Using vapor pressure differential to predict condensation in tents

[Ryan Jordan]

Companion forum thread to: Using vapor pressure differential to predict condensation in tents

How does vapor pressure differential influence condensation in tents?

[Jerry Adams]

very interesting Ryan

I love the hand drawn picture of tent with heat loss, wrinkled paper, pen drawn lines with felt pen emphasis

and when you leave door open it makes a big difference, but those little vents they advertise on tents aren’t enough.  Like was recently discussed on some other thread

[Jerry Adams]

I’m trying to wrap my head around the vapor pressure differential concept.  I googled and found another source https://www.canr.msu.edu/uploads/resources/pdfs/vpd-vs-rh.pdf

“Instead of relative humidity, the more accurate way to express the driving force of
water loss from a leaf is vapor pressure deficit”

“Therefore, there’s a fairly large gradient between plants (nearly saturated with water) and the air, enabling the plants to transpire and, over time, dry out. A low VPD indicates the air is near saturation.  A VPD of zero means the air is 100% saturated and thus plants cannot transpire effectively”

so the analog to tent condensation is the inside surface of the tent is like a leaf.  If there’s any condensation then the inside surface is at 100% humidity.  The VPD will indicate how easily it will evaporate that water to the air.

that makes sense.  If the air temperature is warmer then the VPD will be larger so there’s less condensation.  That matches my experience.

the bigger factor is how much the inside surface cools off from radiative cooling, as you talked about.  But, if it’s warmer, then you won’t get condensation even if there’s radiative heat loss as explained by the VPD concept.

thanks for this article

[Jon Fong / Flat Cat Gear]

I am looking for additional insight, perhaps you can shed some light on this.  In the articles, you discuss RH which is fine.  But how does temperature play into this with respect to condensation.  Isn’t the absolute humidity an important variable?  While condensation will occur at low temperatures, isn’t the absolute volume of water significantly less?  Given the same VPD, how much water is generated at 30 F compared to 40 F?

[Bill Budney]

The condensation condition that I find most annoying is when temperature is high, like 70F+ with dew point matching the temperature. It is common on Summer nights in the East. Everything becomes damp and soggy, and evaporative cooling slows or stops so I cannot get cool no matter what. Indoors, we just turn on the air conditioner. Outdoors, the only possible salvation is a breeze (and lots of ventilation).

[Ryan Jordan]

Jon – great questions.

Absolute humidity defines the quantity of water vapor in the air, expressed as mass per unit volume (e.g., grams of water / m^3 of air).

Relative humidity is expressed as a ratio of the amount of water in the air relative to the maximum amount of water that said air can hold, and is expressed as a %.

As the temperature of a particular volume of air decreases, its absolute humidity doesn’t change – no water is being added or expelled from that volume (in a closed system). However, its relative humidity changes because as air is cooled, its capacity to hold water (the max possible humidity) decreases.

In a tent occupied by a human, it’s not a closed system and water is being added (via the human) and expelled (via ventilation). In this case, absolute humidity is constantly changing, and this is why continuous sensor monitoring of both T and RH is so critical. It’s very difficult to theoretically model that type of system in the absence of direct measurements. Monitoring RH in real time captures the changes in absolute humidity that are occurring inside an occupied tent.

Given the same VPD, how much water is generated at 30 F compared to 40 F?

It’s not clear to me what you mean by “how much water is generated” – are you asking what the total (max) water vapor capacity of air is at those temperatures at saturation (RH%=100)? This chart shows that (at sea level):

 

[Jon Fong / Flat Cat Gear]

OK, let me see if I can clarify my question.  Say the outside air is 35 F and you mearure the VPD to be 0.8 (moderate).  If the outside temperature is 45 F and the VPD is 0.8, I am assuming that the volume of condensation is significantly higher at 45 F, is this true?  So, how does VPD related to condenstate volume with respect to temperature?  Does that make sense?

[Ryan Jordan]

A VPD of 0.8 at 35 °F occurs at RH of about 91%.

A VPD of 0.8 at 45 °F occurs at RH of about 94%.

But 45 °F air holds more water vapor at saturation than 35 °F air – 7.9 vs. 5.5 g/m^3 (sea level).

So at VPD of 0.8, the 35 °F air contains about 0.91 x 5.5 = 5.0 g/m^3.

And at VPD of 0.8, the 45 °F air contains about 0.94 x 7.9 = 7.4 g/m^3.

Now let’s calculate the remaining capacity of water vapor mass in each of these scenarios.

At a VPD of 0.8, the 35 °F air can only take on an additional 5.5 – 5.0 = 0.5 g/m^3 of water vapor mass.

And at a VPD of 0.8, the 45 °F air can only take on an additional 7.9 – 7.4 = 0.5 g/m^3 of water vapor mass.

The same!

So in each scenario, if you put yourself in a tent, and start exhaling, you’ll see similar rates of condensation accumulation.

And that illustrates why VPD is such a good normalization tool for evaluating the condensation potential in an environment. You can’t capture this through RH or T_dp by themselves.

[Jerry Adams]

A leaf in a greenhouse will be at the same temperature as the inside air temperature of the greenhouse, so your calculation only has to include that one temperature.

For the inside surface of a tent, and the condensed water on it, will be at a different temperature than the inside air temperature.  If you really wanted to calculate the VPD accurately, you’d have to include the temperature of that condensed water as well as the temperature of the inside air.  I think…  Maybe the inside air temperature doesn’t have much effect.

And the VPD would tell you how fast the condensation on the inside of the tent would dry out.  But would this tell you how fast water condenses on the surface?  A little different problem.

Part of the problem is that for part of the night you’ll get condensation, then later it may start drying out, so VPD is still useful.

And the over-all conclusion that if it’s warmer, then there should be less condensation is probably good.  This certainly matches my experience – no condensation if it’s warmer even if it’s a clear night.

Except you people that camp in the East (or South) where it’s more hot and humid

[jscott]

geeze, I just wanted to go out backpacking. Without scientific instruments.

I do like the idea of thinking like a leaf.

I try to avoid the more extreme conditions that Ryan is interested in. I’m a lightweight backpacker, after all. That said, given a double wall tent and sun in the morning, as is usual in California, condensation is more of a mild annoyance than anything.

I’m not gloating. I’m thinking like a Jeffrey pine.

[Jerry Adams]

yeah, you don’t need to understand the physics to go camping.  It’s an exercise for nerds that has some useful application

I think Ryan is not looking at extremes, but the common occurrence of condensation on inside of tent that we most all have encountered.  And vapor pressure differential is fundamental to it.

I’ve never heard of it before so thanks to Ryan for bringing it up.

[John S.]

You’ve never heard of it before because vapor pressure differential is not used by meteorologists. It’s more of a physics term used in other ways (engineering, etc).

[Philip Tschersich]

It has been my general impression that, given a certain set of environmental conditions, pitching a tent under a tree (provided we are talking about a relatively thick conifer or a nicely leafed-out deciduous tree), that less condensation is formed compared with pitching the same tent in the same conditions a few yards away, but not under the tree. It may be that the overhead cover prevents dew forming on the outside of the tent in the morning hours, and I’m just sort of conflating that with condensation inside the fly.

[Todd T]

It has been my general impression that, given a certain set of environmental conditions, pitching a tent under a tree (provided we are talking about a relatively thick conifer or a nicely leafed-out deciduous tree), that less condensation is formed compared with pitching the same tent in the same conditions a few yards away, but not under the tree. It may be that the overhead cover prevents dew forming on the outside of the tent in the morning hours, and I’m just sort of conflating that with condensation inside the fly.

A thick tree canopy shields the tent fly from “seeing” the cold night sky, which significantly reduces radiant heat loss.  Consequently, the fly stays warmer and there will be less condensation.  This effect is quite strong on clear nights, when the cold blackness of outer space is visible.  Not so much on cloudy nights.

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