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Posted at 10:30 AM ET, 07/21/2011
The origins of the heat index and why it’s important
By Jason Samenow
Heat is the top weather related killer in the U.S. But there’s truth in the saying, “It’s not the heat, it’s the humidity.” The body cools itself at a slower rate when the humidity is high. For this reason, the heat index was devised, to provide a measure of how hot it actually feels - hence alternative names such as “apparent temperature,” “feels like temperature,” “real-feel temperature”(AccuWeather trademark), “humiture,” and in Canada, “humidex.”
Perhaps originating from some commentary by radio host Rush Limbaugh, questions have arisen as to whether the heat index is a legitimate scientific measure. I can assure you it is. Moreover, it’s critical for communicating health risks related to the potentially deadly combination of heat and humidity.
The roots of the heat index can be traced to Osborn Fort Hevener (described by the New Yorker as a New Jersey weather buff), who coined the term humiture in the early 1900s. In a 1959 issue of Weatherwise magazine, Hevener wrote: Twenty-two years ago, I was fortunate to coin two words that have found their way into the dictionaries, and to develop a concept that has proven useful and popular. To let the secret out, I am the humiture man.
In the 1957 Thondike-Barnhart dictionary, humiture was defined as ”a combined measurement of temperature and humidity, arrived at by adding degrees of temperature to percentage of relative humidity and dividing by two.”
But this simplistic definition would evolve.
Jacksonsville, Fla. broadcast meteorologist George Winterling, published a revised and adapted version of the humiture in the late 1970s in the Bulletion of the American Meteorological Society and began reporting it on-air.
Based on the work of Robert Steadman, who published several seminal studies on the “assessment of sultriness,” the National Weather Service (NWS) then made operational what became the heat index
The heat index results from a whole research area on weather and its effects on the human body, known as biometeorology. The index is grounded in established relationships describing the exchange of heat and moisture between the human body and the atmosphere.
Consider all the following quantities factored into determining heat index: vapor pressure, dimensions of a human, effective radiation area of skin, significant diameter of a human, clothing cover, core temperature, core vapor pressure, surface temperatures and vapor pressure of skin and clothing, activity, effective wind speed, clothing resistance to heat transfer, clothing resistance to moisture transfer, radiation from the surface of the skin, convection from the surface of the skin, sweating rate, ventilation rate, skin resistance to heat transfer, skin resistance to moisture transfer and surface resistance to moisture transfer.
For practical purposes, all of these quantities can be condensed into an simplified equation for calculating heat index requiring just temperature, relative humidity, and a number of constants.
You can also calculate heat index knowing merely the temperature and dew point (another measure of humidity). The NWS Web site features a handy on-line calculator for determining the heat index based on a combination of either temperature and relative humidity or temperature and dew point. Or, you can consult the chart below.
The NWS cautions that its given heat index values assume shady, light wind conditions and that exposure to direct sunlight can increase the heat index by 15 degrees.
The bottom line is that the heat index is an extremely important measure for understanding heat and humidity’s impact on the body. As the NWS notes: “When heat gain exceeds the level the body can remove, body temperature begins to rise, and heat related illnesses and disorders may develop.”
參考譯文：
炎熱在美國已成為與致人非命有關(guān)的頭等天氣。但這一說法的事實是，“問題不在炎熱，而是潮濕。”當濕度較高時身體會以較慢的速率進行自我降溫。因此之故，炎熱指數(shù)就問世了，它為身體感覺到底有多熱提供了一個度量方法——因此它又被稱為“表觀溫度”、“感覺起來的溫度”、“真實感覺溫度”和“溫濕度”，在加拿大它被稱為“濕潤指數(shù)”。
也許是源自電臺主持人拉什•利姆鮑（Rush Limbaugh）的一些評論，關(guān)于炎熱指數(shù)是否是一個合情理的科學(xué)標準的問題被提了出來。我能確定的就是這個。此外，傳播與炎熱和潮濕的潛在致命結(jié)合有關(guān)的健康風(fēng)險很是關(guān)鍵。
炎熱指數(shù)的根源被追溯到奧斯本•佛特•希維納Osborn Fort Hevener）（他被紐約人稱為新澤西的一個天氣狂），他在二十世紀初創(chuàng)造了溫濕度這一術(shù)語。1959年，希維納在《天氣》(Weatherwise)雜志第12卷第2期的一篇文章上寫道：“22年前，我有幸創(chuàng)造了兩個現(xiàn)已被收進詞典的詞并提出了一個已被證明是有用和受歡迎的概念。為了讓這個秘密傳出去，我就是那個提出‘溫濕度’的人。”
1957年的Thondike-Barnhart詞典，“溫濕度”被定義為“一個結(jié)合了溫度和濕度的度量指標，通過將溫度的度數(shù)加到相對濕度百分數(shù)上除以二得到。”
但是這個過于簡化的定義會繼續(xù)演化。
佛羅里達州杰克遜維爾（Jacksonsville）廣播氣象學(xué)家喬治•溫特靈（George Winterling）在上個世紀七十年代末的《美國氣象學(xué)會通訊》（Bulletion of the American Meteorological Society）上發(fā)表過一個“溫濕度”的修訂和改編版，并開始現(xiàn)場直播。
基于羅伯特•斯泰德曼（Robert Steadman）的工作——他發(fā)表過一些對以后發(fā)展有重大影響的有關(guān)悶熱的研究，國家氣象局（National Weather Service，NWS）接著使什么成為炎熱指數(shù)變得可操作。
炎熱指數(shù)源于對天氣的全面研究和它對人體的影響，亦即生物氣象學(xué)。這一指數(shù)是建立在描述人體和大氣之間的濕熱交換關(guān)系基礎(chǔ)上的。
下述因素都被納入到確定炎熱指數(shù)的參考要素中：水汽壓、人的身材、皮膚的有效輻射面積、人的胸腰圍、服裝遮蓋范圍、體核溫度、體核水汽壓、皮膚和服裝的表面溫度和水汽壓、活動量、有效風(fēng)速、服裝的耐熱性、服裝的耐潮性、皮膚表面的輻射、皮膚表面的對流、出汗率、通風(fēng)率、皮膚的耐耐熱性、皮膚的耐潮性和表面耐潮性。
為了實用的目的，所有這些參量都能被納入到一個只需溫度、相對濕度和一些常量的簡化方程式中。
即使只知道溫度和露點（濕度的另一個度量方式），你可以計算出炎熱指數(shù)。國家氣象局（NWS）網(wǎng)站公布了一個方便的在線計算器便于大家基于溫度和相對濕度或溫度和露點的結(jié)合來計算炎熱指數(shù)。或者，你也可以參考下圖。
國家氣象局（NWS）提醒道，它給出的炎熱指數(shù)值并不可靠，微風(fēng)條件和暴露于陽光直射下都能增加炎熱指數(shù)15度。
基本的一點是，炎熱指數(shù)是一個理解濕熱對人體的影響的極為重要的度量方式。正如國家氣象局（NWS）所說：“當熱增量超過人體能夠排除掉的水平時，體溫開始升高，與炎熱有關(guān)的疾病和身心失調(diào)也會出現(xiàn)。”
原文鏈接：http://www.washingtonpost.com/blogs/capital-weather-gang/post/the-origins-of-the-heat-index-and-why-its-important/2011/07/21/gIQASKrnRI_blog.html
