Talk:Heat index

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I'm in the process of writing an application that is going to read all available data from the US National Weather Service and produce a report. As part of checking my code to make sure it was functioning correctly I implemented the first two algorithms on the page and compared the output to that of the KWeather program and also the data on weather.com. The first algorithm matched within 2 degrees fahrenheit, while the second algorithm was consistently 10 degrees fahrenheit above the first (with a .2 degree margin). The code I wrote to test it is available. What I've been wondering since spotting this difference is: Is this difference known and (aside from the equation involving the matrix math) which is actually preferred? -- Dshadowwolf 02:10, 4 August 2007 (UTC)

The part I'm guessing at, in case anyone's wondering, is how they figure the numbers for the heat index. It might be that it's based on theoretical modeling of the thermodynamics of the human body, but I'm thinking they would have taken a "does it feel hotter in this room, or in that one?" approach, or something like it. The links provided don't seem to give any information how they arrived at the curves, so if anyone else can find anything more authoritative than my guesswork, help yourself. -- John Owens 05:22 May 9, 2003 (UTC)

Is this link answer your question? The mathematical forge:Humidex
MiguelTremblay 18:01, 22 May 2005 (UTC)
First of all, look for copies of "The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science" by R. G. Steadman (published July 1979 in the Journal of Applied Meteorology, Volume 18 pp. 861-873 as well as "The Assessment of Sultriness. Part II: Effects of Wind, Extra Radiation and Barometric Pressure on Apparent Temperature" same author same volume, pp. 874-885.
Second, the formula that the NWS uses for its Heat Index calculations is a simplification of Steadman's work assuming things like a constant wind speed, a person of a certain height, etc. Basically they hold everything constant except the temperature and the relative humidity. Clegett 01:03, 16 July 2006 (UTC)

What's the difference between heat index and dew point?

I'm not an expert, but they are related but quite different. Roughly, the dew point is the temperature at which condensation forms, given the air's relative humidity and barometric pressure. The heat index is how hot it feels, depending on the temperature and relative humidy, because if it is humid, water can't evaporate from your skin as easily to cool you down. Generally, as the dew point goes up the heat index also goes up, but they are not linearly related. Bubba73 18:03, July 19, 2005 (UTC)
The heat index is defined by the National Weather Service as "the temperature the body feels when heat and humidity are combined." This is due to the fact that the higher the moisture content of the air, the slower sweat will evaporate off the body and therefore the slower evaporative cooling will take place.
On the other hand, the dew point is defined by the American Meteorological Society to be "the temperature to which a given air parcel must be cooled at constant pressure and constant water vapor content in order for saturation to occur." In other words, "all things being equal, how cold does the air have to be before it can't hold any more water vapor than it already does."
One is a measurement of how hot it feels, the other a measurement of atmospheric moisture content. Clegett 01:03, 16 July 2006 (UTC)

Where is there a similar table showing the comfort effects of Dew Point like the table in the Heat Index article?

All I know about dew points and comfort what is mentioned in the Dew point article. I've written a Windows program to calculate heat index, dew point, and wind chill. I will make it available soon as an external link, within the next few days. Bubba73 14:44, July 27, 2005 (UTC)
I've added an external link to my webpage where the program can be downloaded. Bubba73 03:05, August 1, 2005 (UTC)

Contents

[edit] Contradiction

The article formerly stated "The humidex was developed by Canadian meteorologists in 1965"; however, the highest humidex was recorded in 1953. I've removed the former statement. ‣ᓛᖁ♀ᑐ 19:34, 15 August 2005 (UTC)

They could have calculated it retroactively. Bubba73 19:45, August 15, 2005 (UTC)
It was calculated retroactively; relative humidity was recorded even then, although it was measured using hair... Jeffrey 23:53, 15 August 2005 (UTC)
It was calculated by holding certain variables constant from the calculations contained in "The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science" by R. G. Steadman (published July 1979 in the Journal of Applied Meteorology, Volume 18 pp. 861-873 as well as "The Assessment of Sultriness. Part II: Effects of Wind, Extra Radiation and Barometric Pressure on Apparent Temperature" same author same volume, pp. 874-885. Clegett 01:03, 16 July 2006 (UTC)

[edit] Danger zone

Many sources give a heat index of 130F as the start of "extreme danger", which probably comes from the US National Weather Service. I'm surprized that it is so high. Based on my experience with 105-110 heat index, I'd expect "extreme danger" to start at 120F or maybe 115F. Bubba73 23:48, August 25, 2005 (UTC)

The National Weather Service merely states that above a heat index of 130F heat stroke becomes highly likely with continued exposure. Clegett 01:03, 16 July 2006 (UTC)

I seriously question the Dhahran stats. I've visited Weather Underground several times just to check on Dhahran figures, and the humidity is never anywhere near 67 percent. I suggest either broken equipment or a typo and suggest the whole section of Dhahran info be removed.

More reason to question the July 8, 2003 Dhahran stats. According to Weather Underground, max/min/average relative humidity that day were 100/58/81 percent.

Let's compare that to a day before, a day after, a year before, a year after.

July 7, 2003: 84/15/47. July 9, 2003: 84/24/43. July 8, 2002: 70/10/30. July 8, 2004: 46/12/30. On all four of these dates, the AVERAGE relative humidity is less than the MINIMUM was on the allegely record-shattering July 8, 2003.

Remove the Dhahran comments, unless they can be substantiated by another weather organization measuring from ANOTHER SITE in Dhahran. —The preceding unsigned comment was added by 24.155.166.81 (talk • contribs) 19:06, 16 July 2006.

I've seen dewpoints in the upper 80s and even low 90s F in other coastal cities in Saudi Arabia, on both coasts actually (Red Sea and Persian Gulf). Two cities that seemed to always be very steamy were Jeddah(June 2006) and Gizan (or Jizan)(June 2006). I'm sure I'm forgetting other locations though. The humidity here is highly dependent on wind direction, as some of the most humid conditions preceed a change in wind direction from the dry desert, whereas a wind from off the sea brings very humid conditions. Keep in mind that some of the highest open water sea surface temperatures in the world have been recorded in the Persian Gulf (which Dhahran is next to), in the mid to upper 90s F, so air coming off of this body of water could possibly have a dewpoint in the mid 90s when it reaches the coast.
It appears that the internet now has many references to this episode in Dhahran, some of which are clearly mirror sites, whereas others are probably not (e.g. USA Today). The source here is a valid one and to use your above analysis to try to invalidate it would be original research. However, if you feel that this issue is worth pursuing, I would suggest that you contact a meteorological agency to get them to change the data or something equivalent. I have done this before in the United States and it can be painstakingly slow, but it is possible (at least in the U.S.). I'm not sure if you could contact anyone in Saudi Arabia; perhaps you could try the World Meteorological Organization. Good luck. Ufwuct 21:51, 25 October 2006 (UTC)

[edit] Inverse relationship

This clause is confusing: thus there is an inverse relationship between maximum potential temperature and maximum potential relative humidity, therefore making, say, a simultaneous temperature of 120 °F (50 °C) and 90% relative humidity physically impossible. There cannot be an inverse relationship between two maximums. And it is not clear why the second part is proven by the first. I am not sure what this sentence is trying to accomplish, but if you do I urge you to rewrite it without the "relationship" language. -- cmh 03:43, 27 May 2006 (UTC)

[edit] cleanup

this article doesn't sound like encyclopedic entry. i am not a native speaker of english and can't fix it properly. the informative aspects are good, although occasionally confusing. thus i put up the cleanup tag.

[edit] Heat index = humidex?

I've added a disputed tag because I don't think heat index and humidex are the same thing. The NWS heat index is calculated with temperature and relative humidity, while the Environment Canada humidex is derived from temperature, dewpoint and vapor pressure (http://www.weatheroffice.ec.gc.ca/mainmenu/faq_e.html). -- Mwalcoff 01:37, 1 June 2006 (UTC)

Side note on that: Relative Humidity can be calculated by using vapor pressure and a constant. They could be basically the same thing going through slightly different math.

IMO this isn't a dispute, this is you not liking a sentence in the article. (A dispute is where two parties can't come to an agreement.) It makes no sense to tag this whole article as disputed just because you think "heat index and humidex" are not synonymous... the rest of the article is fine. I have reworded the first sentence to address your concern and removed the tag. In future, if you think something is wrong please simply edit the article to improve it, rather than putting dispute tags on things. -- cmh 03:00, 16 July 2006 (UTC)

I'm not a meteorologist and was hoping that experts could fix the article. I've removed the references to the humidex until this can be sorted out. -- Mwalcoff 03:05, 16 July 2006 (UTC)

[edit] Disputed?

The top of the page mentions how the "factual accuracy of the article is disputed". I don't see any ongoing dispute here on the discussion page though, what am I missing?

See the comment above yours. -- Mwalcoff 23:06, 6 July 2006 (UTC)
I agree that humidex does not seem to equal heat index. The formula given for heat index does not match the one posted on the Canadian Weather Office page here http://www.weatheroffice.ec.gc.ca/mainmenu/faq_e.html#weather6
The Humidex formula is based on the work of J.M. Masterton and F.A. Richardson at the Atmospheric Environment Service (now MSC) of Environment Canada in 1979. It is a standard for Canada, but variations are used around the world. The dew point temperature should be given in kelvins (temperature in K = temperature in °C + 273.1) for the formula to work. The magic number 5417.7530 is a rounded constant; it's based on the molecular weight of water, latent heat of evaporation, and the universal gas constant.
e = vapour pressure in hPa (mbar), given by:
e = 6.11 * exp [5417.7530 * ( (1/273.16) - (1/dewpoint) ) ]
h = (0.5555)*(e - 10.0);
humidex = (air temperature) + h
--DRead 00:37, 12 July 2006 (UTC)

[edit] Confusing language

I can't tell what this means:

"...essentially the same temperature colder than which wind chill is thought to commence"

I'd fix it myself if I knew what the author was trying to say. 72.130.129.220 04:28, 24 July 2006 (UTC)

[edit] Heat index at high temperature and humidity

I was just wondering if anyone had any information as to why heat index tables, such as the one on NOAA's webpage: [1] don't consider the result of high humidity at high temperatures. For example, I'm currently in Qatar and the temperature is about 113F(45C) with a humidity of around 90%. As I've read in some of the other comments, I'm sure some would say this isn't possible, but believe me it's true. Using a seemingly unofficial online calculator at: [2] these values yield a heat index of: 272F(133C). Realizing that even though it's really hot here, this isn't possible, why isn't the equation designed to consider extremes such as this? Any information would be appreciated.

All I can say is, no, it's not possible. Something is wrong with the humidity reading. It may have been 90% humidity in the morning when the temperature was lower, but it won't be 90% humidity at mid-day because the amount of moisture in the air would have to increase drastically for that to occur, and that just doesn't happen in the real world. Haplolology 19:40, 25 May 2007 (UTC)

[edit] Vapor Pressure Units

The article defines V, "vapour pressure", but doesn't specify the units to be used. ?mmHg ?mbar

'Tis done ʍαμ$ʏ5043 17:33, 30 May 2007 (UTC)

[edit] Kent

I removed the hypothetical about Kent because the humidity was nowhere near 50% that day at the time the 101.3F was recorded. If it had been, the dewpoint would have been about 80F. In London, on the same day (8/10/2003), the dewpoint was in the low 40s Fahrenheit and when the temperature was 98.6F, yielding a relative humidity below 20% and a heat index of about 95F.[3] So, there's no reason to get into hypotheticals. Ufwuct 21:05, 25 October 2006 (UTC)

[edit] Weather Underground

All I want to say is that I would take any reading coming from Weather Underground very very cautiously, even if it looks correct. I could give you a list of dozens of readings and long-term averages that anyone should be able to see are plainly wrong but which they list as officially correct. In their defense, however, they have tagged many of the more unreliable stations as being "not official NWS values". Haplolology 19:43, 25 May 2007 (UTC)

[edit] regionalism

Are the terms "heat index" or the colloquial "feels-like temperature" used otuside of the U.S.? -FZ 15:03, 12 July 2007 (UTC)

[edit] WikiProject class rating

This article was automatically assessed because at least one WikiProject had rated the article as start, and the rating on other projects was brought up to start class. BetacommandBot 09:53, 10 November 2007 (UTC)