What the heat index is and how the National Weather Service calculates it

What the heat index means

The NWS Amarillo office calls the heat index the apparent temperature. It is what the temperature feels like to the body when relative humidity is combined with air temperature. The national NWS heat page puts it another way. It calls the heat index a measure of how hot it really feels when relative humidity is factored in with the actual air temperature.

The NWS glossary says relative humidity (RH) is a ratio, expressed in percent, of the moisture present relative to the amount that would be present if the air were saturated.

The glossary adds a warning. Relative humidity by itself does not directly indicate the actual amount of moisture in the air, because it depends on temperature too. The glossary calls the dew point a measure of atmospheric moisture. The NWS glossary defines dew point as the temperature to which air must be cooled to reach saturation, assuming air pressure and moisture content stay constant. A higher dew point means more moisture in the air.

Indoor humidity is a separate question from the outdoor forecast. For that, see our guide to good humidity for a house.

Why humid air feels hotter

The reason is sweat. The NWS Amarillo page says that when the body gets too hot, it begins to sweat to cool itself off. When sweat evaporates off the body, it effectively reduces the body's temperature.

Humidity slows that down. The same page says that when relative humidity is high, the rate of evaporation from the body decreases. In other words, the human body feels warmer in humid conditions.

Dry air works the other way. The page says the body actually feels cooler in arid conditions. When the relative humidity is low, the apparent temperature can actually be lower than the air temperature.

The Centers for Disease Control and Prevention (CDC) explains what happens when this cooling breaks down. The CDC's National Institute for Occupational Safety and Health (NIOSH) says heat stroke occurs when the body can no longer control its temperature. In heat stroke, it says, the sweating mechanism fails and the body is unable to cool down.

How the National Weather Service calculates it

The NWS Amarillo page says that to read the heat index from its chart, you need to know the air temperature and the relative humidity.

The NWS also runs an online calculator through its Weather Prediction Center, part of the National Oceanic and Atmospheric Administration (NOAA). That calculator has one version that takes relative humidity and another that takes the dew point. It accepts temperatures in Fahrenheit or Celsius.

There is also an equation that the NWS Amarillo page says gives a very close approximation to the heat index. The Weather Prediction Center says the computation refines a result from multiple regression analysis by Lans P. Rothfusz. Rothfusz described it in a 1990 NWS Technical Attachment.

The equation uses two inputs, temperature in degrees Fahrenheit and relative humidity. Its result is the heat index, expressed as an apparent temperature in degrees Fahrenheit.

The Weather Prediction Center says the Rothfusz equation is not appropriate when the heat index would be below about 80 degrees. For those cases a simpler formula is used. In practice, the simple formula is computed first and averaged with the temperature. If that value is 80 degrees or higher, the full equation is applied, along with any adjustments.

The equation is close but not exact. The NWS Amarillo page says it has an error of plus or minus 1.3 degrees Fahrenheit.

What the number does not include

The heat index assumes shade. The national NWS heat page says heat index values were devised for shady, light wind conditions. It says exposure to full sunshine can increase heat index values by up to 15 degrees Fahrenheit. The NWS Amarillo page agrees and notes that this surprises many people.

The values also assume light wind. The national page warns that strong winds, particularly with very hot, dry air, can be extremely hazardous.

For people working or exercising outdoors, the NWS points to a second measure. It is called the Wet Bulb Globe Temperature, or WBGT. The NWS says WBGT uses temperature, humidity, wind, solar radiation and other weather factors. The NWS calls WBGT a particularly effective indicator of heat stress for outdoor workers and athletes.

What the heat index bands mean for your body

The NWS Amarillo page sorts heat index values into four bands. For the first three bands, the page lists these effects 'with prolonged exposure and/or physical activity'.

Caution runs from 80 to 90 degrees Fahrenheit. There, fatigue is possible.

Extreme Caution runs from 90 to 103 degrees. Heat stroke, heat cramps or heat exhaustion are possible.

Danger runs from 103 to 124 degrees. Heat cramps or heat exhaustion are likely, and heat stroke is possible.

Extreme Danger is 125 degrees or higher. In that band, heat stroke is highly likely.

The page says heat indices meeting or exceeding 103 degrees can lead to dangerous heat disorders with prolonged exposure and/or physical activity. It also says that in the Panhandles, low relative humidity values make it somewhat unusual to see heat index values that high. Our reading: 108 degrees falls inside the Danger range of 103 to 124 degrees.

Heat alerts in Tampa Bay

Heat alerts are not set the same way everywhere. The NWS says its local offices often work with local partners to decide when to issue an alert. As its own example, it says Florida residents are much more prepared for 90°F-plus weather than residents of Alaska.

The NWS Tampa Bay office lists its criteria on its Daily Heat Index page. It issues a Heat Advisory when daytime heat index values are expected to be between 108 and 112 degrees Fahrenheit. It lists its higher warning and watch levels at 113 degrees or more.

That page still uses the older names Excessive Heat Warning and Excessive Heat Watch. The NWS renamed those products Extreme Heat Warning and Extreme Heat Watch, effective March 4, 2025.

The Tampa Bay page says its heat index forecasts come from NWS forecasters weighing computer models, satellite, radar and other observations. It says the forecast graphics are updated at least twice per day.

During a Heat Advisory, the NWS says to consider postponing outdoor activities, especially during the heat of the day. If you must be outside, it says to drink plenty of water and take frequent breaks in the shade. It also says to stay in a cool place. During an Extreme Heat Warning, it says to stay indoors with air conditioning as much as possible, including overnight, and check on family and neighbors. For an Extreme Heat Watch, it says that if you do not have air conditioning, find the nearest cooling shelter or stay with family or friends who have it.

If your air conditioning (AC) is struggling on those days, see why an AC can't keep up with the heat. For ways to stay cooler without it, see keeping a house cool without AC.