The heat index, or apparent temperature, is a “feels-like” temperature measurement that measures air temperature against relative humidity to determine what apparent conditions feel like to the human body.
Heat is the leading cause of weather-related deaths in the United States – but the number on the thermometer doesn’t completely explain an individual’s heat risk in their particular environment. Below, Perry Weather’s Director of Science Dr. David Martin explains what heat index is and what heat index measures, while sharing how to calculate heat index and providing heat index charts to determine the heat risk at specific conditions.
What is Heat Index?
Heat index is a temperature measurement that combines dry-bulb air temperature and relative humidity, in shaded areas, to provide a more accurate measure of temperature as perceived by humans. The theory of heat index, and its supporting equations, was developed in 1979 by Robert G. Steadman. That same year, the National Oceanic and Atmospheric and Administration’s (NOAA) National Weather Service (NWS) adopted a simplified version of the heat index into its operations using only air temperature and relative humidity as inputs for the calculation.
Today, heat index is used to guide heat product policy and criteria, including the NWS’s Excessive Heat Watch and Excessive Heat Warnings. But there are several limitations to heat index as a measurement of how heat feels – in both the assumed calculation constants and the way different bodies perceive heat.
“The problem with heat index is the model assumes a standard person who weighs about 147 pounds, is 5 foot 7, is walking at about 3 miles per hour, and is wearing light clothing in the shade,” says Perry Weather Director of Science Dr. David Martin. “It doesn’t account for the added metabolic demands of activity, such as the intensity of labor or exercise for an athlete.”
What Does Heat Index Measure?
Heat index measures a human’s perception of heat by combining shaded air temperature and relative humidity. When the body gets too hot, it begins to sweat to cool off. But if the perspiration isn’t able to evaporate due to high humidity, the human body will feel warmer. That’s why a day with 95°F conditions and high humidity feels much hotter than another 95°F day with lesser humidity.
Heat index calculator
Heat index calculators use air temperature and relative humidity to show what the heat feels like to the human body. This calculator uses the Rothfusz equation, which is what the NWS uses to calculate the heat index.
How to Calculate Heat Index
To calculate heat index, the NWS uses a simplified version of the Rothfusz equation to calculate values consistent with Steadman’s results.
“Heat index estimates heat stress using just two variables, air temperature and humidity,” says Dr. Martin. “You only need those two numbers, and they’re available from any weather forecast.”
The formula itself is based on many assumptions, including body mass, height, clothing, physical activity, shaded conditions, and consistent wind speed. This means it doesn’t adjust for an individual’s size, exertion, protective gear, or actual sun and wind exposure.
Heat index calculation
You can use the following equation to calculate heat index by hand.
HI = 0.5 * {T + 61.0 + [(T-68.0)*1.2] + (RH*0.094)}
The above equation is not an appropriate heat index calculation for temperatures above about 80°F. In that case, scientists are suggested to use the full regression equation, along with relevant adjustments. Heat index calculation adjustments account for deviations in humidity levels within specific temperature ranges. Below is an example of a humidity adjustment.
HI = -42.379 + 2.04901523*T + 10.14333127*RH – .22475541*T*RH – .00683783*T*T – .05481717*RH*RH + .00122874*T*T*RH + .00085282*T*RH*RH – .00000199*T*T*RH*RH
where T is temperature in degrees Fahrenheit and RH is relative humidity in percent. HI is the heat index expressed as an apparent temperature in degrees Fahrenheit.
Heat Index Chart
A heat index chart shows how heat index goes up as temperature and relative humidity increase. The following heat index chart can be used to manually calculate heat index using values for apparent temperature and relative humidity.
How to read a heat index chart
Find your air temperature on the left axis and the relative humidity on the top axis of the graph. Follow the corresponding row and column until they meet. That number is the heat index, which is what the temperature feels like in shaded conditions.
For example, an air temperature of 96°F and relative humidity of 50% produce a heat index of approximately 108°F. The chart’s color at that intersection indicates the heat risk category, which is red, or in the danger zone, according to the NWS.
Heat Index Levels
Different heat index levels can be used to determine different levels of heat risk, and the NWS uses color-coded categories on a heat index chart to help organizations assess risk.
“By measuring the microclimate in your area and matching it to a risk category, we can make modifications,” says Dr. Martin. “Guidelines include reducing the total amount of time someone works, providing rest periods to let their core temperature settle, increasing hydration breaks, and limiting the intensity of the work or moving it to a cooler part of the day.”
Here are the NWS guidelines for each heat index level.
What are the symptoms of heat illness at different heat index levels?
Individuals may experience several different heat disorders when exposed to different heat index levels. Humans may encounter:
- Sunburn: Sunburn results in redness and pain. In severe cases, it might also cause skin swelling, painful blisters, fever, and prolonged headaches.
- Heat cramps: Heat cramps are painful spasms that usually occur in the legs and abdominal muscles. It’s associated with heavy sweating and caused by dehydration.
- Heat exhaustion: Heat exhaustion is characterized by heavy sweating and general weakness. It is usually associated with cold, pale, and clammy skin with a thready pulse. Some people may faint and vomit when suffering from heat exhaustion.
- Heat stroke: Heat stroke occurs when individuals experience a body temperature of 106°F as a result of exposure to heat. It’s usually accompanied by hot dry skin as well as a rapid and strong pulse.
“Symptoms don’t line up neatly with a specific heat index number,” cautions Dr. Martin. “They depend on how hot someone’s core temperature gets, which is driven by how hard they’re working, how acclimatized they are, and the environment. For a general rule, the higher the environmental heat stress is, the more likely serious heat illness becomes.”
How to Measure Heat Index
There are several ways to measure heat index. Organizations can manually calculate it using current weather data aggregated from online or an onsite weather station, or use an app like OSHA-NIOSH Heat Safety Tool App or Perry Weather to measure current heat index and plan accordingly for heat risk.
There are several different monitoring tools available for heat index.
- Digital thermometers and hygrometers provide measurements for temperature and humidity, which can be used to calculate heat index.
- Wearable devices can be used to track personal exposure to heat and provide real time data on body temperature and surrounding environmental conditions.
- Weather stations, whether on-site or nearby, can monitor local temperature and humidity across a given area.
- Monitoring sensors are devices that record data on temperature and humidity and transmit it to a central location for analysis.
Heat index vs WBGT
Both heat index and WBGT measure how hot temperature feels to your body. The two measurements are designed for different environments, and confusing them could lead to dangerous decisions.
Dr. Martin cautions that heat index is just a part of the bigger heat-stress picture.
“Heat index doesn’t account for the metabolic demands of activity, such as the intensity of labor or exercise for an athlete,” said Dr. Martin. “This is why wet bulb globe temperature is a better index.”
Read the full heat index vs WBGT comparison.
How Can Heat Index Be Used to Prevent Heat Illness?
Heat index can help prevent heat illness by identifying hot, humid conditions that call for additional precautions. Organizations should use heat index alongside their heat safety policy and reassess conditions throughout the day – and add more frequent rest breaks, access to water and shade, and changes to protective gear and clothing as precautions, as suggested by applicable governing bodies.. For strenuous outdoor work or sports, WBGT provides a more complete assessment of environmental heat exposure.
When to use heat index
You can use heat index to have a more accurate perception of heat and plan to keep your team, workers, or community safe at different temperatures. The NWS has four different heat index zones designed to help. Here’s a refresher:
- Caution – 80°F–90°F: Basic heat safety and planning measures are suggested. Fatigue is possible with prolonged physical activity or exposure to this heat index.
- Extreme Caution – 90°F–105°F: Precautions should be taken. Sunstroke, heat cramps, and heat exhaustion are possible.
- Danger – 105°F–130°F: Sunstroke, heat cramps, or heat exhaustion is likely, and heat stroke is possible with exposure over time. Additional precautions are needed.
- Extreme Danger – 130°F or higher: Heatstroke and sunstroke are very likely with continued exposure, and aggressive protective measures are needed.
Heat index policy
There is no specific standard for using heat index to address heat-related hazards for the workplace, or in athletics. Still, safety administrators regularly use the metric to assess risk for heat-related hazards.
Generally, Dr. Martin suggests creating thresholds for heat so organizations know when to modify activity. “It’s important at each heat index value that you choose to be very descriptive about how the activity is going to be modified,” said Dr. Martin. “Things like the length of the rest breaks, how much total time someone is going to work, and what cooling methods are being put into place.”
How construction safety personnel should use heat index
Understanding heat index is critical for keeping construction and other outdoor workers safe in the workplace. Construction teams can use heat index forecasts to anticipate hot conditions, schedule strenuous tasks during cooler hours, and prepare water, shade, and rest breaks. Because heat index doesn’t account for direct sunlight, workload, or protective clothing, it should be a starting point for assessing risk. On-site WBGT monitoring provides a more complete picture of environmental heat exposure to help guide work-rest decisions.
While OSHA does not currently have a federal heat standard, its National Emphasis Program targets heat hazards through resources, inspections, and outreach.
How athletic trainers and directors should use heat index
Coaches and athletic trainers can use heat index forecasts to flag potentially dangerous conditions and plan practice times, hydration access, and shaded recovery areas. For decisions about practice intensity, equipment, and activity restrictions, use on-site WBGT readings alongside your governing organization’s heat safety policy. WBGT accounts for sunlight and wind, helping teams assess conditions athletes actually experience on the field.
Current heat policy for athletics varies by governing body.
How cities and governments should use heat index
Emergency response and preparedness personnel depend on heat index measurements to predict and address heat-related crises. NWS Extreme Heat Watch alerts signal that dangerous heat is possible and gives departments time to prepare cooling centers, coordinate outreach, and review outdoor event schedules. A Heat Advisory or Extreme Heat Warning signals a need to put protective measures into action, including public safety messaging and access to cool spaces. Alert criteria vary by location, so departments should follow their local NWS forecast office’s guidance.
Heat index limitations
A lower heat index doesn’t guarantee that outdoor work or exercise is completely safe. Heat index has inherent limitations – most prominently, the fact that the calculation only accounts for shady, calm weather, and doesn’t factor in direct sunlight, changes in wind, and radiant heat from nearby surfaces or equipment. It also doesn’t account for things like strenuous activity, heavy clothing, or protective equipment, which can increase heat risk for certain groups.
While some scientists, such as David Romps and Yi-Chuan Lu at the University of California, Berkeley, have attempted to amend heat index calculations and revisit Steadman’s original research, the National Weather Service and other groups just use multiple tools to evaluate extreme heat, including wet bulb globe temperature.
“It’s really the default for a lot of employers,” says Dr. Martin. “Partly because it’s what OSHA references and partly because it’s easy to get. But at the end of the day, if we are assessing the environmental risks and then modifying activity based on those measurements, I am happy. Do I believe those modifications are more effective with WBGT? Yes, absolutely. But a modification is better than none.”
The Bottom Line
Heat index helps explain how hot shaded conditions feel to individuals. It’s a useful starting point for planning outdoor activities and preparing for extreme heat, but it doesn’t capture every factor that affects heat risk. Use the heat index chart and heat index calculator above to better understand weather conditions, and pair that information with your organization’s heat safety policy and WBGT monitoring when appropriate.
Frequently Asked Questions (FAQ)
What is the difference between heat index and air temperature?
Air temperature measures how hot or cold the air is. Heat index combines air temperature and relative humidity to estimate how hot shaded conditions feel to the human body. Higher humidity slows sweat evaporation, so the same air temperature can feel hotter on a humid day.
Is heat index the same as feels-like temperature?
Heat index is a type of feels-like temperature used to describe the combined effects of heat and humidity. However, “feels-like” is a broader term: weather forecasts may also use wind chill in cold conditions or other calculations that include sun and wind.
Does heat index account for direct sunlight?
No. Standard heat index values assume shade and light wind, so they can underestimate heat exposure in direct sunlight. The National Weather Service notes that full sunshine can increase apparent heat by up to 15°F. This is not a fixed correction to add to every reading.
What heat index is dangerous?
The National Weather Service heat index chart identifies 105°F–130°F as the Danger range, where heat exhaustion is likely and heat stroke is possible with prolonged exposure or physical activity. Heat illness can also occur at lower readings, particularly during strenuous activity or direct sun exposure. Heat index is not a universal stop-work or practice-cancellation threshold.
Can heat index be lower than air temperature?
Yes. When relative humidity is low, sweat can evaporate more readily, and the calculated heat index can be lower than the air temperature. For example, the National Weather Service gives a heat index of about 96°F for an air temperature of 100°F and relative humidity of 15%. A lower heat index does not mean conditions are safe.
When should an organization use WBGT instead of heat index?
Use wet bulb globe temperature (WBGT) when assessing environmental heat exposure during strenuous outdoor work or sports, especially in direct sunlight, or when your governing policy requires it. WBGT accounts for humidity, radiant heat, air temperature, and air movement. Interpret the reading alongside workload, clothing, acclimatization, and applicable heat safety requirements.
