -  
Atmospheric_thermodynamics Physical_quantities Psychrometrics