Calculating the Heat Index

We often check the high temperature forecast to see how hot the day will be. But that figure often doesn't tell the whole story. Another number—the relative humidity—frequently affects the way we perceive the air temperature, especially in the summer, a different temperature value that takes the humidity into account is just as important in knowing how hot we 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 might be on a given day and at a given time for heat-related illnesses. There are three ways (other than regular forecasts, which sometimes give air temperature and heat index) to find out the heat index value:

• Look at an online heat index chart.
• Use an online heat index calculator.
• Calculate it by hand using an online heat index equation.

Here are explanations of these three ways to check the heat index:

Read a Chart

Here's how to read a heat index chart:

1. Use your favorite weather app, watch your local news, or visit your National Weather Service (NWS) local page to find the air temperature and humidity where you live. Write them down.
2. Download the NWS heat index chart. Print it in color or open it in a new Internet tab.
3. Put your finger on the air temperature in the column on the far left. Next, run your finger across until you reach your relative humidity (rounded to the nearest 5%) by following the numbers across the top row of the chart. The number where your finger stops is the heat index.

The colors on a heat index chart tell how likely you are to suffer heat illness at specific heat index values. Pink areas indicate caution; yellow areas suggest extreme caution; orange areas predict danger; and red areas warn of extreme danger.

Keep in mind that heat index values on this chart are for shaded locations. Being in direct sunlight can feel up to 15 degrees Fahrenheit hotter than what is listed.

Use a Calculator

Here's how to determine the heat index using the NWS calculator:

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

Calculate by Hand

Here's how to come up with your own calculation (if you're looking for a challenge):

1. Use your favorite weather app, watch your local news, or visit your NWS local page to find the air temperature (in degrees Fahrenheit) and humidity (percentage). Write these down.
2. Plug your temperature and humidity values into this equation and solve.

Source

• "What is the heat index?" National Weather Service.