The formula shown in the Heat Index(HI) requires that the relative humidity should be equal to or greater than 40%, so how to calculate the heat index when the humidity is less than 40%?

Or is there some other formulas can represent the "real feel temperature" without these limitation factors?


1 Answer 1


Read the "Formula" section of that Wikipedia article again. It gives a second set of coefficients which are good for humidities from 0-80%.

  • $\begingroup$ but it requires that "all temperatures between 70 and 115 °F", how to calculate the heat index when, say 53°F and 40% humidity? $\endgroup$
    – Yriuns
    Commented May 3, 2016 at 3:12
  • 1
    $\begingroup$ Is the heat index even relevant in that situation? The heat index is a measure of the human ability to regulate body temperature in various conditions. Radiation alone is plenty of heat loss to maintain a good body temperature at a low temperature like 53 fahrenheit. No one is going to feel hot at all in such circumstances... $\endgroup$ Commented May 5, 2016 at 0:53

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