Talk:Dew point

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What are the dew point comfort ranges?

[edit] Calculating dew points?

What is a good empirical formula for calculating dew points based on temperature and relative humidity?

See the third and fourth external links. I wrote a little program that caluclated the dew point and heat index, and I used the third one. Bubba73 23:09, July 19, 2005 (UTC)

I added the formula to the article. Bubba73 02:41 and Wind Chill can be downloaded. Bubba73 03:08, August 1, 2005 (UTC)

[edit] Questions

I have two questions / suggestions for extra material.

1. I understand the diagram shows the percentage of water vapour that can be carried as a component of air at a given temperature. But what is it a percentage by? Percentage weight, percentage volume, percentage of moles, something else?

I believe (and this is from reading the axis on a Psychrometrics) it is w/w.

2. Are the physiological reasons known why a high dew point is held to be oppressive? Notinasnaid 15:51, 11 November 2005 (UTC)

Because a high dew point -> higher wet bulb temperature -> sweat evaporates less -> less cooling from sweat -> body shuts down to prevent overheating and heat stroke. Humans can survive surprisingly high ambient temperatures as long as it is dry. I recall some army guy got in an oven and cooked himself at something like 120C for 5 minutes and survived by drinking water and sweating like crazy. njh 10:12, 11 January 2006 (UTC)

[edit] Formula seems inaccurate

I used this formula vs. wunderground.com's ambient conditions, and the formula was incorrect. -- 130.18.64.104

The formula matches one in a book I have, perhaps the wunderground algorithm is wrong? furthermore, if I let Tdp = T I get RH = 1 which makes sense. Can you give more precise examples where it fails? njh 23:15, 14 February 2006 (UTC)

[edit] Dew Point what?

This article needs to be re-named. Dew point is ambiguous. There are two phycial properties that are closely related Dew Point Pressure AND Dew Point Temperature. The article clearly is speaking of the latter.

When one says "the dew point" it is often correct to assume that the dew point temperature is being spoken of, however, this is not always the case.

In addition the dew point temperature of a gas or vapor is dependent on the absolute pressure of the gas - that is to say: dew point temperatures must always be refrenced to a pressure and dew point pressures must always be referenced to a temperature.

The article doesn't explain or ellude to this important relationship.

This "dew point pressure" you mention goes by another, more common, name: saturation vapour pressure. mdf 19:09, 17 July 2006 (UTC)

[edit] Dew point calculators

I prefer to use Javascript calculators to ones that need to be downloaded. There is one presently available at http://www.decatur.de/javascript/dew/index.html that seems to work fine.

[edit] Figure 1

This figure looks like it was done in about 1 minute with very little effort, should this figure not be polished up significantly? chiefhoser 08-08-2006

[edit] Question

Hoe can the dewpoint be changed?

[edit] remark

the article defines very nicely what dewpoint temperature is and how it can be calculated. What i'm missing here is some background information on what determines the dewpoint. (Relative humidity, yes, but what influences RH? You can't just say RH is determined by DP, and DP is determined by RH, you have to give information on what influences the relative humdity/DP which should be considered as one factor, since RH is just the consequence of the DP and T)

[edit] Dew point and temperature

I'm confused. The article says that "If the temperature rises without changing the absolute humidity, the dew point will rise accordingly." Since the absolute humidity is "the mass of water in a particular volume of air" and the dew point temperature is "the temperature to which the parcel must be cooled, at constant barometric pressure, for water vapor to condense into water", it seems to me that for a fixed, isolated parcel of air, the dew point temperature is independent of the current temperature of this parcel.

So I do not understand why the dew point should rise?

[edit] Temperature Range on Dew Point Formula?

I think that range 0 < T < 60 is only due to the sensors on the reference page equipment... the RTD temp sensor device. Otherwise, I think that derivation and formula should be valid up until 100C? I have changed this on the page. Does anyone know better? --Phatmattbaker 16:31, 30 August 2006 (UTC)

[edit] High RH

In Melbourne today - Saturday January 20th 2007 at 8:00 pm the RH is 88 and the dew point is 20.9C (figures from the University of Melbourne's weather site at http://www.earthsci.unimelb.edu.au/weather_page.htm )

203.222.159.54 09:12, 20 January 2007 (UTC)

[edit] Record Dewpoint

Anyone know the highest dew point ever recorded? I haven't seen anything above 86F.

Answer from mtn-man... The record high dew point in the world is 93 degrees, observed near Dubai, on the Persian Gulf in the United Arab Emirates. 22:20, 31 January 2007 (UTC)

Comment on 24 October 2007 ... mtn-man answer of 93°F near Dubai, UAR is often suggested as the world record highest dewpoint temperature; however the date of 31 January 2007 is suspect because the weather records for that date at Dubai International Airport and at Sharjah International Airport indicate a maximum recorded dewpoint on that date of 16.8°C (62.2°F). In US Air Force report AFCRL-70-0563 dated 6 October 1970 Grantham and Sissenwine of the Air Force Research Laboratories reported a 33.6°C (92.5°F) record dewpoint for Sharjah in the month of July. The year of this event in vague and some evidence suggests that it was in the period 1940-1953. One problem in determining the record high dewpoint is that most world meteorological stations record temperatures every three hours in contrast to some who claim a high dewpoint record only list a single instantaneous reading often not at an official meteorological station. Many claims for record dewpoints originate at desert locations when an approaching thunderstorm thrusts a short duration burst of moisture into a location.K289g 15:14, 24 October 2007 (UTC)

[edit] usefulness

55 comfortable
60 sticky
65 muggy
70 oppressive
75 unbelievable
80 I saw only once here.
source: Todd Gross WHDH-TV NBC Boston Channel 7

it looks like this chart explains why the dew point is useful. who really cares what the dew point is unless it can give you useful immediate information. shouldn't this be added to the article? Kingturtle 03:28, 17 September 2006 (UTC)

The Houston Chronicle has a good article about the uselessness of dew point at http://www.chron.com/disp/story.mpl/headline/features/4100421.html Ewlyahoocom 18:58, 18 February 2007 (UTC)

[edit] Dew point and absolute humidity

The beginning of the article says that dew point is the temperature to which humid air must be cooled, while maintaining the same pressure, to cause its water vapor to begin to condense: i.e., the saturation temperature of the water in the air at whatever the partial pressure of water vapor is. This matches up with what I remember from thermodynamics. If the temperature is increased while maintaining the same absolute humidity (mass of water per volume of air), the partial pressure of water vapor has been increased (although water vapor close to saturation is not an ideal gas by any means, it still exhibits the same trends, so if mass and volume are held constant while temperature increases, pressure must increase). This, in turn, means that the saturation temperature must increase (the mixture of water vapor and dry air now does not need to be cooled as much to cause the water vapor to start condensing).

   Dbrunner 01:11, 12 July 2007 (UTC)

[edit] Dew point, Real Feel Temperature, Heat index, Wind chill, Humidity

What are the differences and similarities of the calculations?... for (a) Dew Point?... (b) Real Feel Temperature?... (c) Heat Index?... (d) Wind Chill?... (e) Humidity?... dsaklad@zurich.csail.mit.edu 11:43, 22 July 2007 (UTC)

The "calculations" aren't really similar, as they are all measures of different things, excepting that dew point is one measure of humidity. See the articles on humidity, heat index and wind chill. "Real Feel" is a made up thing from Accuweather. --skew-t 09:15, 12 October 2007 (UTC)

[edit] Pictures of dew?

The photos "Dew on a spider web", "Dew On Webs" and "Dew On plants" are appropriate for the article about dew, but don't contribute anything to the understanding of dew point. They should be deleted. Opinions? dwboston 05:59, 29 July 2007 (UTC)

Agreed; I thought the same thing when I saw them. --skew-t 09:17, 12 October 2007 (UTC)

[edit] Celsius

What's with the Fahrenheit? Someone should replace the F:s with C:s - I don't trust my knowledge of F/C conversion enough nor do I have any celsius sources to do it by myself. As the article now stands it's practically unreadable to an international reader. - G3, 14:05, 3 September 2007 (UTC)

I looked at changing the units myself, but I stopped as although farenheight is unknown to me, what unit was the original measurements in?

[edit] Why Only for Air-Water Mixtures ?

This article assumes that the term dew point applies only to air-water mixtures when it can apply to any mixture of two or more components. When cooling a mixture at constant pressure the dew point is the point that the first drop of liquid forms. Similarly when heating a liquid, the bubble point is the temperature that the first bubble is formed. Event Horizon 23:40, 16 September 2007 (UTC)

How about providing some references to papers or texts that use the term dewpoint for condensible mixtures not involving water? If this is a common usage, then a short note extending the article to non-aqueous mixtures would certainly seem appropriate to me.Easchiff 15:59, 17 September 2007 (UTC)

The term is used extensively in the Chemical Engineering field. Any text on Mass Transfer or distillation will refer to the bubble point and dew point of the mixture being distilled. The classical text is "Mass Transfer Operations by Trebal" I do not have access to a copy unfortunately. Maybe someone else can help out. Event Horizon 23:36, 17 September 2007 (UTC)

I'll try to get hold of R.E. Treybal's book, or something equivalent.Easchiff 14:54, 19 September 2007 (UTC)

Here are two references:

Gatley, D., Understanding Psychrometrics, ASHRAE (2003), pages xi and 269

Treybal, R., Mass-Transfer Operations. 3rd Edition, McGraw-Hill (1981, page 229

Quote. Some broaden the definition of psychrometrics to cover mixtues of the gas of one substance (any dry gas component or a mixture of dry gases) and the condensable vapour of a second substance. Examples include hydrogen-water, hydrogen-carbon tetrachloride, oxygen-carbon tetrachloride, air-toluene, air-bezene.

The book "Handbook of Psychrometric Charts, D.C. Shallcross, Kluwer (1997) contains over 300 charts of which 38 apply to air-water vapour and the remainder are other hydrocarbons and other chemical substances.

Since the Wikipedia "Psychrometrics" section begins with "Psychrometrics or psychrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures." this should be sufficient justification to change the wording in dew point {temperature}. k289gK289g 16:15, 4 November 2007 (UTC) K289g 13:48, 5 November 2007 (UTC)

[edit] WikiProject class rating of Start

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 09:48, 10 November 2007 (UTC)

If no inline references are added to this article, it will remain start class, and therefore will be less likely to be used within wikipedia by users. Wikipedia users are getting more savvy in checking out the class of articles in order to determine their usefulness. Thegreatdr (talk) 15:29, 6 April 2008 (UTC)

[edit] ugly excel graph

Is there no better alternative for this ugly MS Excel graph? --85.146.199.125 (talk) 22:40, 15 January 2008 (UTC)

[edit] "Feels like the western US"

That is not much use to the vast majority of the world who've never been to the western US! 86.143.48.55 (talk) 16:02, 29 January 2008 (UTC)

[edit] Denver and New York?

From the 'Constant pressure' para:

"For this reason, the same dew point in New York and Denver (which is at a much higher altitude) will imply that a higher fraction of the air in Denver consists of water vapor than in New York."

From the 'Varying pressure' para:

"Coming back to the New York - Denver example, this means that if the dew point and temperature in both cities are the same, then the mass of water vapor per cubic meter of air will also be the same in those cities."

How can these both be true? —Preceding unsigned comment added by 82.70.243.134 (talk) 09:37, 5 February 2008 (UTC)