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To capture the real impact of a heat wave, the National Weather Service uses its heat index. It portrays what the temperature and humidity really "feel like" to the human body. So if the temperature is 88 degrees and the humidity is at 75%, it really feels like 103 degrees.
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The heat index only shows what temperatures feel like in the shade, without the added heat from standing in the sun.
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As pivotal as the heat index research was, it had a flaw. It models how humans physiologically feel heat, but at higher temperatures, the model broke down and it couldn't calculate the effects of high heat and humidity. To complete the heat index, the National Weather Service extrapolated using the lower temperature to fill in the gaps for the higher temperatures.
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In addition to the heat index, the National Weather Service releases an "excessive heat warning" when a heat wave gets dangerous. But critics say that language is too general and not specific enough for vulnerable groups.
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