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  #1  
Old 06-19-2017, 09:18 AM
nogreenthumbs nogreenthumbs is online now
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Default Rel. Humidity vs dew point

Why does everyone use the relative humidity values when talking about orchid culture rather than dew point?

The reason I ask is that 70% humidity at 60° is a very, VERY different humidity than 70% humidity at 80°. But if the dew point is 65° at both 65° and 85°, then the same amount of moisture is in the air. i would think that we are most concerned with moisture content.
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  #2  
Old 06-19-2017, 09:33 AM
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When I looked up the definition of dew point this is what I got:

dew point
noun
The atmospheric temperature (varying according to pressure and humidity) below which water droplets begin to condense and dew can form.

This is the definition for relative humidity:

relative humidity
noun
The amount of water vapor present in air expressed as a percentage of the amount needed for saturation at the same temperature.

For both definitions, just do a Google search and those two definitions will show up in a box.

These links seem to be a good one for those who understand the math and the graphs:

Relative Humidity:
Relative Humidity

Dew Point:
Relative Humidity

Note: Both links say "Relative Humidity", but they are not mistakes made when I copied and pasted the links nor are they identical.

(Physics was something I sucked at as a subject matter, btw. Looking at those formulae and graphs were kind of intimidating.)

When I'm growing an orchid, I'm not interesting in creating conditions that form dew droplets on my orchids, especially when they are being grown in my room! I kind of understand the saturation of moisture part of it, but in order to talk about dew point properly, from what I understand, you're also talking about the temperature at which it is needed to drop to form dew droplets.

The amount of water vapor in the air is the thing that keeps the orchid's roots hydrated when it is not being rained on or watered. In many areas throughout the tropics, the air is humid. Does dew form during certain times of the day there? Yes, of course. But that is temporary because the conditions that allow for that to happen change. Relative humidity change throughout the day as well, but it has a longer duration between changes, and it is a bit more consistent.

In cultivation, talking about dew point is difficult, because to recreate those conditions are not that easy, and it is of a smaller value to an orchid.

I think the concept of saturated vapor pressure is also something to think about.

Vapor Pressure

I also believe absolute humidity is something to understand too.

There are actually 3 different kinds of humidity:

1) absolute humidity
2) specific humidity
3) relative humidity

The following article discusses briefly more about relative humidity than absolute humidity, but it does touch on it a little.

What is relative humidity and how does it affect how I feel outside? | HowStuffWorks

This is the definition of absolute humidity:

absolute humidity
Absolute humidity is the measure of water vapor (moisture) in the air, regardless of temperature. It is expressed as grams of moisture per cubic meter of air (g/m3). The maximum absolute humidity of warm air at 30°C/86°F is approximately 30g of water vapor - 30g/m3.

(Absolute vs. Relative Humidity - What’s the Difference?)

At this point, I'm not sure how specific humidity works. Maybe someone else can chime in. Perhaps someone who is good at physics.

In the meantime, here is the definition of specific humidity:

specific humidity
noun
The ratio of the mass of water vapor in air to the total mass of the mixture of air and water vapor.

(Specific humidity | Define Specific humidity at Dictionary.com)

This link talks about all 3 kinds of humidities.

http://imnh.isu.edu/digitalatlas/cli...ging/humid.htm
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  #3  
Old 06-19-2017, 10:17 AM
nogreenthumbs nogreenthumbs is online now
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Dew point using the combined temperature and rel humidity gives you something more comparable across various temperatures.

Think about it this way. If you take a quart of cold water and you try to dissolve a cup of sugar in the water, you probably won't be able to. Maybe only 1/2 cup dissolves. So if you're trying to make rock candy, and you need 1cup per quart to do it, you can't do it with cold water. Now you probably know that if you heat/boil the water, you can dissolve more sugar in the water, so if you heat the water to say 150° the whole cup dissolves.

My thinking is this, if the orchids require 1 cup of water per cubic yard of air, you may not be able to accomplish that at 60°, but at 80° it isn't a problem. That's the way to think about the dew point as providing a better measure of how much water content is in the air.

This morning, I was looking at the weather. When I took the dogs outside it was 79° and 99% humidity with a dew point of 79°. 30 mins later, it was 81° and 95° humidity with a dew point of 79° and then another hour after that, it was 85°, 88% humidity and a dew point of 78°, IIRC.

So, despite the temp change, the same amount of moisture was in the air.

Maybe it's not that important since our growing temps and humidities are in a fairly narrow range, but I would think that when you see recommendations for 50-70% humidity, I assume that's when the daytime temps are 70-80°. If folks are doing shooting for the same thing in the winter at 60-65°, then the environment will be much drier, especially if their yearly range is 80°-60°. But then don't most orchids prefer/expect drier winter weather. Maybe that's the key, that they usually want/need more humidity in the summer and less in the winter and so relative is a great measure.

I don't think it's common to see absolute humidity reported anywhere. If it was, that would probably be great for us. I suspect the only places you'll find that sort of measurement is in the scientific community.

Another reason why we use rel humidity is probably because it's not hard to find something to measure that, but getting the dew point is much less likely without an actual weather station, where I think you can pick up a cheap temp sensor that includes rel humidity.
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Old 06-19-2017, 11:36 AM
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It's important because the evapotranspiration increases with lower %HR, ie plants and medium dry faster.
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Old 06-19-2017, 01:51 PM
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Orchids (and plants in generall) are sensitive to the ability of air to take on water. It does matter in absulute terms how much that is, what matters to the plant is, how fast it can('t) conduct water through its tissues. For that they need a certain difference in water potentials around their roots and around their leaves. The relative humidity dictates, how fast the air ''sucks'' the moisture out of the plant. Closer you are to the dew point, the harder it is for the plants to do it (or on the other hand the stress of drying out lessens).
The absolute ammount of moisture (what you'd write down as g/m3 or equivalent) is important only in the relationship with temperature. It doesnt tell you much, just that there is, eg 5 g of water in a cubic meter of air. With temperature, you can know, how much more water can air take on, that is, how easy it is to dry stuff (like plant tissues). Dew point is simply a measure of absolute ammount if water, and it states a temperature at which the current water ammount constitutes 100%.
It's much more pratctical to operate in relative water content, because it gives more information with less measurements (you dont need temp.).

It's like; when you want to drink tea, it the ammount of sugar is not important, it's it concentration. If you put the same ammount (let say a spoon) of sugar in a gallon, a cup, or a thimble, and then drink it all. only one will be tasty - the cup. Here would be the sugar solubility (dew point) although the same, the ammount of sugar drunk (absoulte humidity) also same, for these containers usless measure of tastiness, concentration (relative humidity) is where it's at.

So let's say, we have an orchid, that does not fare well in low humidity. Let's say that the dew point would be 8°C. At 8 degrees, the relative humidity is 100% at 8 g H2O per cubic meter. Than temperature rises, but no moisture enters te air. So at 30°C , the abslute humidity is the same, the dew point is the same, but relative humidity is 28%. The plant will desiccate, as it is not adapted to fast pumping of water through its tissues.
It no need to say, that dew point helps us not here (except if we take it, that the dew point must be as close to the actual temp. as possible).
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Old 06-19-2017, 08:32 PM
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Excellent post, thanks tons. That explains it well.

My assumption is that the plant needed for there to be moisture in the air, but it's more about the difference and how the water enters and leaves the plant.

Great info!
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Old 06-19-2017, 09:17 PM
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Plants are water balloons. They depend on water pressure inside the plant to stand upright. With inadequate water they wilt, then collapse.

Water enters through the roots.

Water evaporates from the pores in the leaves.

There is a continuous column of water in the plant's vascular cells. This column extends from the roots to the pores on the leaves. Water evaporating from the pores pulls water up from the roots.

When there is insufficient water taken up at the roots, and water continues to evaporate from the pores, the pressure inside the plant decreases. Stiff-bodied plants and leaves, as in orchids, wrinkle. Plants with softer stems and leaves droop. Water continues to evaporate from the pores, pulling on the column of water in the plant's vascular system.

If there is insufficient water at the roots, the pulling on the water column will break it. The column will cavitate and become discontinuous. Once broken, the continuity cannot be re-established. The part of the plant receiving water from this part of the vascular system dies above the point of cavitation.

Many people have, at some time, forgotten to water a coleus plant. The plant droops. When rewatered, the top of the plant dies, but perhaps it resprouts from the base. The cavitation was at the point where the top of the plant died, and the water column was intact below this, permitting the plant to continue growing.

The higher the relative humidity, at any air temperature, the less water will tend to evaporate from a plant. Therefore the plant will not have to draw up as much water to maintain its water content at a higher relative humidity. Water stressed plants survive longer with higher relative humidity.

Some plants survive drought by being good at closing their pores during times of water stress. Succulents are like this. Other plants survive drought by being very good at sucking water out of dry soil. Creosote bushes are like this.

Many plants have no mechanisms to survive drought. They die quickly when water stressed. Many plants from areas with plentiful rain are like this.

Dew point reflects condensation of water from the air. It has little bearing on plant water economy. It has a bearing on whether evaporative coolers do a good job coolinlg.
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