How To: Calculate Heat Index (And Why You May Feel Miserable Today)

With the National Weather Service predicting a heat index as high as 104 Sunday, it’s helpful to know what that number means, aside from realizing that the day could be pretty yucky.

The heat index is what the temperature feels like to the human body when relative humidity is combined with the air temperature.

Here’s how it is calculated, how you can figure this out at home and how the numbers are reached.

People sweat to cool off when our bodies get too hot. That evaporative process lowers our temperatures. But if the air is too moist, the rate of evaporation from our bodies decreases and we don’t cool off as well, making us feel hotter.

As the National Weather Service notes, using the accompanying chart, you need to know the air temperature and the relative humidity.  According to the chart, a temperature of 90 degrees combined with 65 percent humidty puts the index at 103. (Here’s a chart to do the calculations:  And, the NWS says, the heat index values in the chart  are for shady locations.  If you are in direct sunlight, the heat index value can increase by as much as 15 degrees.




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