# 1:3.5.1 Mean Monthly Relative Humidity

Relative humidity is defined as the ratio of the actual vapor pressure to the saturation vapor pressure at a given temperature:

$$R\_{hmon}=\frac{e\_{mon}}{e^o\_{mon}}$$                                                                                                                                                          1:3.5.1

where $$R\_{hmon}$$ is the average relative humidity for the month, $$e\_{mon}$$ is the actual vapor pressure at the mean monthly temperature ($$kPa$$), and $$e^o\_{mon}$$ is the saturation vapor pressure at the mean monthly temperature  ($$kPa$$). The saturation vapor pressure, $$e^o\_{mon}$$ , is related to the mean monthly air temperature with the equation:

$$e^o\_{mon}=exp\[\frac{16.78\*\mu tmp\_{mon}-116.9}{\mu tmp\_{mon}+237.3}]$$                                                                                                                        1:3.5.2

where $$e^o\_{mon}$$ is the saturation vapor pressure at the mean monthly temperature ($$kPa$$), and $$\mu tmp\_{mon}$$ is the mean air temperature for the month ($$\degree C$$). The mean air temperature for the month is calculated by averaging the mean maximum monthly temperature, $$\mu mx\_{mon}$$, and the mean minimum monthly temperature, $$\mu mn\_{mon}$$.

The dew point temperature is the temperature at which the actual vapor pressure present in the atmosphere is equal to the saturation vapor pressure. Therefore, by substituting the dew point temperature in place of the average monthly temperature in equation 1:3.5.2, the actual vapor pressure may be calculated:

$$e\_{mon}=exp\[\frac{16.78\*\mu dew\_{mon}-116.9}{\mu dew\_{mon}+273.3}]$$                                                                                                                        1:3.5.3

where $$e\_{mon}$$ is the actual vapor pressure at the mean month temperature ($$kPa$$), and $$\mu dew\_{mon}$$ is the average dew point temperature for the month ($$\degree C$$).                     


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