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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.”
参考译文：
炎热在美国已成为与致人非命有关的头等天气。但这一说法的事实是，“问题不在炎热，而是潮湿。”当湿度较高时身体会以较慢的速率进行自我降温。因此之故，炎热指数就问世了，它为身体感觉到底有多热提供了一个度量方法——因此它又被称为“表观温度”、“感觉起来的温度”、“真实感觉温度”和“温湿度”，在加拿大它被称为“湿润指数”。
也许是源自电台主持人拉什?利姆鲍（Rush Limbaugh）的一些评论，关于炎热指数是否是一个合情理的科学标准的问题被提了出来。我能确定的就是这个。此外，传播与炎热和潮湿的潜在致命结合有关的健康风险很是关键。
炎热指数的根源被追溯到奥斯本?佛特?希维纳Osborn Fort Hevener）（他被纽约人称为新泽西的一个天气狂），他在二十世纪初创造了温湿度这一术语。1959年，希维纳在《天气》(Weatherwise)杂志第12卷第2期的一篇文章上写道：“22年前，我有幸创造了两个现已被收进词典的词并提出了一个已被证明是有用和受欢迎的概念。为了让这个秘密传出去，我就是那个提出‘温湿度’的人。”
1957年的Thondike-Barnhart词典，“温湿度”被定义为“一个结合了温度和湿度的度量指标，通过将温度的度数加到相对湿度百分数上除以二得到。”
但是这个过于简化的定义会继续演化。
佛罗里达州杰克逊维尔（Jacksonsville）广播气象学家乔治?温特灵（George Winterling）在上个世纪七十年代末的《美国气象学会通讯》（Bulletion of the American Meteorological Society）上发表过一个“温湿度”的修订和改编版，并开始现场直播。
基于罗伯特?斯泰德曼（Robert Steadman）的工作——他发表过一些对以后发展有重大影响的有关闷热的研究，国家气象局（National Weather Service，NWS）接着使什么成为炎热指数变得可操作。
炎热指数源于对天气的全面研究和它对人体的影响，亦即生物气象学。这一指数是建立在描述人体和大气之间的湿热交换关系基础上的。
下述因素都被纳入到确定炎热指数的参考要素中：水汽压、人的身材、皮肤的有效辐射面积、人的胸腰围、服装遮盖范围、体核温度、体核水汽压、皮肤和服装的表面温度和水汽压、活动量、有效风速、服装的耐热性、服装的耐潮性、皮肤表面的辐射、皮肤表面的对流、出汗率、通风率、皮肤的耐耐热性、皮肤的耐潮性和表面耐潮性。
为了实用的目的，所有这些参量都能被纳入到一个只需温度、相对湿度和一些常量的简化方程式中。
即使只知道温度和露点（湿度的另一个度量方式），你可以计算出炎热指数。国家气象局（NWS）网站公布了一个方便的在线计算器便于大家基于温度和相对湿度或温度和露点的结合来计算炎热指数。或者，你也可以参考下图。
国家气象局（NWS）提醒道，它给出的炎热指数值并不可靠，微风条件和暴露于阳光直射下都能增加炎热指数15度。
基本的一点是，炎热指数是一个理解湿热对人体的影响的极为重要的度量方式。正如国家气象局（NWS）所说：“当热增量超过人体能够排除掉的水平时，体温开始升高，与炎热有关的疾病和身心失调也会出现。”
原文链接：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
