# Slope of Saturation vapor pressure versus temperature-A parameter in Penman's equation

How is mm of Hg per degree Celsius converted to kPa per degree Celsius? I also need the equation for computing the value of A (slope) in mm of Hg per degree Celsius.

• Is this a homework question?
– Neo
Sep 22 '14 at 17:51
• This is not a homework question. Sep 23 '14 at 5:01

You can convert mm Hg to kPa with the conversion factor $0.133322368\ \mathrm{kPa\ mmHg^{-1}}$.

Saturation vapor pressure as a function of temperature is

$$e_s(T) = e_{s0}\exp\left[\left(\dfrac{L_v(T)}{R_v}\right)\left(\dfrac{1}{T_0} -\dfrac{1}{T} \right)\right],$$

where $L_v(T)$ is the specific enthalpy of vaporization, $R_v$ is the specific gas constant for water vapor, $T_0$ is reference temperature (273.15 K) and $e_{s0}$ is the saturation vapor pressure at $T_0$ (611 Pa).

You can find the slope of this curve by taking the derivative $\dfrac{d e_s}{dT}$.

Note that the units of $e_s$ and $T$ are Pa and K respectively in the equation above.

• he could also do it through wolfram alpha if its only a few conversions =)
– Neo
Sep 22 '14 at 20:39
• Well, Thank you for replying. I know the relation between kPa and mmHg. But I need to express the slope in mmHg/degree celsius instead of kPa/degree celsius. I need an equation for finding that. Sep 23 '14 at 5:05

finally i could find out that both units are not compatible to convert. Little bit speculated about the fact.
The equation for finding the slope in mmHg/°C = 4098*(4.584*exp(17.27*T/237.3+T))/(T+237.3)^2.