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How to Calculate Heat Index

Before you burn up in the scorching hot weather, calculate how hot it actually feels and avoid serious heat-related illnesses.


  • Step 1: Find out where the air temperature and relative humidity level as a percentage intersect on the heat index chart -- this is the heat index, which is how hot the air actually feels to your body.
  • Step 2: Add another 10 to 15 degrees if you are standing in direct sunlight or if there is little to no breeze to the find the final head index temperature.
  • FACT: Because the Earth's atmosphere traps heat, the average global temperature hovers around 57 degrees. Without the atmosphere, the temperature wouldn't climb above zero.
  • TIP: A hygrometer may take up to 2 hours to provide an accurate reading.
  • Step 3: Take the outdoor air temperature from a reliable thermometer that is not in direct sunlight, which will skew your measurement.
  • Step 4: Measure the humidity level using a hygrometer, which will tell you how much moisture is in the air relative to the maximum amount of moisture the air can hold at that temperature.
  • TIP: Don't confuse the dew point with the humidity level. Humidity is measured as a percentage out of 100.
  • Step 5: Track down a heat index chart by searching on the internet.

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