Calculating Vapor Pressure Deficit (VPD) using Temp in in Celsius and RH

[gcohlson]

Hi there,

I'm wondering if anyone has a formula for calculating vapor pressure deficit from temp in Celsius and relative humidity.

It seems like other threads are using temp in rankine.

Thank you for your help!

[MrShorty]

I almost always do my vapor pressure calculations using temperature in Kelvin. It just depends on what my chosen vapor pressure equation requires.

From a Wikipedia article (https://en.wikipedia.org/wiki/Vapour-pressure_deficit ), here's how I understand the process:

1) Compute vapor pressure of water at T using your favorite vapor pressure equation for water. Whose vapor pressure equation for water do you prefer?
1a) If your preferred vapor pressure equation uses T in R, you can use the CONVERT() function to convert your temperature units (assuming your version of Excel supports Rankine as a temperature unit -- my older version does not). CONVERT(T in C,"C","Rank") will perform the conversion, or use T in R = (T in C + 273.15)*9/5
1b) If you do not have a preferred vapor pressure equation, Wikipedia lists a few different equations for vapor pressure of water (in addition to the one given on the VPD page) here: https://en.wikipedia.org/wiki/Vapour_pressure_of_water DIPPR also has a reliable vapor pressure equation for water (if you have access to their database) and NIST can calculate vapor pressure of water in their REFProp programs (online version here: https://webbook.nist.gov/chemistry/fluid/ ).
2) The computation in 1 represents the partial pressure of water at a relative humidity of 1. You can compute the actual partial pressure of water as RH*result of 1.
3) According to Wikipedia, the vapor pressure deficit is simply the difference of the partial pressure of water at saturation (100% RH) and the actual partial pressure of water. VPD=result of 1-result of 2=result of 1-(RH*result of 1)=result of 1*(1-RH).

I will admit that the Wikipedia article left me somewhat confused -- especially their choice of vapor pressure equation. Compared to the equations I usually use for the vapor pressure of water, their equation results in saturated vapor pressures about 60% of other vapor pressure equations. VPD seems to be specifically used in meteorology/atmospheric sciences, so maybe they have a drastically different meaning for "saturated vapor pressure" than I am used to.

At this point, the big question is what vapor pressure equation for water do you want to use?

[CK76]

Wait, is this Excel question? Sounds more engineering/physics question...

At any rate... VPD is calculated as Saturation Vapor Pressure (SVP) - Actual Vapor Pressure (AVP)

SVP is calculated as... Where T = temperature in Celsius.
SVP = 0.6108 * EXP(17.27*T/(T+237.3))

AVP is calculated as...
AVP = RH/100 * SVP

Then VDP is...
VPD = SVP - AVP

This can be simplified to...
= 0.6108 * EXP(17.27*T/(T+237.3))*(1-RH/100)

ex: Temp 24C, RH 68%
= 0.6108 * EXP(17.27*24/(24+237.3)*(1-68/100)
= 0.954769709... kPa

Edit:
You can test it using https://www.dimluxlighting.com/knowl...pd-calculator/

Edit2: Typo on one of the constant. 237.3 not 238.3 (fixed), and below may be more accurate calculator (gives more decimal place).
https://cals.arizona.edu/vpdcalc/

[gcohlson]

This was super helpful! Thank you both for your help. I was able to get the formula in excel and it's working great.