Vapour Pressure Deficit (VPD: the measure of the drying power of the air) is very useful in plant physiology and ecology.
To calculate VPD for a given temperature (degC) and relative humidity (RH%)
(1) Ascertain the saturated vapour pressure (SVP) for a given temperature (see list below)
Temperature (degC) - SVP (Pa)
0 - 611
1 - 657
2 - 706
3 - 758
4 - 813
5 - 872
6 - 935
7 - 1002
8 - 1073
9 - 1148
10 - 1228
11 - 1312
12 - 1402
13 - 1497
14 - 1598
15 - 1705
16 - 1818
17 - 1937
18 - 2064
19 - 2197
20 - 2338
21 - 2486
22 - 2643
23 - 2809
24 - 2983
25 - 3167
26 - 3361
27 - 3565
28 - 3779
29 - 4005
30 - 4242
31 - 4492
32 - 4754
33 - 5029
34 - 5318
35 - 5621
36 - 5940
37 - 6273
38 - 6623
39 - 6990
40 - 7374
41 - 7776
Source: http://physics.holsoft.nl/physics/ocmain.htm
See: Murray FW (1967) On the computation of saturation vapor pressure. J. Appl. Meteorol. 6: 203-204.
Monteith JL, Unsworth MH (1990) Principles of environmental physics. Arnold.
SVP (Pascals) = 610.7*107.5T/(237.3+T)
(2) As VPD is the saturated vapour pressure minus the actual vapour pressure (SVP - VPactual), and VPactual = (RH*SVP)/100
we may apply the formula
VPD = ((100 - RH)/100)*SVP
(alternatively
VPD = (1 - (RH/100))*SVP
)
where RH is relative humidity and SVP is saturated vapour pressure
For example take 80% RH at 25 C. From the list above, SVP = 3167 Pa
100-RH = 20
20/100 = 0.2
0.2 * 3167 = 633.4 pascals (Pa)
Note: for convenience VPD may be given in kilopascals (kPa). For this example 0.6334 kPa.
An easy way to visualize VPD is in the form of a thermohygrogram. The example on the right is taken from: Cronk, Q. (2000) The Endemic Flora of St Helena. Anthony Nelson Ltd, Oswestry; page 12.
This shows the diurnal march of temperature (hours 0-24) at two locations (CP and SB) on the same day. RH is plotted against temperature, overlain with lines of equal VPactual and VPD, as shown. In this example the units of the latter are in millibars rather than pascals (1 mB = 100 Pa). This gives a more or less complete picture of atmospheric humidity parameters. An excel spreadsheet for drawing diagrams of this kind is available from QC.
Another example of this thermohygrogram in use is Figure 1 of this article:
Canty, R., Ruzzier, E., Cronk, Q., & Percy, D. (2016). Salix transect of Europe: patterns in the most abundant chrysomelid beetle (Coleoptera: Chrysomelidae) herbivores of willow from Greece to Arctic Norway. Biodiversity data journal, (4).
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139136/
Legend:
Figure 1. Collecting conditions (temperature and relative humidity) at the sites (data plotted from Table Table 2). In this graph lines of constant absolute humidity (AH; g/m3) and vapour pressure deficit (VPD; kPa) are plotted as dashed lines. VPD is a measure of the drying power of the air. Circles (red line) mark collection localities 1-20 (April 2015) while the triangles (blue line) mark sites 21-41 (June 2015). Note that the environmental conditions during collection are very similar between Central Europe (sites 18-20) in April and Arctic Europe (sites 30-41) in June.