Calculating the Heat Index

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You check the high temperature to see how hot the day will be. But in summer, there's another temperature besides air temperature that's just as important for knowing how hot you should expect to feel -- the Heat Index.

The Heat Index tells you how hot it feels outdoors and is a good tool for determining how at risk you may be on a given day and time to heat-related illnesses. How can you find this summer temperature?

There are 3 ways (other than looking at your forecast) to find out what your current Heat Index value is: 

  • look at a Heat Index chart,
  • use a Heat Index weather calculator, or
  • calculate it by hand using the Heat Index equation.

Here's how to do each.

Reading a Heat Index Chart

  1. Use your favorite weather app, watch your local news, or visit your NWS local page to find the current air temperature and humidity where you live. Write these down.
  2. Download this NWS Heat Index chart. Print it in color or open it in a new internet tab.
  3. To find the Heat Index temperature, put your finger on your air temperature. Next, run your finger across until you reach your relative humidity value (round to the nearest 5%). The number you stop at is your Heat Index.  

The colors on a Heat Index chart tell how likely you are to suffer heat illness at specific Heat Index values. Light yellow areas indicate caution; dark yellow areas, extreme caution; orange areas, danger; and red, extreme danger.

Keep in mind that Heat Index values on this chart are for shaded locations. If you're in direct sunlight, it can feel up to 15 degrees hotter than what's listed.

Using a Heat Index Weather Calculator

  1. Use your favorite weather app, watch your local news, or visit your NWS local page to find the current air temperature and humidity where you live. (Instead of humidity, you could also use dew point temperature.) Write these down.
  1. Go to the online NWS Heat Index Calculator.
  2. Enter the values you wrote down into the correct calculator. Be sure to enter your numbers in the correct boxes -- either Celsius or Fahrenheit!
  3. Click "calculate." The result will be displayed below in both Fahrenheit and Celcius. Now you know how hot it "feels" outside!

Calculating Heat Index By Hand

  1. Use your favorite weather app, watch your local news, or visit your NWS local page to find the current air temperature (in °F) and humidity (percentage). Write these down.
  2. To approximate the heat index value, plug your temperature and humidity values into this equation and solve.

Edited by Tiffany Means

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Oblack, Rachelle. "Calculating the Heat Index." ThoughtCo, Apr. 23, 2017, thoughtco.com/calculating-the-heat-index-3444309. Oblack, Rachelle. (2017, April 23). Calculating the Heat Index. Retrieved from https://www.thoughtco.com/calculating-the-heat-index-3444309 Oblack, Rachelle. "Calculating the Heat Index." ThoughtCo. https://www.thoughtco.com/calculating-the-heat-index-3444309 (accessed January 20, 2018).