Talk:Humidity

From Wikipedia, the free encyclopedia

	Portal This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.
Start	This article has been rated as Start-Class on the assessment scale.
???	This article has not yet received an importance rating within physics.
Help with this template This article has been rated but has no comments. If appropriate, please review the article and leave comments here to identify the strengths and weaknesses of the article and what work it will need.

	This article related to meteorology and/or specific weather events is part of WikiProject Meteorology and Weather Events, an attempt to standardize and improve all articles related to weather or meteorology. You can help! Visit the project page or discuss an article at its talk page.
Start	This article has been rated as Start-Class on the assessment scale.
High	This article has been rated as High-importance within WikiProject Meteorology.

[edit] Wrong definition of Specific humidity??

I think that specific humidity is the ratio of the mass of water vapor to the mass of MOIST air. This article has it as the ratio of mass of water vapor to mass of dry air. Moist air is the total mass (dry air + water vapor) and dry air is the air without the water vapor...so it makes a difference. I just noticed it. If I'm right, how can it be changed? (sorry if I'm not signing correctly.)jack 17:51, 22 February 2008 (UTC)

[edit] does this graph help ??

http://en.wikipedia.org/wiki/Image:Abs_humidity.gif#file

please note that it may be erroneous, it is based upon formulae by Sonntag/Wexler, although my application could be incorrect, however I have checked it and to me it appears correct, any confirmation would be gratefully received though :) Teeteetee 21:00, 8 August 2006 (UTC)

[edit] humidity and density

A few changes here - liquid water is heavier than air yes, but is it more dense? as a liquid yes, but not so simple any more. Also, cleaned up some of the writing, we don't need to take the author on a story about the cubic metre of gas, after all, if we have a fixed volume of a cubic metre, and flow in and out of it, you can in fact have more molecules than you previously had in that volume (maybe for a short time more molecules collide and bump each other into that box, than in the previous timeframe). Anyway, doesn't matter, as the point is, water vapour is lighter than N2 and O2, the whole cubic metre thing and the flow doesn't really help the explanation imho. --Phatmattbaker 13:43, 24 July 2006 (UTC)

{FAOL tags) -- deleted - Hard Raspy Sci 01:57, 15 December 2005 (UTC)

The above can be deleted, I read German, and nothing in the english wiki source is left out here from other wiki topics (english version). Ie. some stuff in the German article may be considered off topic in the English version...as other articles cover the same stuff...see water vapor. The only thing I can see is that this article needs editing, if it is to remain combined from 3 articles. ---- Hard Raspy Sci 05:04, 15 November 2005 (UTC)

[edit] Request for more information :-/

How is the ideal humidity

Yes, what is the ideal humidity, for comfort/health, indoors? That is why I came to this page (humidity), but I can't find an answer

Agreed. A bit abstract, guys. I came here so I could understand the weather forecasts.

Thermal_comfort has the info, but it's in jargonese. According to Weatherman Steve [1] people feels best at a dew point temperature between 50 - 60° (which he translates as 31-41% relative humidity if the air is 90° ) Gaviidae 23:14, 21 November 2006 (UTC)

[edit] Keep it simple

Absolute humidity is the quantity of water vapor in the air expressed by weight.

Specific humidity is the quantity of water vapor in the air expressed by a ratio of water vapor to dry air.

(Such a ratio might be 1:200, for example). That would specify that the air contain one part water vapor and 200 parts dry air.

Relative humidity is the quantity of water vapor in air expressed by the use of a percentage.

This method is the cause of a great amount of confusion of the subject of humidity. Now that computers have been created which can access a database and display absolute humidity figures and specific humidity ratios in the blink of an eye, the relative humidity method may soon be replaced by one of the other methods, which do not introduce any confusive quality.

Each percentage point is applicable to the qualities of the air which are present at a specific moment. Any change in air temperature or air pressure instantaneously changes the capacity of the air to hold water vapor. Every combination of air temperature/air pressure has its own maximum amount of water vapor that the air will hold.

At the same air pressure, the maximum amount of water vapor in air (expressed as 100%) is much greater at high temperatures.

At the same air pressure, very cold air at -40 degrees C (-40 degrees F) has the capacity to hold very little water vapor. Nevertheless, the maximum amount of water vapor will be expressed as 100% relative humidity.

The same amount of water vapor that is expressed as 100% relative humidity at -40 degrees will be expressed as only 20% or 10% or 5% or 1% (et cetera) at certain other warmer air temperatures.

Supersaturation, where the capacity of air to hold excessive quantities of water vapor is present, is an unnatural condition which may be ignored since it does not ever occur in the Earth's atmosphere, normally. Keep it simple.

141.151.140.116 11:51, 16 August 2005 (UTC)

I agree with "keep it simple" and to the point, however Relative Humidity will never go away, especially due to its highly important property that defines condensation. Absolute and Specific do not do that, but that does not degrade their importance either.

Hard Raspy Sci 04:45, 15 November 2005 (UTC)

Ok, did a lot of edits...don't pay attention to the summaries on the history page, they may not make sense. Still needs more work though...

Hard Raspy Sci 06:13, 15 November 2005 (UTC)

[edit] NSDL

NSDL is temporarily down do not edit unless permanent. Thanks - Hard Raspy Sci 01:55, 15 December 2005 (UTC)

[edit] The opposite of sweating

Is it possible to extract heat from humid air by extracting the H2O ? [Is it possible to extract heat from humid air, then use this heat to warm the air, draining the water (now a liquid) that was once just a vapor (ie liquidise the vapor)] Teeteetee 12:05, 16 March 2006 (UTC)

Of course :) Air conditioners do this :P Gaviidae 23:05, 21 November 2006 (UTC)

Yeah, ... :) ...but do Air conditioners use the heat(energy) extracted from the air to warm the air being conditioned ?(ie not cooling the inside air, but warming it)Teeteetee 05:52, 8 February 2007 (UTC)

Why would you want it to? The purpose is to cool the air. If air conditioners warmed the air in a room, then why would you have one? They do warm the air behind them (in a window-shaker) which is usually outside. A cooler or refrigerator usually warms the air behind it or under it (warms my feet).

If you are asking about people using an air conditioner to DRY the air but not cool it, then you really just want a dehumidifier :) They don't specifically heat the air using the energy they've gained, but the heat does turn the air vapour into water (the little dish underneath that you have to empty periodically). Gaviidae 10:12, 29 April 2007 (UTC)

some a/c units are capable of being run in reverse to warm a room. Operation is via the unit swapping the flowpaths of the room and outside air streams so the outside air gets cooled and inside air picks up the exhaust heat and is blown back into the room. They are usually only used in warmer climates because they tend to be inefficient compared to gas or resistive heating (and the efficiency goes down as the outside temperature gets colder). Mostly they are used in window/wall units in small, lower-cost apartments (such as studios), as the initial cost of the unit is normally lower and the added costs (due to inefficiency) are absorbed by the tenants.128.158.214.39 19:34, 8 August 2007 (UTC)

[edit] practical question

In that warmer air holds significally more water than cooler air. The practical question seems to be - how much more water is held in the air at increasingly varing temperatures.

Given usual and similar pressure and other conditions, what is the relationship of the relative amount of water that can be held at say 90% humidity at 70F degrees versus 80F, 90F, 100F and 110F.

Is there a general formula? Does air at 90F hold twice as much water as 70F?

Air does not hold water.

The Antoine equation is a emperical formula that can be used to estimate the saturated vapor pressure of water (and other substances for tha matter) at different temperatures and pressures.

Go here: http://antoine.frostburg.edu/chem/senese/101/liquids/faq/antoine-vapor-pressure.shtml.

For the question asked. At 90F the saturated vapor pressure of water is 0.699 psi(a); at 70F it is 0.363 psi(a). 0.699/0.363 = 1.9. So an air/water mixture at 100% RH at 90F has approximatly 1.9-times the that of a mixture at 70F.

(A good rough rule of thumb is that a 10 degree (C or K) increase in temperature roughly doubles the saturated vapour pressure, i.e. the amount of water held as vapour in the air. -DGB)

[edit] Measurement

I added a section on remote sensing of humidity, and an external link to a more detailed discusssion. KonaScout 23:55, 10 October 2006 (UTC)

[edit] Footnote

Can we get that Optiks footnote out of the middle of the page and moved down to the bottom? I can't figure out why it is where it is now, but it looks stupid. 82.93.133.130 09:07, 29 November 2006 (UTC)

Done. Vsmith 11:53, 29 November 2006 (UTC)

Thanks-- now that I see how you did that, I can do it on the next one : ) 82.93.133.130 22:55, 2 December 2006 (UTC)

I would like to ask if there is a 'league' table of relative humidity around the globe. for example where is the most humid place on earth and were is the least humid place on earth. Also what is the Relative Humidity of these places. Phil Wainwright 11:39, 26 April 2007 (UTC)

Phil_Wainwright

[edit] Vandalsim

Since I forgot to put them in my edits, here are my three revert reasons: User:210.3.0.120 added "Popo" to Humidity and Air Density. User:58.107.131.189 changed "volume" to "wettness" in Absolute Humidity and User:64.228.217.130 changed "humidity" to "humidty" under Specific Humidity. Dikke poes 10:05, 29 April 2007 (UTC)

[edit] Negative humidity stuff

Moved from article:

== Why do we see reports of negative humidity? ==

Often, the dew point is used to report relative humidity of the air. The dew point is measured using a wet thermometer. Unlike a dry thermometer, water on the thermometer evaporates, thus a wet thermometer is cooler than a dry one, which can lead to a negative value.

This above explanation is wrong.

The dew point cannot be used to report the relative humidity. The dew point reports the dew point!

The dew point is NOT measured using a wet thermometer. A wet thermometer measures the wet bulb temperature. The dew point may be calculated from the wet bulb temperature if one also knows the ambient temperature and the air pressure. If the gas is not pure air there is usually not enough data known to do the calculation.

If by a negative value the question refers to the dew point, then the answer is simple, under dry conditions it is often the case that the gas must be cooled well below 0°C before water condenses (usually as ice). The dew point (or frost point) is a negative temperature in Celsius.

The lower temperature of the wet thermometer does not result in a negative humidity. The whole point of a wet thermometer is that it shows a lower temperature (the wet bulb temperature). It is from the wet and dry temperatures that the relative humidity and other units are calculated. If the question refers to a negative relative humidity this is impossible and can only arise when measurement errors are reported by people who do not understand the physics. A negative relative humidity implies that some water must be added before there is none!

Bob Pragnell 10:21, 17 May 2007 (UTC)

It was a bit out of place in the article and quite confused. Bob's comment seems on target, but belongs here rather than in article space - discuss here. Vsmith 11:06, 17 May 2007 (UTC)

A wet bulb thermometer and a dry bulb thermometer are identical except that a "sock" made of a suitable fabric is slipped over the bulb of the thermometer that is chosen to be the one which will be dipped into water. The "sock" is tied in place with string at both the top and the bottom of the "sock" and remains on the thermometer for months or until it becomes dirty or soiled. A "sock" is about three-fourths of an inch (18mm) in length.

Side-by-side, both thermometers are lashed to a thin metal plate. A swivel and a handle are attached to the metal plate, forming a utensil called a "sling psychrometer" which is used to move the thermometers through the air by waving the "sling psychrometer" in a circle. Some of the water on the wet bulb "sock" will evaporate and in doing so will cool the wet bulb thermometer. The dry bulb temperature and the wet bulb temperature are used to calculate the dew point temperature and the relative humidity. Air pressure must be considered, too, however, a given geographic location will nearly always remain at the same air pressure (i. e., within acceptable limits), such as "30 inches of mercury," "29 inches of mercury," etc.

Long ago, scientists calculated relative humidity values for various readings of dry bulb temperatures, wet bulb temperatures, and air pressures. Their calculations have been entered on slide rules, therefore, it is easy to manipulate the slide rule and produce the dew point and the relative humidity. An experienced worker needs about five seconds to manipulate the slide rule and see the values.

I'd never heard of "negative humidity" until I saw it in Wikipedia (above). I suspect that someone has poor hearing and thinks that the words "relative humidity" is "negative humidity."Velocicaptor 11:21, 31 May 2007 (UTC)

A more modern way to obtain evaporation of the water on the wet bulb is to use a battery-powered device equipped with a small fan that blows air onto the moistened wet bulb. The "sling psychrometer" is old-fashioned.Velocicaptor 11:47, 31 May 2007 (UTC)

[edit] Extra

humidity is water

[edit] Incorrect formula

It appears that the formula in Section 1, "Absolute Humidity," is incorrect. The right hand side is identical with the formula for "Mixing Ratio" in Section 3. Moreover, the text says absolute humidity has units of density, while the formula gives a dimensionless ratio.

I don't know what the correct formula for absolute humidity is. It would be great if someone could correct this mistake.

Jmkinder1 13:13, 4 June 2007 (UTC)

[edit] Humidity and sweating

In the Middle East, it is very common during summar days that the air temperature reaches 45 to 50 Deg C. On some of these days, the relative humidity also increases, especially on areas near the sea. The result is a very uncomfortable humid state in the late evenings and early mornings. The people go out will sweat, of course. My question is this..at an air temperature of 45°C, for example, suppose the air is saturated with water vapor and the RH is very high. Human body is at a much lower temperature than the air, at this condition. So, the excessive sweat we experience in such a condition is produced from the body itself or IS IT A CONDESED WATER VAPOR from the air itself on the body (which is at a colder temperatuer)? Someone's clarification would be highly appreciated.

213.42.2.22 05:29, 17 June 2007 (UTC) penguine_s June 17, 2007

This is close to a subject I have been trying to understand for many months. Unfortunately I do not yet understand it fully, however...
I get the impression that humidity measurement is not as exact as a digital display often indicates. The real/true/exact humidity reading of air can vary quite significantly from one place to another, even if the two places are only a short distance apart(eg. 30mm). So the humidity of the air right next to your skin may well be different to that of your measuring device. In addition, salts and other compounds on the skin will probably be affecting humidity on/near the skin see Critical relative humidity for further information. I hope this helps more than it confuses. I believe this is a confusing subject even for experts, as evidenced by the lack of experts making the subject clear to non-experts. —Preceding unsigned comment added by 80.42.171.167 (talk) 14:46, 26 September 2007 (UTC)
Well, it seems pretty simple. If the dewpoint is higher than the temperature of your skin, then not only will your own sweat not evaporate, but extra water from the air will condense on it. Skin temperatures aren't that warm, so this should be a somewhat common occurrence in tropical regions. And it seems that 35°C is near the upper limit of recorded dewpoints on Earth (see, for example, http://www.usatoday.com/weather/resources/askjack/archives-weather-extremes.htm), so your hypothetical condition of a 45°C dewpoint would probably only exist in a steam room :)

75.69.32.42 (talk) 16:55, 23 April 2008 (UTC)

[edit] Redirects

I'm not sure "mixing ratio" should redirect to this page. While a valid topic for humidity, it has many other applications, such as chemical solutions and combustion (air/fuel ratio). For example, I got linked here from an article on liquid rocket engines. Maybe a disambiguation page would be appropriate, I don't know how to set this up though.

[edit] WikiProject class rating

This article was automatically assessed because at least one WikiProject had rated the article as start, and the rating on other projects was brought up to start class. BetacommandBot 09:54, 10 November 2007 (UTC)

[edit] First line possible error

The first line currently states "Humidity is the amount of water vapor in a sample of air compared to the maximum amount of water vapor the air can hold at any specific temperature.". I believe that is the definition of relative humidity. NOT humidity. In common language, humidity is usually taken to mean relative humidity, but not necessarily.Nick R Hill (talk)

In first para, the sentence "Warm water vapor has more thermal energy than cool water vapor and therefore more of it evaporates into warm air than into cold air." needs more explanation, to be moved to a paragraph of it's own, checked for duplication of info in article, or to be removed.Nick R Hill (talk)

[edit] Humid redirect

This is problematic, since this article only discusses humidity as the degree of water vapor saturation of the air. But (see arid), "humid" or "humidity" can also mean that there is generally more precipitation than evapotranspiration in a region, seen in the long run. See "Humidity and sweating" discussion above, where it becomes clear that humidity in the scope of the article can also very much apply in a region that by itself is a textbook example of not being humid.

Basically, the article is at present only about the meteorological definition of humidity, not the climatological Thousands of biological and geographical articles link here, but they of course need the climatological definition. (In biology, we usually simply use "wet" for "moist" for meteorological humidity and "humid" for climatological humidity. E.g. "moist cloud-forest in a generally arid region" or "a dry rain-shadow in the humid tropics".) Dysmorodrepanis (talk) 20:43, 28 March 2008 (UTC)

[edit] How should humidity be stated?

I have found in some other articles that the humidity of a province/state is stated in degrees Farinheit or Celsius. Everywhere else when humidity is reported, (on tv, in the newspaper), it is stated in % (percent), which makes more sense. Am I wrong or should stating the humidity of a region's climate be stated in %s (percents)? —Preceding unsigned comment added by 209.47.31.6 (talk) 05:19, 9 June 2008 (UTC)