$\textit{Relative Humidity}$ is the ratio of the partial pressure of water vapor in an air-water mixture to the saturated vapor pressure of water at a prescribed temperature.
The relative humidity of air depends not only on temperature but also on the pressure of the system of interest.
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$\textit{Relative Humidity}$ is also referred to as $\textit{Relative Partial Pressure}$.
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Relative partial pressure is often referred to as $RH$ and expressed in percent.
Applicable units are those of quantitykind:RelativeHumidity
http://en.wikipedia.org/wiki/Relative_humidity
$\varphi = p / p_{sat}$, where $p$ is partial pressure of vapour, $p_{sat}$ is thermodynamic temperature and $V$ is its partial pressure at saturation (at the same temperature). Relative partial pressure is often referred to as $RH$ and expressed in percent. $\textit{Relative Humidity}$ is also referred to as $\textit{Relative Partial Pressure}$.
http://www.iso.org/iso/catalogue_detail?csnumber=31890