Talk:Relative humidity

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Totally incorrect. The relative humidity has absolutley nothing to do with soluability of water in air - it is governed by the partial pressures. Furthermore we need to clarify units. If absolute humidity is to be expressed as a percentage - then a percentage of what verses what? grams verses m^3? That makes no sense! Volume H2O verses volume of air? How does temperature effect this? Mass of H2O verses mass of air?

Clearly there is a degree of confusion here.

Terrell Larson (minor update to link in my log in)

A very helpful article! what I found missing here was reference to typical values of relative humidity, e.g. perhaps is it 0% only in the desert? And is it 100% in a rainforest? etc.

Judging by the appearance of the graph I don't see how relative humidity could be inversely proportional to temperature. The current graph would have to be flipped about the vertical. The graph appears to be exponential, but I would appreciate if somebody in the field or in the know would write the equation(s) that relate relative humidity (dew point etc) and temperature.

Thanks for writing above the contents everybody. And not signing your comments. It really helps foster a sense of chaos. Good work. —Preceding unsigned comment added by Laikalynx (talk • contribs) 23:59, 20 March 2008 (UTC)

Contents

[edit] Condensed water on car window - heating or cooling?

Especially in winter time it's common for car windows to get a thick layer of condensed water vapor on them. My brother and I were aguing what's better to do in that situation: he said it's better to put on the heating, so that the windows start to warm up and the car's interior can hold up more water vapor. I argued that cooling the car will bring the whole system (outside - window - inside) in a temperature equilibrium, thus the water vapor has no specific preferred place to condensate.

What whould you say is better? Heating or cooling? --Abdull 6 July 2005 09:55 (UTC)

Heat the windshield to bring its temperature up to above the dew point of the car's interior. Vsmith 6 July 2005 15:47 (UTC)
I agree absolutely with Vsmith, but if your heater doesn't work, open the windows to remove the (extra human-made) water vapour from the car-- assuming the outside air is dry winter air.82.93.133.130 20:14, 9 November 2006 (UTC)

This comment of units is bit funny - if it's percentage then the units are the same, i.e. the present mass of the water (vapour) in relation to the mass of the water (vapour) in the saturated air. One thing bothers me though. It says there, that in saturated air the water won't lose its mass - I think I know what the author meant, but it may seem to somebody, that water vapours because it actually loses it's mass (literally, physically), which is not true.

[edit] Not Completly Accurate

The verbiage in this article leads the reader to believe that solubility plays a role in relative humidity. This is not the case.

Relative humidity has noting to do with solubility of water in air. This statement must be removed: The warmer air is, the more water vapor it can "hold."

Air does not "hold" water.

Furthermore:

The term Dew Point needs to be better articulated. Dew Point Temperature and Dew Point Pressure need to be distinguished in the discussion of Relative Humidity.

The Relative Humidity of a gas is a function of not only the absolute pressure but also temperature of the gas.

So for simplicity's sake, should we tell the reader right off that we're assuming we're at sea level-760mmHg?? Gaviidae 22:11, 21 November 2006 (UTC)

As for the condensation problem in a vehicle: What is going on? When the water content in the cabin air inside the vehicle increases the dew point temperature of this water/air mixture also increases. When this gas mixture contacts a surface (such as the window) which is below the dew point temperature of the mixture condensation will occur.

What is the "best" thing to do? Do both. Remove water vapor from the air by operating the air conditioner and heat the internal surfaces of the cabin by turning on the heater so they remain above the dew point temperature of the cabin air.

In practice turning on the air conditioner will immediatly reduce window condensation because this action will remove water from the cabin air inside the vehicle. Turning on the heater alone will also reduce condensation but not as readily because the heater must heat the cabin air and then this thermal energy needs to be transfered to the internal surfaces of the vehicle. This is a slower process than cooling air with the vehicles A/C.

[edit] Thanks for the changes...

I like some of the changes users have made to the intro, overall the article is shaping up nicely. Some changes however:

The following verbiage does not make sense ...under conditions of vapor saturation at a given temperature (see: common misconceptions below). and has been removed.

I have removed the incorrect phrase: more technically more technically than what? It does not seem to fit here.

I noticed the definition was reverted back to the concept of air-holding water for relative humidity - PLEASE avoid explaining the concept of relative humidity in these terms. This is absolutely incorrect.

Also removed the reference Technical definition and replaced it only with Definition. The adjective technical implies there are other definitions to relative humidity, this being the technical one. Infact this is the definition of relative humidity.

I've always been told also that warmer air "holds"more moisture-- that, air pressure being the same, more water will stay in droplet form in warmer air than colder air. If I have 2 beakers at room temperature, and they both have the same % of water relative to air volume, and I heat one beaker, there will be less condensation than in the other... but now of course I've changed the pressure in that beaker... whoops. : )
But my question is with this sentence: Water vapor is a lighter gas than air at the same temperature, so humid air will tend to rise by natural convection. Huh? Why do clouds have to "rain" to get over the mountains (old school lesson, I know)? Isn't the windy side of the mountain range getting more yearly rainfall than the leeward side?? because the clouds are too heavy (or dense) to rise over the mountaintops?? The article didn't make this clearer, esp not this sentence. Humid air is "lighter" (less dense?) than dry air?? Gaviidae 20:24, 9 November 2006 (UTC)

Ok, this is what I got from How Stuff Works [1]: Absolute humidity is the mass of water vapor divided by the mass of dry air in a volume of air at a given temperature. The hotter the air is, the more water it can contain. Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain... Since HSW is also just another website, how does this definition compare to say, a real college textbook? Why doesn't it mention saturation and air pressure in mb? Gaviidae 20:29, 9 November 2006 (UTC)

Me again-- HSW not well written. Found two better sources, but I'm afraid to use one on Wikipedia because I dunno what gsu.edu IS-- I only found their database, great for basic stuff of all sorts http://hyperphysics.phy-astr.gsu.edu/hbase/ and the humidity stuff at http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/relhum.html and also friendly meteorologist http://www.shorstmeyer.com/wxfaqs/humidity/humidity.html
The way they put it (hyperphysics good for diagrams, Shorstmeyer good for basic energy concepts), air is air and liquid will enter air when liquid molecules get enough energy to fly and be free and all that. So the reason air doesn't "hold" water is that the water holds itself with its energy. Warmer air CAN HAVE more moisture because the warmth in the air is energy that, if harnessed by any liquid water laying around, is used to launch water molecules into the air. Cool. Colder air has less energy, and as water molecules in the air bounce into and off from colder molecules, they lose their kinetic energy and can no longer fight hydrogen bonds and van der waals-- forces that keep water molecules together as water. Wow. Gaviidae 22:08, 21 November 2006 (UTC)

[edit] Lack of citations

Hi all. This article fails to cite its sources per WP:REF. I therefore added the tag for this on the article page. Not citing sources is substandard for Wikipedia articles. Moreover, the recent revert war may have thereby been avoided. CyberAnth 18:12, 8 October 2006 (UTC)

[edit] Wet cities and dry cities

Is there anywhere on the internet where I can compare relative humidity in different places in the world, perhaps using a map or table or something?

I did not look for tables with cities, but as I was looking for better explanations of humidity, I found this meteorologist's site,and he does mention places with highest recorded dew point temps-- http://www.shorstmeyer.com/wxfaqs/humidity/humidity.html Gaviidae 22:01, 21 November 2006 (UTC)

[edit] 'Lightness' of Water

In reference to the "lightness of water"-- what I meant was the weight of water molecules (oxygen=16 + 2 Hydrogen = 18) versus N2 (28) and O2 (32). Assuming the water molecules have the energy to stay unattatched to other water molecules (and thus form a droplet too heavy to stay afloat), they have less density (or maybe mass, not sure) than the surrounding air. Make sense or did I butcher this completely? Also, if the lightness of the water molecules has nothing to do with RH and only AH, then it's also mentioned under Other Important Facts

I believe I understand what you are trying to communicate. What your describing in part relates to the reason that water is a liquid at standard conditions compared to materials such as methane (molecular mass of 16). Based on molecular weight alone water should exsist as a gas at standard conditions. The reason, in part, for the stability of liquid water at normal temperature and pressure is because of the extensive hydrogen bonding that occurs among the molecules in the liquid phase.

Physical properties of water such as density and molecular mass and body forces such as buoyancy do not adequately explain concepts relating to relative humidity, absolute humidity, relative saturation, or the like. The concepts introduced in discussions of single component phase equilibria (a topic of interest in the chemical engineering profession) better explain the reasons for relative humidity. Although the topics in phase equlibria are advanced for a person not educated in the field the ideas are not difficult to understand.

In my experience I have found that people make the explanantion of RH unnecessarily complicated. If you understand the information that is presented in a steam table then you know all there is to know about Relative humidity —The preceding unsigned comment was added by D-dawg (talk • contribs) 16:42, 17 February 2007 (UTC).

[edit] metric

Could we have units in metric (in addition to imperial, if necessary), please?

[edit] Level of the article on Relative Humidity

Who is / should this article aimed at?

While I'm sure everything in the article is correct, I found it of little use to me as a "common-man".

I have Degree in Science and I work IT, I wanted some understanding of relative humidity in relation to my server rooms. I found the article to be difficult to read and a scientific discussion of RH and of little practical use to most people.

By all means have the detailed scientific discussions but it should be preceded by and general introduction and practical application of the term.

[edit] A bit dogmatic

I am afraid to say that I am not really sure that this article is very helpful as it stands. This may be due to inconsistencies due to different aithors and various reverts.

I am also not convinced by the insistence of some people that the concept of relative humidity as a proportion of the amount of water that air could contain vs the amount it does contain is fundamentally wrong. Relative humidity is about air and how much water it holds/contains/whatever word is correct. This is how relative humidity is explained on countless websites, including the likes of the UK Met Office who say "Relative humidity (expressed as a percentage) is a measure of the amount of water vapour in the air compared to the maximum that could be contained by the air at the same temperature. "

I also disagree with the assertion that relative humidity is calculated using that big equation. In the real world relative humidity is very often calculated using wet and dry bulb thermometers and a set of tables.

Now, perhaps there is a difference between the pure scientific understanding of relative humidity, and the meteorological approach. If this is the case, then this is how the article should be structured. Explain what meteorologists mean and how they measure it, and how it effects the comfort of warm days, then explain the pure science. This will make the article both accurate and useful.Ewan carmichael 22:25, 21 June 2007 (UTC)


I agree. This article seems incorrect. Relative humidity is RELATIVE because air at different temperatures has different densities. Cold air is dense, and typically contains less water vapor than warm air, which is less dense. A room of cold air containing X amount of water vapor, when heated up, contains the exact same amount of water vapor (X) but the RELATIVE humidity has changed, because the warm air has a greater capacity for water vapor. For example, let's just say X is 5. The cold air has a capacity of 10. So in the cold room, you have 50% RELATIVE humidity. The warm air has a capacity of let's say 25; in the warmed room, the RELATIVE humidity is 20%, but there is still 5 water vapors.  ;) Get it? I studied meteorology. I wasn't aware there were different definitions of relative humidity.

"Air doesn't HOLD water" people keep saying. Uh.. I beg to differ! -Laikalynx (talk) 00:07, 21 March 2008 (UTC)


Relative humidity is RELATIVE because the quantity compares the measured vapor pressure of water in air RELATIVE to the staurated vapor pressure of water at the temperature prescribed. The density of air has nothing to do with relative humidity; relative humidity is defined without any representation of the density of air in the formula.
The effect of a temperature change on relative humidity is that the saturated vapor pressure to which the measured vapor pressure is compared to changes (i.e. the reference point changes; the denominator) but the measured quantity of water vapor remains the same. To state that air has a greater capacity for water vapor suggests that mixtures of water vapor and air behave similar to solute-solvent systems. This is simply not the case.
There is only one definition of relative humidity and it is accuratly communicated in the article. Air does not hold water.
Conduct the following thought experiment. Consider a closed system (no mass in, no mass out) at some temperature, pressure, and 100% relative humidiy. If the pressure of the system is decreased the relative humidity will decrease; if the pressure of the same system increases rather than decreases the relative humidity will remain at 100%.
Why would a decrease in pressure cause air to hold less water?; Why would an increase in pressure cause air to hold the same amount of water (i.e. relative humidity stays at 100%)?
Answer: The notion of air holding water cannot explain these observations. Such a model fails to describe the system behavoir. In fact AIR has nothing to do with relative humidity. Relative humidity is soly related to the physical properies of water and the outcomes of the thought experiment above are readily explained by physical property data of water vapor found in steam tables. —Preceding unsigned comment added by 99.248.233.45 (talk) 03:17, 13 May 2008 (UTC)


[edit] Work to be done

Having read this article and these comments many times now, I thought I'd contribute my suggestions here, in hopes that we can improve this article...

I agree with the above comment, this article is not very helpful. By jumping right into the real scientific definition, I think most readers are lost from the start. Think of a young grade-school student doing a simple project on the weather, trying to comprehend partial pressures? I strongly believe that the explanation of air "holding water" should be included, but then immediately qualified as not being the true definition. The fact is, most people understand RH that way, if only as a quick answer. If we want to promote the "true scientific definition," then we must address the "incorrect" way in the article, not dismiss it as bad science and simply delete all occurances of it.

Lastly, regarding Ewan Carmichael's previous comment about using "that big equation" to calculate RH, versus simple temperature measurements and tables. "That big equation" is right, as is another version using mole fractions (from the ASHRAE Handbook). The tables referred to are psychrometric charts, which are made using that equation (and many others). The fact is, it's a lot easier to measure two temperatures and refer to a chart, than it is to carry around gas/vapor analysis machinery. :-) This, too, should be explained in the article.

So, if no one objects, I'll take a pass at this article soon, trying to follow all the previous comments posted here. CS 11:02, 10 July 2007 (UTC)


[edit] Please be Careful

Carmicheal be careful citing the volume of web sites that state a certain concept - one hundred web sites that state a concept incorrectly are still incorrect. In fact RH is one of those concepts that are more often explained incorrectly.

"the big equation" IS the definition of relative humidity. I strongly believe that it should remain. Don't confuse the method of measurement (i.e. a psychrometer - dry and wet bulb temps) with the definition. A psychrometer, hygrometer, FTIR, and many other instruments can be used to measure partial pressures of water vapor but relative humidity is defined independent of the nature of measurement.

The meteorological and scientific understanding of relative humidity are the same - thier applications differ. In the former it is used to express a feeling of heat - in the latter it is used most often to estimate the water content in air (as an example for HVAC applications)

I strongly oppose the introduction of "air holding water" in the discussion of RH. Including a concept such as this in the definition is mis-leading. Furthermore this has nothing to do with how RH is defined. Please see the `bad clouds` link in the article - I don`t believe the integrity of the article is preserved if the air holding water concept in introduced when references within the article are introduced that refute that concept.

I have no problem with a section explaining how RH is measured. In fact this probably would be very useful. I do, however, believe that the definition of RH must stand. RH is what it is and nothing else. D-dawg (talk • contribs) 16:42, 12 July 2007 (UTC).

[edit] WikiProject class rating

This article was automatically assessed because at least one WikiProject had rated the article as start, and the rating on other projects was brought up to start class. BetacommandBot 10:02, 10 November 2007 (UTC)

[edit] Correction on %Humidity

There was a discussion inserted into the body of the article that was critical of the math there.

The article stated that a relationship between % Relative Humidity and Temperature existed such that an increase of 20 degrees F would cut the % Relative Humidity percent in half. Then it stated that a change in temperature from 40F to 70F would reduce relative humidity from 80% to 10%, which is clearly not consistent with the stated relationship. Someone then pointed out the bad math right there in the article, but did not correct it. I assume that they just didn't want to bother figuring out the correct number exactly.

I just plotted Log10(%Relative humidity) vs. degrees F, using log(80%) and 40F as my starting point, and using the above stated relationship to plot a few other points (60F and log(40%), 80F and log(20%)). The "correct" value of the % Humidity was then calculated by simply plugging 70F into the line equation obtained from the plotted points. I say "correct" because all I did was correct the consistency of the math - but I have no idea whether the math is now consistently right or consistently wrong. --Majorsheisskopf (talk) 01:16, 7 February 2008 (UTC)<small (talk • contribs) 07:33, 5 February 2008 (UTC)