To compute the VPD, we need the ambient (greenhouse) air temperature, the
relative humidity and, if possible, the canopy air temperature. We must then compute the saturation pressure. Saturation pressure can be looked up in a
psychrometric chart or derived from the
Arrhenius equation; a way to compute it directly from temperature is : vp_\text{sat} = e^{A/T + B + CT + DT^2 + ET^3 + F\ln T}, where :vp_\text{sat} is the saturation vapor pressure in PSI, :A = -1.0440397 \times 10^4, :B = -11.29465, :C = -2.7022355 \times 10^{-2}, :D = 1.289036 \times 10^{-5}, :E = -2.4780681 \times 10^{-9}, :F = 6.5459673, :T is temperature of the air in the
Rankine scale. To convert between Rankine and degrees Fahrenheit: T[\text{R}] = T[^\circ\text{F}] + 459.67 We compute this pressure for both the ambient and canopy temperatures. We then can compute the
partial pressure of the water vapour in the air by multiplying by the relative humidity [%]: :vp_\text{air} = vp_\text{sat} \times (\text{relative humidity})/100, and finally VPD using vp_\text{sat} - vp_\text{air} or vp_\text{canopy sat} - vp_\text{air} when the canopy temperature is known, or simply :VPD = vp_\text{sat} \times (1-\text{relative humidity}/100). It can easily be seen from this formula that if T rises (which raises vp_\text{sat}), but relative humidity remains constant, VPD will increase. == Climate ==