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.

Rush Limbaugh’s commentary on heat index

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.

Heat index chart (National Weather Service)

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.”

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