Talk:Relative humidity

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

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. 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.

Not Completly Accurate[edit]

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.


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.

Thanks for the changes...[edit]

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)

ASHRAE Handbook Fundamentals defines relative humidity as the ratio of the mole fraction of water vapor in the air to the mole fraction of water vapor in saturated air of the same temperature and pressure. Other references say similar things, but often putting it into terms of mass instead of moles. Many references use vague terms like "amount" of water vapor. The references that define relative humidity as the the ratio of partial pressure of water vapor in the air to the partial pressure of water vapor in saturated air are conflating a formula for calculating relative humidity with its'Italic text' definition, albeit a formula that is accurate enough at atmospheric conditions for most work, but not exact. —Preceding unsigned comment added by (talk) 02:31, 1 June 2010 (UTC)
Me again-- HSW not well written. Found two better sources, but I'm afraid to use one on Wikipedia because I dunno what IS-- I only found their database, great for basic stuff of all sorts and the humidity stuff at and also friendly meteorologist
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)

Lack of citations[edit]

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)

Wet cities and dry cities[edit]

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-- Gaviidae 22:01, 21 November 2006 (UTC)

'Lightness' of Water[edit]

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 (talkcontribs) 16:42, 17 February 2007 (UTC).

Totally incorrect[edit]

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)

Typical values[edit]

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 (talkcontribs) 23:59, 20 March 2008 (UTC)


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

Thankfully this seems to have now been done. The article is now readable to non-Americans. It's about time Wikipedia banned the use of imperial measurements especially strange systems like fahrenheit, unused except by Americans and a few old British people.-- (talk) 23:37, 9 June 2009 (UTC)

Level of the article on Relative Humidity[edit]

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.

A bit dogmatic[edit]

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 authors 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 (talk) 03:17, 13 May 2008 (UTC)

Work to be done[edit]

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)

Please be Careful[edit]

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 (talkcontribs) 16:42, 12 July 2007 (UTC).

WikiProject class rating[edit]

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)

Correction on %Humidity[edit]

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 (talkcontribs) 07:33, 5 February 2008 (UTC)

Graph of Grains/KG v. temp incorrect????[edit]

It looks as though it should be grams/kg???Artmario2001 (talk) 22:10, 2 July 2008 (UTC)

It is. Vsmith (talk) 01:42, 3 July 2008 (UTC)

Estimating relative humidity[edit]

Sorry, but one equation is just blatantly wrong. E can be eliminated and we are left with 17.269xTd = 273.3 + Td. So the dew point temperature is actually constant - oops.....

If the e on the right hand side is actually the natural logarithm, then certainly a p subscript on the left hand side is needed (as well as in the equation above it), but ideally a variable other than e should be used. I've fixed accordingly but not changed e, perhaps "p" should be used instead.

Djp (talk) 03:18, 7 July 2008 (UTC)

The equations for saturated pressure vs T were still wrong, as of 2009-Jan-11. They had no units, no reference and do not match the tabulated data in Perry or the CRC Handbook of Chem and Phys. The form p = e ^{\frac {\alpha \times T} {\beta + T}} will always return p=1 at T=zero. Evaluating the eqn using the quoted parameters returned 101.3 at 100°C, which is correct for units of kPa. But it means a factor of 0.611 is needed to return the correct pressure at the ice point. That then makes the use of 237.3 instead of 273.3 correct. Rather than stick in yet another equation, I changed the section to refer to other pages of Wikipedia (and elsewhere) where more precise and referenced equations are presented. Alloy730 (talk) 09:42, 12 January 2009 (UTC)

Stemming the tide of confusion[edit]

Hi, I don't feel qualified to rewrite this article, but I hope I can offer some constructive suggestions to make the article more accessible, which will hopefully reduce the amount of misunderstanding.

Careful not to introduce further confusion into this article; some points below are well taken while others may further confuse readers. D-dawg (talk) 04:51, 30 October 2008 (UTC)

Firstly, I think the article summary is misleading. It refers to "the amount of water vapor that exists in a gaseous mixture of air and water", which introduces the incorrect feeling that RH is something to do with the ratio of water to air, and fails to mention that it is proportional to the saturated vapour pressure. I think it is worth clarifying this, even at the expense of duplicating some of the definition. My attempt would be something like:

I'm not certain others readers would read the definition in the same manner. The ratio of air to water is never mentioned in the phrase: "the amount of water vapor that exists in a gaseous mixture of air and water". Of course RH is related to the saturated vapor pressure of water, however, it isn't clear how including this observation adds clarity to the definition. D-dawg (talk) 04:51, 30 October 2008 (UTC)
  "Relative humidity measures the amount of water vapor in an atmosphere, relative to the point of complete saturation"
Another definition may be helpful, however, I think this attempt adds ambiguity. First, RH is neither a measurement nor does it measure the amount of water in air. Second, the words relative and saturation have no referents (relative to what?; saturation of what?). Finally, the word saturation is a superlative so the modifier complete is redundant. D-dawg (talk) 04:51, 30 October 2008 (UTC)

I would then suggest a short section intending to explain the basic implications of this definition:

  • 0% implies a complete lack of water vapour (may be this is too obvious!)
  • 100% implies complete saturation. At this point, liquid water will not evaporate: it is in equilibrium with the atmosphere.
At 100% relative humidity water will not evaporate because the vapor is at it saturation conditions NOT becasue it is in equlibrium with the atmosphere. D-dawg (talk) 04:51, 30 October 2008 (UTC)
at 100%RH the thermodynamic equilibrium is between water vapor and liquid water (or ice, at suitable temperatures) Alloy730 (talk) 10:14, 12 January 2009 (UTC)
  • The RH is completely independent of the pressure of any other gases in the atmosphere (ie. it doesn't change with air pressure)
RH is NOT independent of pressure of other gases; RH DOES change with air presssure. If you compress air without a change in temperature (isothermal compression) the RH will approach 100%. D-dawg (talk) 04:51, 30 October 2008 (UTC)
The saturation vapor pressure is very weakly dependent on pressure. But the whole question is context-dependent. For an engineer designing air conditioning systems, pressure is an important factor; for somebody climbing a mountain it is independent. I have added a paragraph illustrating the second point (at the risk of confusing things further). I think the critical point is that the relationship between partial pressure and mixing ratio is pressure dependent and so those who work with these parameters regularly must take pressure into account.Alloy730 (talk) 10:14, 12 January 2009 (UTC)
Saturation vapore pressure may be weakly dependent on pressure, however, relative humidity is not. While I agree the significance of a change in RH is context dependent it is not insignificant. An air-water system which is near saturation at normal pressure will become saturated with a small change in pressure and it is this change to 100% saturation which is normally very significant - it is the difference between water condensing from the vapor phase and into the liquid phase; often it is small changes such as these which are the impetus to measure (or control) relative humidity.D-dawg (talk) 00:17, 10 September 2009 (UTC)

  • The saturation point increases with temperature. If the water pressure in the atmosphere is held constant then the RH will decrease as temperature increases.
If saturation point means the saturation vapor pressure of water then it does indeed increase with temperature. D-dawg (talk) 04:51, 30 October 2008 (UTC)
  • Decreasing the temperature will increase the RH. For a given water pressure, the temperature at which the atmosphere becomes fully saturated is called the "dew point".
This is NOT the definition of the dew point temperature. The atmosphere does not become saturated with water vapor. Water vapor reaches its saturation pressure as a result of the decrease in system temperature. D-dawg (talk) 04:51, 30 October 2008 (UTC)
  • Condensation is likely to form on any surface which is at a temperature below the dew point.
  • RH is related to wind chill, since lower RH means moisture will be absorbed more rapidly.
Moisture is not absorbed (by air); water will evaporate at a rate related to the vapor pressure of water in the atmosphere.
Adsorption and absorption have very specific meanings both of which are unrelated to relative himudity, evaporation, and saturated vapor pressure. D-dawg (talk) 04:51, 30 October 2008 (UTC)
  • RH can be measured by a variety of instruments: ...

Some of these points are repeated through the article, but gathering them together in one place near the top of the article should make it easier for people to grasp the rest of the page. Doing this removes the need for the 'common misconceptions' section.

I hope this helps. AndrewBolt (talk) 19:07, 26 October 2008 (UTC)

Is relative humidity really completely independent of the concentrations of gases other than water vapor?[edit]

Recently I wrote a new introduction to the article, in which I said, among other things, that the RH is “very nearly independent” of gases other than water vapor that are present in the mixture. But D-dawg objected to this statement, and on grounds of this objection changed my edit. In the comment to the change, he or she wrote,

The change to the definition is inaccurate. RH does not depend on other gases AT ALL. The words 'very nearly' imply this to not be the case.

But doesn't the fact that there is such a thing as a pressure enhancement factor (which is discussed in the article, in the section Pressure dependence) show that the other gases do matter, albeit only slightly? Here is a simple argument that says that they do indeed matter:

The section titled pressure enhancement factor is not entirely accurate. The terminology the author uses confuses the saturated vapor pressure, the vapor pressure of water and the enhancement factor. What the author is basically stating is that the vapor pressure of water in a closed system depends on the system pressure. This fact, however, is only indirectly related to the enhancement factor.
To be clear the proper terminology is enhancment factor NOT pressure enhancement factor. The enhancment factor is defined as the ratio of the water vapor partial pressure of saturated, moist air to the saturation vapor pressure of water. The quantity is commonly used in meterology to estimate the saturated vapor pressure or vapor pressure from the dew point temperature.
Where an enhancement factor would be used is in the measurment of relative humidity using a psychrometer. The most simple psychrometers will provide two temperature readings: a wet bulb and a dry bulb temperature. The user then refers to a psychrometric chart to estimate the relative humidity. However, psychrometric data (charts) are most often referenced at sea level. Therefore, in the case where a wet bulb temperature was obtained at an elevation other than sea level it would have to be corrected to properly calculate RH from standardized tables of psychrometric data.--D-dawg (talk) 05:37, 29 October 2009 (UTC)

Consider a sample of nH2O(g) moles of water molecules in a volume V at temperature T. Now let us vary the concentrations of other gases present in the sample, while keeping nH2O(g), V, and T constant. As we do so, the partial pressure of water vapor remains constant, but saturated vapor pressure of water changes (slightly), due to the pressure enhancement factor. Thus RH=(partial pressure)/(saturated vapor pressure) changes as well.
Considering this example in two parts.
Part 1 - the partial pressure of water vapor. Provided that the total number of water molecules in the vapor phase (nH2O(g)) are constant, as presented in the problem statement, the partial pressure of water vapor in the system depends on two quantites only:
(1) the total pressure of the system; and
(2) the total moles of molecules in the vapor phase.
As long as these two quantities remain constant the partial pressure of water vapor will be invaraint to any other changes in the system.
Part 2 - the saturated vapor pressure of water. The saturated vapor pressure of water vapor has nothing to do with this (or any) system; it is a physical property of water that depends only on temperature.
The relative humidity of this system is calculated by dividing the partial pressure of water vapor by the saturated vapor pressure of water at the system temperature (T). --D-dawg (talk) 05:37, 29 October 2009 (UTC)
Therefore, in the above process, RH was (slightly) changing solely due to the changes in the concentrations of the other gases present; the concentration and temperature of water vapor were not changing. Surely, given this, it is incorrect to say that “RH does not depend on other gases AT ALL?”
Relative humidity depends only on the partial pressure of water vapor of a system and the saturated pressure vapor of water. 'Other gases' will change the relative humidity of a system only to the extent that the vapor pressure of water is changed by the 'other gases' (the saturated vapor pressure of water depends only on temperature - nothing else). --D-dawg (talk) 05:37, 29 October 2009 (UTC)

That's the argument. Is there anything wrong with it? Reuqr (talk) 02:03, 26 October 2009 (UTC)

The inconsistency in the argument is that the saturated vapor pressure of water in some way is related to the system of interest. It is not. It is a property of water and the perturbations to the system that were presented in the example do not change the saturated vapor pressure of water
Relative humidity is a very simple concept. However, for some reason it is one that seems to be shrouded in confusion. Keep things simple: RH has two parts - a numerator and a denominator. The denominator (the saturated vapor pressure of water) depends on temperature and nothing else. The numerator (the vapor pressure of water) depends on a lot of different properties . However, keeping things simple (yet accurate), the numerator represents the concentration of water vapor in air. So anything that changes the water vapor concentration will change the relative humidity of the system. --D-dawg (talk) 05:37, 29 October 2009 (UTC)

(Since we can't keep indenting the text forever, let me reset the left margin; I hope that's all right.)

O.K., we do agree at least on the numerator in the definition of RH, i.e. on what the partial pressure of water vapor is. You wrote

provided that the total number of water molecules in the vapor phase (nH2O(g)) are constant, as presented in the problem statement, the partial pressure of water vapor in the system depends on two quantites only:
(1) the total pressure of the system; and
(2) the total moles of molecules in the vapor phase.

I agree; we have pH2O(g) = (nH2O(g) / nTot) × pTot. Of course, at the same time we have pH2O(g) = nH2O(g)RT/V; this follows because pTot/nTot = RT/V. In the statement of the problem, I said that V and T are kept constant; but that means that pTot/nTot is kept constant, too. So indeed, one way or the other, since nH2O(g) is constant as well, it follows that pH2O(g) is constant through the course of the experiment described.

As far as the denominator, we appear to agree at least on the following: at a given temperature, the following two things are not the same:

(1) the water vapor partial pressure of saturated, moist air (let's call this  p^{\scriptscriptstyle \text{SMA}}_{\text{w}}, where “SMA” is short for “saturated moist air”);
(2) the saturation vapor pressure of pure water vapor (let's call this  p^{\scriptscriptstyle \text{PWV}}_{\text{w}}, where “PWV” is short for “pure water vapor”).

Indeed, at any given temperature, we have that  p^{\scriptscriptstyle \text{SMA}}_{\text{w}} > p^{\scriptscriptstyle \text{PWV}}_{\text{w}} . The enhancement factor f_{\text{w}}(T,p) is given by  f_{\text{w}}(T,p)=\frac{p^{\scriptscriptstyle \text{SMA}}_{\text{w}}}{p^{\scriptscriptstyle \text{PWV}}_{\text{w}}}>1. This much we all agree on, correct?

Now we come to the remaining disagreement. If I am understanding you correctly, you are saying that the denominator in the definition of RH should be  p^{\scriptscriptstyle \text{PWV}}_{\text{w}} . I claim that the denominator should in fact be  p^{\scriptscriptstyle \text{SMA}}_{\text{w}} .

It is a question of fact as to what the definition of relative humidity is. A number of bona-fide sources that I have access to provide a clear and simple definition. Perry's Chemical Engineers Handbook (the same source listed in the article) I present to be the best of these sources states:
Percent relative humidity is defined as the partial pressure of water vapor in air divided by the vapor pressure of water at a given temperature. Thus RH = 100p/pS."
Preceeding this text, Perry provides definitions for p and for pS that are consistent with my understanding of these quantities.
I considered several of your points; suffice to say we still have disagreement. However, for a meaningful discussion to occur the definition of RH has to be agreed upon. Once this is established the remaining points can be constructivley discussed. I suggest we work together in this regard. --D-dawg (talk) 18:03, 31 October 2009 (UTC)
I completely agree that we need to reach an agreement on the definition of RH, and that the only thing that matters is what definition is used in reputable sources. Since the debate now switches to the debate abut what reputable sources say or not say, I will continue it in a new section, below. Reuqr (talk) 03:07, 1 November 2009 (UTC)

Let x_{v} be the mole fraction of water vapor in a given sample of moist air characterized by pressure p and temperature T, so that the partial pressure of water vapor is x_{v}p. Let me define the following quantities:

\text{RH}_{\scriptscriptstyle \text{D-dawg}} = 100 \times \frac{x_{v}p}{p^{\scriptscriptstyle \text{PWV}}_{\text{w}}}, and
\text{RH}_{\scriptscriptstyle \text{Reuqr}} = 100 \times \frac{x_{v}p}{p^{\scriptscriptstyle \text{SMA}}_{\text{w}}}.

Can we agree on the following statements:

1. You claim that the real RH is RHD-dawg, and I claim that it is RHReuqr.
2. RHD-dawg does not “depend on the presence of other gases,” but RHReuqr does.

(By “depend on the presence of other gases,” I really mean this: in the experiment I described above, as we vary the concentrations of other gases present in the sample, RHD-dawg remains the same, but RHReuqr changes.)

If we can agree on this, then we can proceed; if not, then don't read any further—tell me what is it that you disagree with in what I said thus far. Everything that follows assumes that we agree on 1. and 2.

Now, why do I think that RH=RHReuqr. First, there is an a priori reason: it seems to me that the one thing we definitely want the definition of RH to do is to give 100% just at the point when water vapor starts to condense (more precisely, RH should be 100% at the exact point when the water vapor reaches equilibrium with the liquid, or possibly solid, phase of water). And this will be so only if RH=RHReuqr, and not if RH=RHD-dawg.

Second, while I haven't done anything like an exhaustive search, my impression is that the standard definition of RH is indeed one where the denominator is  p^{\scriptscriptstyle \text{SMA}}_{\text{w}} and not  p^{\scriptscriptstyle \text{PWV}}_{\text{w}} . The most explicit example I have been able to find is in Handbook of psychrometric charts: humidity diagrams for engineers by D. C. Shallcross (Blackie Academic and Professional, London, 1997), Eqs. 2.13. and 2.10 on p. 12 and Eq. 2.7 on p. 11; note the explicit appearance of the enhancement factor in Eq. 2.10.

At this point I could list at least six other sources that seem to be saying the same thing, more or less explicitly. They are mostly in one or another kind of engineering. In general, every source I looked at that mentioned the existence of the enhancement factor seems to be saying that RH=RHReuqr. As far as atmospheric science books, they typically would not mention the enhancement factor, which is not surprising because in that science the enhancement factor can presumably be assumed to be 1. So it is unlikely that an atmospheric science source could shed any light on the RHReuqr vs. RHD-dawg issue.

However, as best as I can tell, the other kinds of sources are pretty much all saying that RH=RHReuqr. That's how things look from my end. Reuqr (talk) 05:23, 31 October 2009 (UTC)

What is in the denominator of the definition of RH?[edit]

This section is the continuation of the previous one, which has become a bit too long for comfortable editing.

The issue is to clarify which of the following two quantities appears in the denominator of the definition of RH:

(1) the water vapor partial pressure of saturated, moist air (let's call this  p^{\scriptscriptstyle \text{SMA}}_{\text{w}}, where “SMA” is short for “saturated moist air”);
(2) the saturation vapor pressure of pure water vapor (let's call this  p^{\scriptscriptstyle \text{PWV}}_{\text{w}}, where “PWV” is short for “pure water vapor”).

Depending on which one of these is chosen to be in the denominator, we get the two competing definitions for RH, which above I labeled RHReuqr and RHD-dawg. For readability by people other than D-dawg and me, let us relabel them RHSMA and RHPWV.

D-dawg, of course I agree that whatever the merits of my “a priori arguments,” the definition of RH in the article must be the one used in reputable sources. Let me postpone the discussion of what Perry's Chemical Engineers Handbook says for a moment. Whatever Perry's says, can you at least acknowledge that there is indeed at least one reputable source that says RH=RHSMA, namely Shallcross's Handbook of psychrometric charts: humidity diagrams for engineers that I referenced above? The links I provided are to Google Books; if you have trouble accessing them, I can send you page images.

Shallcross defines relative humidity in Equation 1.7 on page 2 as:
"...the ratio of the partial pressure of the vapor pressure of the vapor component at some temperature to the vapor pressure of the component at that same temperature.
This definition is entirely consistent with Perry and indicates that the denominator is, in fact, the vapor pressure of water. I'm not sure what more can be said in this respect; Shallcross along with Perry provide clear, unambiguous, and equivalent definitions for relative humidity.
On the enchancement factor and RH. Shallcross presents Equation 2.7, however, the passage preceeding Equation 2.7 clearly states that the equation is an approximation:
"...When the partial pressure of the condensing component reaches is vapor pressure at a given temperature the gas is saturated. If the system were to be ideal then we could write the expressions..."
Then Shallcross presents Equation 2.7. Therfore the underlying assumption of Equation 2.7 is ideality. The enhancement factor is introduced into Equation 2.7 to account for the non-idealities of real systems (some of which are mentioned on the top of page 12). The outcome is Equation 2.10, which includes an explicit reference to the enhancement factor. But this is not the definition of relative humidity; relative humidity is defined by Equation 1.7.
Furthermore, if Equation 1.7 and Equation 2.10 (or any derevation of this equation thereof) are coupled then all subsequent relationships will carry the assumption of ideality introduced by Equation 2.7. Including the enhancement factor via Equation 2.10 does not relieve the assumption of ideality introduced by Equation 2.7.
It is important to note that relative humidity is defined without reference ideality. Mathematically, Equation 2.10 or Equation 2.7 can be coupled with the defintion of relative humidity (Equation 1.7). However, it would be incorrect to conclude that the relative humidty implicitly depends on the enhancement factor; doing implies that relative humidity somehow assumes ideality in the system of question, which is not the case. --D-dawg (talk) 04:04, 2 November 2009 (UTC)

What made the definition in Shallcross so unambiguous is that it clearly included the enhancement factor in the definition of RH (this follows immediately from Eqs. 2.7, 2.10, and 2.13). Here is another source that also explicitly includes the enhancement factor in its definition of RH: S. Hasegawa and J. W. Little, “The NBS Two-Pressure Humidity Generator, Mark 2” (J. of Research of the Nat. Bureau of Standards-A. Physics and Chemistry 81A, 81-88, 1977), publicly available here. They say RH=(xv / xw)Pc,Tc× 100, where

xv = “the mole fraction of water vapor in a given sample of moist air characterized by pressure, pc, and temperature, Tc,” and
xw = “the mole fraction of water vapor in the saturated mixture at the same values of pressure, pc, and temperature, Tc.”

This already sounds like RHSMA (they talk of saturated mixture), but just to make sure, let's try to see the explicit appearance of the enhancement factor.

This humidity generator works as follows: first one creates a fully saturated air sample at high pressure ps (note that s here stands for “saturator”). Then one expands the gas while keeping the same temperature, so that the pressure drops to pc (c stands for “chamber,” as in “test chamber”). Clearly the new state of moist air is unsaturated; so, what is its RH? Hasegawa and Little say that it is this:

RH = \frac{f_{\text{w}}(T_{s},p_{s})}{f_{\text{w}}(T_{c},p_{c})} × \frac{p^{\scriptscriptstyle \text{PWV}}_{\text{w}}(T_{s})}{p^{\scriptscriptstyle \text{PWV}}_{\text{w}}(T_{c})} × \frac{p_{c}}{p_{s}} × 100.

That's Eq. (3) in the paper, where instead of  p^{\scriptscriptstyle \text{PWV}}_{\text{w}}(T) they use the symbol e_{\text{w}}(T); also, in the actual machine, the temperatures of the saturator and the test chamber are the same, T_{s}=T_{c}. Here f_{\text{w}}(T,\, p) is just the enhancement factor, and is given by their Eq. (2),

f_{\text{w}}(T,p)=\frac{x_{w} p}{p^{\scriptscriptstyle \text{PWV}}_{\text{w}}(T)},

in complete agreement with the definition of f that you and I agreed on in the previous section of this discussion.

Now look at their expression for RH. It can be rearranged as

\text{RH} = 100 \times \frac{\left(\frac{f_{\text{w}}(T_{s},p_{s})p^{\scriptscriptstyle \text{PWV}}_{\text{w}}(T_{s})}{p_{s}}\right) \times p_{c}}{f_{\text{w}}(T_{c},p_{c})p^{\scriptscriptstyle \text{PWV}}_{\text{w}}(T_{c})}.

From the definition of f, the term in the parentheses in the numerator gives (x_{w})_{p_{s},\,T_{s}}, i.e. the mole fraction of water vapor in the saturator (since in the saturator the air-water mixture is saturated). After the expansion, the total pressure drops to p_{c} but the mole fraction remains the same. Therefore, the numerator as a whole is simply the partial pressure of water vapor in the test chamber. Finally, the denominator is clearly  p^{\scriptscriptstyle \text{SMA}}_{\text{w}} in the test chamber. Thus the definition of RH used in this paper is the one where the numerator is  p^{\scriptscriptstyle \text{SMA}}_{\text{w}} and not  p^{\scriptscriptstyle \text{PWV}}_{\text{w}}.

The discussion in Hasegawa and Little is restated basically verbatim in another reputable source: Water Vapor Measurement: Methods and Instrumentation by P. R. Wiederhold (Marcel Dekker, New York, 1997), on p. 180. On p. 69 of that same source, we find a definition of RH whose denominator is “the water the [sic] vapor pressure if the air were saturated”; I think the reference to air is significant, and it says that Wiederhold thinks that it is  p^{\scriptscriptstyle \text{SMA}}_{\text{w}} and not  p^{\scriptscriptstyle \text{PWV}}_{\text{w}} that is in the denominator (after all, only in this case will his definition on p. 69 be consistent with the discussion of the humidity generator on p. 180; yes, it is possible for a book to be internally inconsistent, but I don't think this book is, about this).

D-dawg, before we turn to the discussion of Perry's Chemical Engineers Handbook, may I assume that we agree that the following three sources, at least, are (a) all reputable and (b) all say that RH=RHSMA:

1. Shallcross's Handbook of psychrometric charts;
2. Hasegawa and Little's “The NBS Two-Pressure Humidity Generator, Mark 2”; and
3. Wiederhold's Water Vapor Measurement?
We do not agree that these sources define as RH=RHSMA. Shallcross clearly does not define relative humidity in this manner (as previously noted). Hasegawa defines relative humidity in terms of mole fractions, not partial pressures (page 82, line 4) and the enhancement factor is NOT referenced in Hasegawa with respect to the definition of relative humidity. Wiederhold defines relative humidity on Page 19 (Equation 2.24) in precisely the same manner as Perry AND Shallcross - they incorporate the saturation vapor pressure of water.
Be careful distingushing between definitions and approximations. Shallcross and Wiederhold both define relative humidity and then make approximations by substiuting the appropriate quantities.
I do agree that Hasegawa defines relative humidity different from Perry, Shallcross, and Wiederhold. In fact, others also define relative humidity in the same manner as Hasegawa and it is fair to say that relative humidity is NOT uniquely defined in the literature. However, I have yet to find a source which incorporates the enhancement factor in the definition of relative humidity.--D-dawg (talk) 08:04, 7 November 2009 (UTC)

Turning to Perry's: yes, from the definitions on p. 12-4, it would seem that they say RH=RHPWV. But now take a look at the last paragraph of the section “Calculation formulas” on p. 12-5, the one where the enhancement factor gets mentioned. Also note that the Sonntag equation is given a bit above in the same column, as an equation for \log \, p_{s}. Now ask yourself this: if the authors truly believed that ps is strictly (rather than approximately) p^{\scriptscriptstyle \text{PWV}}_{\text{w}}, would they really write the sentence “The Sonntag equation strictly only applies to water vapor with no other gases present”? Isn't this an odd thing to say?

I don't know why it would be odd to say that and in fact, this is exactly what the Sonntag equation is stating. It is an emprical relationship to estimate the vapor pressure of water. Perry also suggests the Antoine equation or the Magnus equation all of which are empricial relationships used to estimate the pure component vapor pressure of water.
The vapor pressure of a substance is the pressure of the gaseous phase over the condensed phase with no other gases present. That is the definition of vapor pressure.
I cannot guess what the authors were thinking, why they made a choice of language, what they meant to say, or what they believe. I can only interpret the content objectively and my assumption is, that in a creditable, peer reviewed source such as Perry's, the content is detailed, complete and accurate. I suggest we not use our thoughts, feeling, or beliefs as a basis for resolving the discussion on RH. As I stated previously, it is a question of fact as to what the definition of RH is. What you and I think or feel is not relevant.--D-dawg (talk) 05:21, 2 November 2009 (UTC)

After all, to what else but “water vapor with no other gases present” would they possibly want to apply the Sonntag equation, given that it is an equation for ps, which, as we are assuming, is strictly p^{\scriptscriptstyle \text{PWV}}_{\text{w}}? Wouldn't they rather write something like the following: “From the knowledge of ps as given [exactly, for all practical purposes] by the Sonntag equation, one can also compute another quantity [as in, distinct from ps] which is occasionally of interest, namely p^{\scriptscriptstyle \text{SMA}}_{\text{w}}. All that is required is to multiply ps by an enhancement factor f, etc.”?

There is an alternative that makes more sense. Try this exercise: imagine that in the minds of the authors, ps always really meant p^{\scriptscriptstyle \text{SMA}}_{\text{w}}, and that the definition of p. 12-4 was an approximate one. Now look at the sentence “The Sonntag equation strictly only applies to water vapor with no other gases present”. It no longer looks odd, does it?

Consider also in what manner the authors treat the enhancement factor: as a mere afterthought. It's just one paragraph, no approximate equations but only a reference to British Standards, and the closing statement that “it [the enhancement factor] is therefore usually neglected for engineering calculations.” Don't you think that this is what's going on: the authors know full well that ps in the definition of RH is in reality indeed p^{\scriptscriptstyle \text{SMA}}_{\text{w}}, but they simply didn't want to complicate the discussion of basic definitions by something about which they will eventually say that it can be neglected!

I do not concur with this conclusion. Not at all. I believe your suggestion that Perry's is concerned with NOT complicating discussions and as such is neglecting to include relevant information in thier discussions is nonsense. In fact, ANY source which does so without qualifying such simplifications is NOT creditable.
The enhancement factor is not excluded from the definition of relative humidity becase it would complicate the discussion. The enhancement factor is not included in the definition because, in fact, relative humidity is defined without reference to the enhancement factor.--D-dawg (talk) 05:21, 2 November 2009 (UTC)

Reuqr (talk) 02:54, 1 November 2009 (UTC)

Hi, although this conversation seems to be a month old, I feel the need to chime in here. The misunderstanding seems to lie in the definition of partial pressure. Imagine a closed container with gases a, b, and c. The partial pressure of component a is defined as the pressure it would exert if it were alone at that temperature and volume. Therefore, p_sma is defined as p_pwv. They are equal. (talk) 01:55, 13 December 2009 (UTC)

Partial pressures of gaseous components in a mixture and their pure component vapor pressures are, generally, NOT equal.
The question goes beyond partial pressures. In the context of your example the question becomes: Can the pure component vapor pressures of a, b, and c be used as a suitable proxy to estimate the partial pressures of the three components in the mixture?
If the components a, b, and c behave as ideal gases the answer is yes. If water is one of the components a, b, or c then p_sma and p_pmv are identical. This is the case, however, only when the mixture is assumed to be ideal. In fact, p_sma and p_pmv are NOT identical in the general case.
The nut of the discussion, however, is how is relative humidity defined. Is it in terms of the pure component vapor pressure of water or the measured vapor pressure of water in saturated moist air (these two quantities are NOT identical, hence the definition of the enhancement factor, which is defined as their ratio and has a value different than unity). Reuqr has presented several sources that corroborate that relative humidity is defined with respect the pure component vapor pressure of water vapor. A source that defines RH in terms of vapor pressure of water in saturated moist air has yet to be presented.
Although the vast majority of sources present RH as described above it is fair to say the quantity is not uniquely defined in the literature. D-dawg (talk) 21:58, 14 December 2009 (UTC)
After reading the (very long) discussion above I must agree with Reuqr. Not only the sources mentioned here but also the first five google hits (after this article) for "relative humidity" all talk about the saturated mixture or the amount of water vapor the air can hold (Some are ambiguous in some sentances but clear in others). I will not enter into a discussion similar to the one above, but would like to recomend a change in the definition in the article so that it follows these reputable sources. D-dawg, can you please read the definition sentences in the sources again with an open mind. Ulflund (talk) 23:28, 15 July 2012 (UTC)
I don't know what more can be said. Reuqr's argument is not consistent with the sources that were cited. I suggest you review (with an open mind) the sources cited: Shallcross, Hasegawa, Wiederhold, and Perry's. The aforementioned discourse breaks it down. The sum total of Reuqr's argument comes down to the assertion of what authors were thinking and thier desire not to complicate the discussion of RH. That is absurd.
You are suggesting that the first five Google hits are reputable sources? I propose we use Perry's, Shallcross, or Wiederhold as a reputiable source for the definition of RH as they are academic, peer reviewed sources. Air don't hold water - no matter what Google says. — Preceding unsigned comment added by (talk) 05:44, 14 October 2012 (UTC)

Buck equation[edit]

The Buck (1996) equation as written doesn't seem to give sensible answers, and differs from what is presented at (talk) 07:44, 23 December 2009 (UTC) Ross

True. I just corrected it (parenthesis mismatch). + added the usual approximation (+ref). 17 Aug 2010; Fabrice NEYRET —Preceding unsigned comment added by (talk) 14:06, 17 August 2010 (UTC)


when the relative humidity is given..??? why is it also important to know the air temperature..??? (7th june 2010) —Preceding unsigned comment added by (talk) 14:10, 7 June 2010 (UTC)

RH defined as the ratio of the partial pressure of water vapopr  \left({e_w}\right) in the air to the vapor pressure of pure water vapor  \left({{e^*}_w}\right) .  {{e^*}_w} exponentially increases with temperature.

So to answer your question. To only say the RH is 75% is not saying much. Why? Because, if the air temperature were 12°C (54°F) then the amount of water vapor present in the air is small. If RH is 75% and the air temperature were 30°C (85°F) then there is a cosiderable amount of water vapor in the air.

The math is straight forward. At 12°C the vapor pressure of water  {{e^*}_w} is 14.0 mbar; at 30°C it is 42.5 mbar. So at 12°C the partial pressure of water vapor  \left({e_w}\right) is 10.5 mbar ( = 14.0 × 75%) whereas at 30°C the partial pressure is 31.9 mbar ( = 42.5 × 75%). This example illustrates that for a given RH (75%) the amount of water vapor in the air can vary considerably (in this case more than 3 times; 31.9 mbar versus 10.5 mbar). (talk) 14:11, 9 June 2010 (UTC)-- (talk) 14:11, 9 June 2010 (UTC)

What does it mean...[edit]

When the article says that, at 0% relative humidity, 24 degrees celsius FEELS like 21, and at 100% humidity FEELS like 27? I followed the ref. hoping to find an explanation, but the info in this article is pretty much a word-for-word quote. The site doesn't explain what is meant by FEELS, and it looks like a wishy-washy random number someone came up with to make "it feels colder when it's arid" look more scientific. (talk) 15:54, 18 August 2010 (UTC)

Are you being serious?
The passage presents two relevant points: The first point is the fact that humans percieve the rate of heat transfer from the human body rather than temperature itself. The second point is the example that if the air is 24 °C and contains saturated water vapor, then the human body cools itself at the same rate as it would if it were 27 °C and dry.
It's clear that the term 'feel' is connecting the rate of heat transfer to humans perception of temperature.
There are no wishy-washy random numbers and in no place is it stated or implied that "it feels colder when it's arid" - that is not what is stated at all. Wow. (talk) 23:00, 24 August 2010 (UTC)
It's 'tards like who have zero reading comprehension that drag WP down into the dumps. -- (talk) 20:44, 28 February 2011 (UTC)
Actually, you guys are begging half the question. The basis for comparison is clear now -- two temperature/humidity pairs feel the same if the rate of cooling (as computed by evaporation rate) is the same. The problem is that the WP article is not being as precise as you are being in your well-considered comments : the article compares a temperature/humidity pair to just a temperature: For example, if the air temperature is 24 °C (75 °F) and the relative humidity is zero percent, then the air temperature feels like 21 °C (69 °F). (talk) 00:17, 14 September 2014 (UTC)

A definition in the summary[edit]

At the risk of attracting the ire of all the knowledgeable people above, I've added a simple definition in the summary. Most of the article will make no sense to lay people. (talk) 19:03, 13 October 2010 (UTC)

The definition was removed [2] with a sarcastic edit summary. No, the previous definition was *not* fine, it was too short and too lacking in information, see WP:LEAD. Anyone is welcome to provide a better definition but just cutting it out altogether is not constructive. (talk) 17:39, 30 October 2010 (UTC)

Reverted again, really should read WP:LEAD and then write their own idea of a decent introduction. It's no good leaping from a 1-line lead straight into something far too technical for average users, so that they then need to look for a sensible explanation from another site. (talk) 18:30, 5 November 2010 (UTC)

Consistency of Units[edit]

More than a few times this article has be edited and the % sign from the RH formula has been removed. This edit leaves the RH formula inconsistent. Here is why.

Percent is the expression of a quantity on a 'per one-hundred' basis. Mathematically, the percent symbol (%) can be treated as a constant that is equal to 0.01. Therefore, % = 0.01 = 1/100.

With that in mind, the following is true: 100% = 100 × % = 100 × 0.01 = 1.

In the context of this article it is incorrect to remove the percentage sign leaving the whole number 100 in the definition. This is true because 100 ≠ 1, which implies that 100 ≠ 100%. Therefore, replacing 100% with 100 is not replacing like-for-like. This is, incorrectly, changing the formula for RH.

Removing the quatity 100% from the definition leaving:

 \phi  =  {{e_w} \over {{e^*}_w}}

is acceptable, however, the verbiage leading into the definition would also have to be changed becasue the results from this formula would be decimal (or fractional) rather than on a percentage basis.

I recongnize this is a small detail, however, I thought I'd explain my revert for those who may be interested and (hopefully) prevent future editing out of the % symbol. --D-dawg (talk) 19:51, 14 February 2011 (UTC)

RH and gas mixture composition[edit]

In this edit, some text was deleted, in particular this: According to a common misconception, air is said to be unable to 'hold' any more water vapor than 100% relative humidity. In practice, however, relative humidity can exceed 100%. In nature, it does so in virtually all clouds, where it is often around 101%. In a laboratory, it is not difficult to fill a container with pure water vapor at 300% relative humidity. I have restored it (but in the Misconceptions section rather than the introduction paragraph) since it does not seem to be incorrect to me; it seems to have been discussed in depth earlier on the talk page; I didn't take the time to check in detail. Han-Kwang (t) 14:11, 29 April 2012 (UTC)


I'm afraid I didn't find this article very enlightening. As I understand it, relative humidity is a measure of the proportion of water vapour in the air. So if relative humidity is 100%, does that mean that the air is 100% water vapour? Surely that can't be so, since that would be unbreathable. I read today that the humidity in Singapore is 140% How is that possible? How can anything make up 140% of anything else? Obviously I am wrong in my understanding, but this article has not clarified the matter for me. Intelligent Mr Toad (talk) 03:56, 11 June 2012 (UTC)

After reading the definition of RH I'm not sure how it can be concluded that RH describes the proportion of water vapor in the air. It does not. This addresses your second question: 140% RH does not speak to the proportion of one thing of another thing.
100% RH means, very simply, that the partial pressure of water vapor in the air is equal to the saturated vapor pressure of water at those same conditions. An air-water mixture at this condition is said to be saturated with water vapor because at the given temperature and pressure the partial pressure of water vapor cannot, under normal circumstances, increase. — Preceding unsigned comment added by (talk) 20:26, 12 June 2012 (UTC)

I'm afraid that means nothing to me. What does "pressure" mean in this context? Pressure of what, on what? If 100% RH means that the air is "saturated" with water vapour, what does 140% RH mean? I shouldn't have to ask these questions, since the article should have been able to make these things clear. The test of an encyclopaedia article on a scientific issue must be that it makes the issue understandable to a reasonably intelligent lay-person, such as myself. As it stands, this article fails that test. Intelligent Mr Toad (talk) 23:48, 12 June 2012 (UTC)

Really? You cannot derive that the pressure referenced is the partial pressure of water vapor. This is written explicitly at least three times.. I suppose if the reader does not have a grasp of the concept of pressure than the article is confusing, but that is hardly a problem with the article. Relative humidity is defined in terms of partial pressures- there is no way around that fact. The reader really needs to have an understand of what a (partial) pressure is. Again not a problem with the article.
I'm not sure how a reasonably intelligent person could not conclude, based on the definition of RH, that an RH of 140% means that the partial pressure of water vapor In air is 1.4 times that saturated pressure of water vapor at the conditions specified. Comprehension really needs to be given a bona-fide attempt, which based on the earlier comments, i think could be improved. Somehow the concept of 'proportion' and the 'percentage of one thing in another' was drawn from the passage. These concepts are not introduced at all in the Wiki of RH, not in the least. — Preceding unsigned comment added by (talk) 05:21, 16 June 2012 (UTC)

OK let's try again. Firstly, could you please sign your comments, as is the usual convention here? Secondly, the purpose of an encyclopaedia is to provide knowledge to those who do not have it, particularly on subjects outside their own areas of expertise. I have a doctoral degree in another field, so I am not uneducated, but I confess to knowing almost no physics. This article must be written in a way that is comprehensible to people with no knowledge of physics, otherwise it is a waste of space. I came to this article because I wanted to know how it was possible for Singapore to have 140% humidity. I understand that my previous belief, that humidity is a measure of the percentage of water vapour in the air, is incorrect. But having read the article several times, I still do not understand what humidity is. The article says: "It is defined as the ratio of the partial pressure of water vapor in the air-water mixture to the saturated vapor pressure of water at the prescribed temperature." That sentence has no meaning for me, because I don't know what "partial pressure" means and I don't know what "saturated vapor pressure" means. There are links to articles on both those terms, but I don't understand them either. The "partial pressure" article says: "In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume." The "saturated vapor pressure" article says: "Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system." So technical terms are explained by recourse to more technical terms. This is not encyclopaedic writing, and I should not have to follow a further set of links to find out what is meant. If it is really necessary to use technical terms, they must be explained in the article: not by recourse to more technical terms, but in plain English. Is that really beyond the ablility of all those working on this article? Intelligent Mr Toad (talk) 12:07, 16 June 2012 (UTC)

Ok, one more time, I suppose. Apparently you are not able or not willing to resolve the meanings of terms such as partial pressure and saturated vapor pressure. These concepts are integral and ancillary to the definition of relative humidity. Without a grasp of these concepts you will fail to understand what RH is. I'm not sure how one can covey the concept of RH void of these terms when the ratio of these quantities is precisely the definition of RH.
The Wiki article on partial pressure has a relatively high B class quality rating by the Wiki community. You don't understand it. You don't feel you should have to investigate further terms and concepts explained in that article or in the article herein. You say that this is not encyclopedic writing, yet Wikipedia characterizes itself as an online encyclopedia. I'm not sure you would garner the support from the Wiki community on these thoughts and objectives.
Your question with respect to Singapope is how is 140% RH possible, previously you asked what does 140% RH mean. These are two different questions, which one are you trying to answer? The latter question is readily explained by the definition of RH, the former question is much more vexing an explanation (which isn't covered well in the RH article). With neither an understanding of chemistry/physics nor the desire/ability to investigate the concepts relevant to the article in question I'm afraid you will remain unenlightened. — Preceding unsigned comment added by (talk) 17:04, 16 June 2012 (UTC)
I have tried to add a sentence to the opening paragraph of the article to clarify the issue raised by IMT above, but I could not find a way to word it that avoids the use of the term 'partial pressure' without creating ambiguity or ending up explaining the concept of partial pressure. I thought of something like Put differently, it is the amount of water vapor relative to the maximum amount of water vapor that, under normal conditions, can exist at a given temperature. However, I don't like the word "amount" because it doesn't imply that it is per unit of volume. I considered "concentration" instead, but this would be misleading as it suggests that it is about the water-air ratio. Han-Kwang (t) 19:59, 16 June 2012 (UTC)
Therein lies the issue. RH is a ratio of pressures - partial pressure and saturated vapor pressure have a specific and unambious meanings. If one uses something else as a proxy for them the result will be confusion, ambiguity, or worse. The failing normally is in the readers: RH is a deceptively simple concept but, in reality, is somewhat abstract. The reader really has to comprehend what is being said without preconceived ideas of what RH is (or is not). — Preceding unsigned comment added by (talk) 20:44, 16 June 2012 (UTC)
Reading through this talk page I see that there are many readers who are dissapointed by this article, thinking that it is too technical. The concept of relative humidity is not complicated and this article should be able to convey it to the layman. Although the strict definition is in terms of partial pressures, the concept can be explained in simpler terms. Above I find the sentence "Relative humidity 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." That single sentence would be a lot more helpfull to most readers than the current lead. It is not as precise, but it is simpler to understand, not false, and it cannot be misunderstood. (I expect that D-dawg will jump at this though) Ulflund (talk) 22:38, 15 July 2012 (UTC)
I tried to make the intro more clear [3], but it was reverted immediately. I agree with you, Ulflund, but I don't want to waste my energy arguing/reverting back and forth. Han-Kwang (t) 09:43, 17 July 2012 (UTC)
Relative humidity is a measure of the amount of water vapour in the air compared to the maximum that could be contained by the air....It is not as precise, but it is simpler to understand, not false...
That sentence is false, that is the problem. Air does not contain, hold, or dissolve water vapor and the maximum amount of water vapor in the air has nothing to do with air itself (it has everything to do with temperature). So that suggested sentance, albiet simple to understand, is not simply imprecise, it is wrong. What good is that? Nobody is defending the need to keep complexity in the article but you can't replace it with false information. — Preceding unsigned comment added by (talk) 05:55, 14 October 2012 (UTC)


I just removed one sentence from the misconception section. I believe it was creating more confusion than it was clearing up. Basically: the removed sentence said that "humidity" can be measured in an air-less environment. That is true, but it is not relevant, because this article is about "relative humidity", not "humidity". Relative humidity cannot be measured in an air-less environment. I am drawing that conclusion based on the definition provided in the 1st sentence of the page, which states that RH refers to a mixture of water vapor and air. If someone wants to re-enter the sentence that I removed, then we would need to change the definition of RH used in this article, because otherwise it is inconsistant. — Preceding unsigned comment added by (talk) 13:28, 11 July 2012 (UTC)

Disagree. RH can be measured in an air-less environment. The definition of RH has no reference to air - it is a ratio of vapor pressures of water. Air is not referenced Yes RH refers to an air water mixture but an air water mixture which has 0% air and 100% water qualifies as a relevant context. The definition does not have to be changed to accommodate this specific cas. There is no inconsistency here. — Preceding unsigned comment added by (talk) 06:11, 12 July 2012 (UTC)

I disagree. You said, "Yes RH refers to an air water mixture but an air water mixture which has 0% air and 100% water qualifies as a relevant context." However, 0% air and 100% water would not fit the definition of the word "mixture", so this would not represent a mixture of air and water. Consequently, RH could not be applied to this situation - at least not with the current wording on this page.

On the page for "Mixture", Wikipedia uses this definition: "In chemistry, a mixture is a material system made up by two or more different substances which are mixed but are not combined chemically."

0% air and 100% water does not contain two or more different substances; therefore, this system is not a mixture. — Preceding unsigned comment added by (talk) 18:30, 21 August 2012 (UTC)

Wrong again. A mixture is considered to be a continuum. It is not being argued that "100% water" is nothing other than water. Qualifying water with 100%, however, is redundant and unnecessary in the general case. In the context of a mixture the possibility exists that air is present (by definition), even though it may not be in the specific case in question. When the notion of a continuum is properly applied the apparent disparity is resolved. — Preceding unsigned comment added by (talk) 05:26, 14 October 2012 (UTC)

Good grief, why can't articles on topics like this...[edit]

be written at least in part for a lay audience? Blah-blah-blah technicalities.

My specific question, the answer to which may well be buried somewhere in this article, but a simple subhead and explanation would help: If the temperature rises or falls, what happens to the relative humidity? From "Relative humidity changes when temperatures change. Because warm air can hold more water vapor than cool air, relative humidity falls when the temperature rises if no moisture is added to the air." Yeah, it's easier to just Google.

Updated Misconception section[edit]

After reading the citations from this section I re-read the section and realized that the third paragraph was stating exactly the opposite of the first two. I re-wrote the third paragraph to be consistent with the first two and also consistent with the information from the citations. I'd like to thank who ever added those citations. It was fascinating reading. I was appalled to find that I was a victim of this misconception and that I have been spreading it. Thank you for the correction. Also, for the person who felt unenlightened, the citations explain how relative humidity can be greater the 100%. I always found that galling as well. In short, it is because the measurement for relative humidity is contrived to be a from flat source of water, where as in nature much of the water sources are very small spheres (water droplets.) For more details, check out the citations.

I liked them too. I particularly liked the section on boiling in the second reference -- a powerful description, even if it takes some thought to get the point. But I edited the section to be more concise and less tendentious. I hope I was able to get the idea across in fewer words. I also reduced the profile of the word "saturation" in the article without eliminating it -- I get the point about this word being misleading, but I don't have the strong emotions about it that some authors have!!
The main thing I got out of the discussion in the two references is that the water-vapor equilibrium process occurs at the water/water-vapor boundary. On the other hand, outside the water body, the fluid is not in equilibrium. In fact, both the temperature and the water-vapor concentration vary (spatially) from their boundary values as you move away from the water, and at least one of them must vary, because the dew point and the temperature (typically) stop being equal when you have left the boundary. This disequilibrium gets varied by convection and diffusion. In any case it is a spatial disequilibrium of temperature and of concentration, not a pointwise disequilibrium of the water/water-vapor phase change. Indeed, that latter is in equilibrium -- if you allow the question to be asked where there is no nearby low curvature water interface -- because the temperature is above the dew point and there is (as required) no water.
Of course there is also the case of being above 100% relative humidity which turns precisely on there being no nearby low-curvature water surface, but we'll leave that for another day... (talk) 00:10, 14 September 2014 (UTC)
The phrase used in the references is "saturation vapor pressure" and it refers specifically to vapor over a flat sheet of clean (and pure) water. "Equilibrium vapor pressure" could be over salt water, or in clouds where the small spheres cause the actual "equilibrium vapor pressure" to vary from the "saturation vapor pressure". These phrases do not mean the same thing. Robert - Northern VA (talk) 09:33, 28 October 2014 (UTC)


Both an increase in temperature, pressure, and, to a much lesser degree, humidity will cause an increase in density altitude. Thus, in hot and humid conditions, the density altitude at a particular location may be significantly higher than the true altitude.

Isn't it the opposite of this for pressure? If the pressure is higher than the assumed standard pressure, then density of the air will be higher, so the density altitude will be lower.

Concerning the humidity, won't larger-than-standard humidity also cause a (small) increase in the density of the air, and thus a decrease in the density altitude? I'm assuming that the air density is defined to include the water vapor. If not, why would humidity have any effect? (talk) 10:33, 13 September 2014 (UTC)

You are correct about the pressure - I have removed it.
Water molecules have less mass the "air" molecules. As a result, humid air is less dense than dry air at the same temperature and pressure. Robert - Northern VA (talk) 09:13, 28 October 2014 (UTC)

Pressure dependence section[edit]

The bulky calculation in this section should really be removed. It looks like one editor's attempt to convince another editor of something.

It could be reduced to a much shorter statement: if the ambient pressure increases without anything else changing (no water added or removed and no temperature change), then the relative humidity will increase in the same proportion.

(Proof: the partial pressure of water vapor will increase in the same proportion as the overall pressure, but the equilibrium vapor pressure of water doesn't change.)

We don't need a heavy-handed treatment of this fact (except in the back room if there is some doubt about it). (talk) 17:42, 14 September 2014 (UTC)