Vapor Pressure Deficit vs Relative Humidity
It wasn’t until recently that I had heard of Vapor Pressure Deficit (VPD).
It is similar to RH, but is simply the difference between saturation and actual vapor pressure at a given temp– as opposed to a ratio.
But VPD is what directly affects evaporation rates, not RH.
It is well-known by horticulturists and my googling yields mostly documents focused on plants.
So my question is: does VPD serve any useful purpose for human comfort, or other non-horticultural applications?
Building scientists like to talk about Dew Point, for good reason, but will bring up RH when talking about interior humidity levels in winter (for example) in regards to human comfort or ‘safe levels’. It seems that technically, VPD would be the more accurate measure for human comfort since it governs the rate that moisture will leave the skin.
Of course, something is only useful if it is widely understood and easily measured. Additionally, if we assume that interior temperatures for human dwellings are kept within a pretty narrow temperature band, RH will give useful comparisons. VPD gives useful comparisons over wider temperature swings (look at the diverging RH lines as you move right on the psychrometric chart to see this).
But there may be some niche times when understanding VPD will help in understanding our environment. If, for example, we compare a house kept at 60F and 35RH, vs a house at 72F and 35RH, the latter will ‘feel’ drier because there is a greater vapor pressure deficit, even though RH is the same for both and even though absolute humidity is higher in the latter case.
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