Talk:Relative humidity

message: 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)
thread_title:  Condensed water on car window - heating or cooling? 

message: 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)
thread_title:  Not Completly Accurate 

message: 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)
thread_title: Lack of citations

message: 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)
thread_title: Wet cities and dry cities

message: 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)
thread_title: 'Lightness' of Water

message: 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)
thread_title: Typical values

message: 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.--217.203.179.64 (talk) 23:37, 9 June 2009 (UTC)
thread_title:  metric 

message: 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)
thread_title: Work to be done

message: 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)
thread_title:  Please be Careful

message: 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)
thread_title:  WikiProject class rating

message: 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)
thread_title:  Correction on %Humidity 

message: It looks as though it should be grams/kg???Artmario2001 (talk) 22:10, 2 July 2008 (UTC)
replies: text: It is. Vsmith (talk) 01:42, 3 July 2008 (UTC)
thread_title:  Graph of Grains/KG v. temp incorrect????  

message: 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)
replies: text:  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)
thread_title:  Estimating relative humidity 

message: The Buck (1996) equation as written doesn't seem to give sensible answers, and differs from what is presented at 
http://en.wikipedia.org/wiki/Arden_Buck_Equation
158.39.20.205 (talk) 07:44, 23 December 2009 (UTC)
thread_title:  Buck equation 

message: 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. 87.80.97.137 (talk) 19:03, 13 October 2010 (UTC)
thread_title:  A definition in the summary 

message: 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)
thread_title:  Consistency of Units 

message: 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)
thread_title:  RH and gas mixture composition 

message: 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 69.174.113.4 (talk) 13:28, 11 July 2012 (UTC)
replies: text:    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 23.16.106.197 (talk) 06:11, 12 July 2012 (UTC)
replies: text:   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 64.180.97.151 (talk) 05:26, 14 October 2012 (UTC)
thread_title:  Misconception 

message: 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... 84.227.241.146 (talk) 00:10, 14 September 2014 (UTC)
replies: text: 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)
thread_title:  Updated Misconception section 

message: 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). 84.227.242.182 (talk) 17:42, 14 September 2014 (UTC)
thread_title:  Pressure dependence section 

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thread_title:  External links modified 

message: I would suggest that the personnel opinion maintenance tage from 2012 could be removed. The article longer looks like personnel opinion and is authoritative. 
John15CM (talk) 14:25, 5 February 2016 (UTC)
thread_title: Remove essay like maintenance tag 

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