Relative humidity and vapor pressure deficit

[dpjm]

I've been thinking about relative humidity and how it relates to temperature. I've been moving the hygrometer between warmer and cooler areas in the enclosure. The warmer areas have lower RH and the cooler areas have higher RH, and explanations for these observations are already well known.

But then, when we give out recommendations for RH in the enclosure of a certain animal, say a tegu, are we talking in the cool end or the warm end, because it makes a difference. So maybe RH is not the best measurement of humidity as it relates to living organisms.

Plant people already know this, but animal keepers are a bit behind, I suppose. People who run greenhouses often try to calculate a different measure of humidity, called the vapor pressure deficit (VPD). VPD is the difference between the vapor pressure at saturation (100% RH) and the actual vapor pressure at the same air temperature. Plant people use this measure because it is a better assessment of the drying capacity of an environment than relative humidity is.

If you have a warm location that is at 75% RH, the VPD will be higher than another location that is cooler but has the same 75% RH. The higher VPD number in the warmer location will reflect that the warmer location will have a greater capacity to suck moisture from the organism, even though the RH is the same.

Here's an example of a warm location and a cool location, where RH is constant and VPD is variable:

90 F, 75% RH - VPD = 12
60 F, 75% RH - VPD = 6

So even though the RH is measured at exactly the same value, the VPD is much greater at 90F than at 60F, meaning that warmer areas will suck more moisture out of your animal than cooler areas when RH is the same at both locations.

So VPD is a much better way to measure humidity as it relates to an animal. To have a constant VPD throughout an enclosure, you would have higher RH in warm areas and lower RH in cool areas.

Same example as above, except now VPD is constant and RH is variable:
90 F, VPD = 12 - 75% RH
60 F, VPD = 12 - <35% RH

This sounds great, but VPD is more difficult to calculate and there are no gauges. To calculate it you need to know the temperature, the saturation vapor pressure for that temperature, and the RH. If you know those things, the calculation is simple: saturation vapor pressure - (saturation vapor pressure x RH/100). It's the saturation vapor pressure that is more difficult to calculate.

This is mostly food for thought. I found it interesting and it cleared up some humidity-related questions that were roaming around in my head.

 

[Walter1]

I've been thinking about relative humidity and how it relates to temperature. I've been moving the hygrometer between warmer and cooler areas in the enclosure. The warmer areas have lower RH and the cooler areas have higher RH, and explanations for these observations are already well known.

But then, when we give out recommendations for RH in the enclosure of a certain animal, say a tegu, are we talking in the cool end or the warm end, because it makes a difference. So maybe RH is not the best measurement of humidity as it relates to living organisms.

Plant people already know this, but animal keepers are a bit behind, I suppose. People who run greenhouses often try to calculate a different measure of humidity, called the vapor pressure deficit (VPD). VPD is the difference between the vapor pressure at saturation (100% RH) and the actual vapor pressure at the same air temperature. Plant people use this measure because it is a better assessment of the drying capacity of an environment than relative humidity is.

If you have a warm location that is at 75% RH, the VPD will be higher than another location that is cooler but has the same 75% RH. The higher VPD number in the warmer location will reflect that the warmer location will have a greater capacity to suck moisture from the organism, even though the RH is the same.

Here's an example of a warm location and a cool location, where RH is constant and VPD is variable:

90 F, 75% RH - VPD = 12
60 F, 75% RH - VPD = 6

So even though the RH is measured at exactly the same value, the VPD is much greater at 90F than at 60F, meaning that warmer areas will suck more moisture out of your animal than cooler areas when RH is the same at both locations.

So VPD is a much better way to measure humidity as it relates to an animal. To have a constant VPD throughout an enclosure, you would have higher RH in warm areas and lower RH in cool areas.

Same example as above, except now VPD is constant and RH is variable:
90 F, VPD = 12 - 75% RH
60 F, VPD = 12 - <35% RH

This sounds great, but VPD is more difficult to calculate and there are no gauges. To calculate it you need to know the temperature, the saturation vapor pressure for that temperature, and the RH. If you know those things, the calculation is simple: saturation vapor pressure - (saturation vapor pressure x RH/100). It's the saturation vapor pressure that is more difficult to calculate.

This is mostly food for thought. I found it interesting and it cleared up some humidity-related questions that were roaming around in my head.

Hi dpjm- thanks for sharing this. Besides measurement being tricky, so os bouyant air. Stagnant is bad regardless of remo and humidity, but warm, humid, and stagnant is particularly bad.

 

[Roadkill]

I get where you're going with this, and I am not saying you're wrong, but some of what you've written is, at face value, misleading.

Here's an example of a warm location and a cool location, where RH is constant and VPD is variable:

90 F, 75% RH - VPD = 12
60 F, 75% RH - VPD = 6

So even though the RH is measured at exactly the same value, the VPD is much greater at 90F than at 60F, meaning that warmer areas will suck more moisture out of your animal than cooler areas when RH is the same at both locations.

Let's address the one issue you're discussing here, relative humidity. As the term indicates, this is a subjective measure because it is RELATIVE, beings that it is a ratio of the current absolute humidity to the maximum capable, which is relative to the temperature, while also being further relative to the barometric pressure. So contrary to your statement "at exactly the same value", 75% RH at 90F is NOT the exact same value as 75% at 60F (although perhaps I'm arguing semantics over the meaning of "value"), only the relative measure is the same. Now, unless you have some wildly huge (or physics defying) enclosure, we can assume barometric pressure to be pretty much uniform throughout the enclosure (let's assume standard sea level), so at 90F you have an absolute humidity of 0.026 kg/m^3 (from a possible max of 0.034 kg/m^3), and at 60F you have an absolute humidity of 0.01 kg/m^3 (from a possible max of 0.013 kg/m^3). If you truly understand RH, then the VPD should be intuitively apparent - for the same RH, higher temperatures have greater absolute humidity but also much greater ability to absorb even more moisture than a lower temperature.

What you're pointing out about VPD is indeed accurate, and I can see how it is of greater value to people growing plants because 1) plants are stationary, and 2) evapotranspiration/transpiration is likely of greater concern with plants than is evaporation with reptiles.

However, this is not to dismiss your discussion, as I have to admit that in a tegu enclosure, I would be merely speculating that as one moves through the enclosure, I suspect absolute humidity likely doesn't waver that much except at the extremes, and that focal hotspots represent a "sink" for humidity. In relation to a tegu's needs....compared with what I measured in their natural habitat, every day of the year (except during some periods of the winter), relative humidity hit 100%, and did so for a significant portion of the day (quite often the majority of the day) - is this necessary for their health? I doubt it. Should you try to have elevated humidity? Yes. The argument is in what is actually optimal, and I don't think we can accurately assess that with what we have.

 

[dpjm]

Thanks for contributing, Roadkill. I just came across this deficit concept about a week ago so it's not exactly clear in my mind yet.

So contrary to your statement "at exactly the same value", 75% RH at 90F is NOT the exact same value as 75% at 60F (although perhaps I'm arguing semantics over the meaning of "value"), only the relative measure is the same.

Sorry, I guess "value" was not what I meant, I did mean measurement on the hygrometer. I can see that "value" would be the absolute humidity, which is not equal between the two temperature examples.

for the same RH, higher temperatures have greater absolute humidity but also much greater ability to absorb even more moisture than a lower temperature

And this is due to the deficit, or the actual amount of vapor that is missing? So for 90F there would be a 0.008 kg/m^3 deficit and at 60F there would be a 0.003 kg/m^3 deficit. I guess that vapor amount (kg/m^3) would be another way of expressing the deficit, instead of vapor pressure.

The argument is in what is actually optimal, and I don't think we can accurately assess that with what we have.

No doubt. There must be some humidity level that they do their absolute best at and now I think that would be a measurement of moisture deficit like VPD, not RH.

I think that when we give out recommendations of humidity levels, it should have a temperature attached to it. Just saying that the enclosure should be, say, 80% RH, without a corresponding temperature could lead a keeper to make errors. Like 1) placing a hygrometer in a basking area and trying to keep high humidity in an area that is really hot - endless misting and ventilation-cutting that is probably unnecessary; or 2) placing a hygrometer in the cool zone, where 80% can be easily maintained but the rest of the enclosure is too dry. I usually try to measure humidity in a area of mid-range temperature. Maybe a better recommendation would to maintain 80% RH at 90F, for example.