Absolute Humidity

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Absolute humidity is the density of water vapor in the air (kg/m3)

To calculate absolute humidity, we first use the dewpoint temperature and the formula following formula; E=6.11*10.0**(7.5*Tdc/(237.7+Tdc)) to calculate vapor pressure in millibars.

Then we convert the vapor pressure in millibars to Pa by multiplying by 100.

Once this is done, we get the vapor pressure in Pa, we can use the gas law to calculate water vapor density (i.e. absolute humidity) by substituting Rw in place of R and by using the vapor pressure in the gas law formula, rather than the total atmospheric pressure that we would use to calculate air density.

Gas law;



P= pressure in Pascals (Pa)


T=temperature in degrees Kelvin

R=gas constant for air=287 (J/kg*Kelvin)

Rw=gas constant for water vapor= 461.5 (J/kg*Kelvin)


Rearranging, the density m/V = P*M/(T*R). The absolute humidity is just the density of water vapor,
where M = 18.02 g/mol, and R = 8.314472 Pa·m³/(mol·K), and T is temperature in kelvins

The absolute humidity is just the density of water vapor,
ea*M/ (T*R)

Density M/V = M/ (T*R). Where M = 0.0015 g/mol, and R = 8.314472 Pa·m³/ (mol·K), and T is temperature in Kelvin =35 +273O c =308k

= 0.0015gmol-1/ (308 K X 8.314472 Pa·m³/ (mol·K)

= 0.0015g/2560.8573 m-3

= 5.8574 x 10 -7 gm-3