A Primer on the Heat Index

This primer is excerpted from Mid-Atlantic Regional Climate Impacts Summary and Outlook: Spring 2025.

Introduction

Extreme heat is the deadliest weather-related hazard in the United States, contributing to more excess deaths¹ than hurricanes, tornadoes, and floods combined.² As scientific understandings of heat risk and impact on human health have evolved, the development of more precise heat measurement metrics has become increasingly vital. One such metric is the heat index.

The heat index serves as a composite metric that integrates ambient temperature and humidity to provide a more accurate, "real feel" estimate of the human thermal experience. It was designed to account for the role of humidity in the body's ability to cool itself, primarily achieved through the evaporation of sweat. This cooling process becomes less efficient as moisture content in the air (humidity) rises. Consequently, the heat index aims to measure the combined impacts of air temperature and moisture, offering a more comprehensive understanding of heat stress than air temperature alone.^3,4

The heat index is used by local and state governments, emergency management organizations, and public health agencies to inform extreme heat preparation and planning, issue heat advisories, and communicate to the public. This section explores the heat index calculation and historical development, examines how the heat index functions in practice through a case study of Washington, D.C., and highlights its strengths and limitations in real-world applications. It concludes by situating the discussion within the broader context of climate change, featuring an interactive graph projecting the number of days at or above 95 degrees F in Washington, D.C. under low, medium, and high emissions scenarios.

The Heat Index Equation and Classifications

The heat index equation was developed by the U.S. National Weather Service based on prior work conducted by R.G. Steadman.⁵ It utilizes relative humidity (RH), expressed as a percentage, rather than absolute humidity. Relative humidity indicates the amount of moisture content in the air at a specific temperature relative to the maximum amount of moisture content that air can hold at that temperature.⁶ In contrast, absolute humidity measures the actual amount of water vapor in the air and is expressed by grams of water vapor/cubic meter volume of air.⁷ The heat index is communicated as an adjustment of ambient air temperature in degrees Fahrenheit (F) based on RH levels. For example, an air temperature of 92 degrees F at 60% RH results in a heat index or "real feel" value of 105 degrees F.

SOURCE: https://www.weather.gov/media/unr/heatindex.pdf

National Weather Service Heat Index Chart

The chart represents the heat index based on relative humidity (%) and temperature (°F) and categorizes the likelihood of heat disorders with prolonged exposure and/or strenuous activity into four risk levels: Caution, Extreme Caution, Danger, and Extreme Danger. The Caution level covers temperatures ranging from 80 to 88 degrees F, through 100% humidity. Extreme Caution covers temperatures 90 to 96 degrees F through 100% humidity. Danger covers temperatures 98 to 106 degrees F through 100% humidity. Extreme danger covers temperatures 108 to 110 degrees F through 100% humidity.

Relative Humidity (%)	80°F	82°F	84°F	86°F	88°F	90°F	92°F	94°F	96°F	98°F	100°F	102°F	104°F	106°F	108°F	110°F
40%	80	81	83	85	88	91	94	97	101	105	109	114	119	124	130	136
45%	80	82	84	87	89	93	96	100	104	109	114	119	124	130	137
50%	81	83	85	88	91	95	99	103	108	113	118	124	131	137
55%	81	84	86	89	93	97	101	106	112	117	124	130	137
60%	82	84	88	91	95	100	105	110	116	123	129	137
65%	82	85	89	93	98	103	108	114	121	128	136
70%	83	86	90	95	100	105	112	119	126	134
75%	84	88	92	97	102	109	116	124	132
80%	84	89	94	100	106	113	121	129
85%	85	91	97	102	112	123	132
90%	86	91	98	105	113	122	131
95%	86	93	100	108	117	127
100%	87	95	103	112	121	132

The National Weather Service (NWS) heat index classifications—Caution, Extreme Caution, Danger, and Extreme Danger—reflect escalating health risks with heat and physical activity: from possible fatigue (Caution), potential heat stroke (Extreme Caution), likely heat cramps or exhaustion without activity (Danger) and a high likelihood of heat stroke even without exertion (Extreme Danger). For more information on the heat index classifications of Caution, Extreme Caution, Danger, and Extreme Danger, see the Fall 2023 Mid-Atlantic Regional Climate Impacts and Summary.

Case Study: Washington, D.C.

Washington, D.C. uses the heat index as a way to activate heat risk reduction measures that are laid out in its Heat Emergency Plan. The District activates its Heat Emergency Plan during the Extreme Caution heat index classification and prior to the "Danger" classification. Specifically, the Plan is activated at 95 degrees F heat index or air temperature.⁸ This means that on a day with air temperature high of 90 F with 45% RH or less, DC's Heat Emergency Plan would not be activated; however, it would be triggered at 90 degrees F with 50% RH. A number of actions are taken when the heat emergency plan is activated, one of which includes opening the city's cooling centers to provide refuge for residents to recover or find relief from the heat.⁹ Washington, D.C. also uses the heat index to set and communicate thresholds for heat advisories, excessive heat watches, and excessive heat warnings to the public (Figure 2).

Heat advisory, watch, and warning messages:

1. Heat advisory: It's going to feel like it's 105 to 109 degrees outside within the next 24 hours. Minimize time outside, stay well hydrated, and check out our tips on how to stay safe.
2. Excessive heat watch: It's possible that it will feel like it's above 110 degrees outside within the next 48 hours. Prepare for extreme heat.
3. Excessive heat warning: It's going to feel like it's above 110 degrees outside within the next 24 hours. Minimize time outside, stay well hydrated, and check out our tips on how to stay safe.

Source: Ready DC¹⁰

It is important to highlight that, in addition to the use of the heat index as a trigger for emergency actions, D.C. also employs an absolute threshold of 95 degrees F for air temperature to activate the District's heat emergency plan. As previously discussed, if the District used a threshold of 95 degrees F air temperature alone, emergency actions would be activated potentially less frequently and at significantly higher "real feel" temperatures—increasing the likelihood of heat disorders with prolonged exposure and/or strenuous activity for residents.¹¹ The inclusion of the 95 degrees F air temperature as an additional threshold, however, ensures that the District's heat risk management strategies are operational even during periods of high air temperature but lower RH. For example, at 95 degrees F air temperature and 20% RH, the heat index is 93 degrees F, which falls into the "Extreme Caution" category;¹² thus, by having a temperature-based second threshold of 95 degrees F, the District would still activate its heat emergency plans.

Data Challenges and Limitations

Despite its importance and advantage over air temperature alone, there are data limitations and challenges to the use and communication of the heat index. For instance, heat index charts are based on real-feel temperatures in the shade.¹³ This is particularly significant for outdoor workers, as exposure to direct sunlight can raise heat index values by up to 15 degrees F.¹⁴ Furthermore, as a scientific measurement based on air temperature and humidity, the heat index does not take into account personal factors such as age, health, and socioeconomic status that can create discrepancies in heat risk. Thus, the heat index is most effective when used in conjunction with education about heat risk and the varied impacts of heat on human health. Other heat measurements address some heat index limitations but pose their own challenges. The Wet-Bulb Globe Temperature (WBGT), for instance, measures heat stress in direct sunlight and is calculated from temperature, humidity, wind speed, sun angle and solar radiation.¹⁵ Nevertheless, the heat index is often used because it is relatively easy to understand and communicate to the public, whereas the WBGT requires additional interpretation from trained specialists.¹⁶

Although there have been advances in heat and health research, more research is needed to better understand the relationship between air temperatures, humidity, and health outcomes.¹⁷ This understanding is crucial for establishing standardized temperature and heat index thresholds for effective heat risk management. Currently, there is no unified definition of an extreme heat event shared by state agencies and jurisdictions,¹⁸ nor are there shared temperature and heat index thresholds consistently used to activate heat emergency plans—even within individual states.¹⁹ Therefore, although the heat index remains a valuable and life-saving tool when compared to air temperature alone, further investigation is essential to improve and coordinate extreme heat planning and management.

A deeper understanding of these relationships is not only crucial for the present; given rising temperatures due to climate change, this information is necessary for developing climate adaptation strategies and data-informed decision making regarding future heat risk. As climate change increases the duration, severity, and frequency of heatwaves, the heat index becomes an even more vital tool. However, while projection of air temperatures at local scales has become increasingly feasible with advancements in climate modeling,²⁰ projecting humidity—and thus the heat index—remains a more complex and still-emerging modeling approach. While downscaling temperature projections requires only temperature data, maximum relative humidity calculations require specific humidity, minimum temperature, and surface pressure.²¹ Recent work has also suggested that the latest generation of global climate models (CMIP6) do not capture some historical humidity trends, particularly in arid and semi-arid regions.²² Downscaled humidity projections, particularly in these geographies, should therefore be considered an estimation.²³

Figure 3 illustrates the projected number of days in which Washington, D.C. is expected to experience maximum temperatures at or above 95F under under medium (SSP2-4.5), high (SSP3-7.0), and very high (SSP5-8.5) emissions scenarios. Due to current challenges in humidity projection, the analysis presented here is based solely on ambient temperature. Given Washington, D.C.'s relatively high summer humidity levels, the graph likely reflects a conservative estimate of the minimum number of days that the District's heat emergency plan would be activated under each climate scenario. Despite this limitation, the data remains valuable for understanding the magnitude of rising temperatures and the necessary heat risk management and mitigation strategies required to safeguard human health.

This figure is a line graph showing projected outcomes across three emissions scenarios from 2030 to 2099. The y-axis represents numerical values from 0 to 90 in increments of 10, and the x-axis represents years, starting at 2030 and ending at 2099.

• Very high emissions scenario (SSP5-8.5): Represented by a red line, this scenario shows a consistent increase over time, reaching nearly 90 by 2099.
• High emissions scenario (SSP3-7.0): Represented by an orange line, this scenario demonstrates an upward trend similar to the very high scenario but ends slightly lower at around 70 by 2099.
• Medium emissions scenario (SSP2-4.5): Represented by a yellow line, this scenario shows a slower and more stabilized growth, reaching approximately 45 by 2099.

The graph suggests that higher emissions scenarios lead to greater increases in the measured outcome, while medium emissions scenarios result in less drastic growth over the same time period.

SOURCE: Localized Constructed Analogs 2 (LOCA2)

Technical Note: Localized Constructed Analogs 2 (LOCA2) is a downscaled climate data product available at 6 km resolution over the continental United States.²⁴ The LOCA2 dataset includes 27 of the climate models available in the Coupled Model Intercomparison Project 6 (CMIP6) archive, for three future climate scenarios: an intermediate-emissions future (Shared Socioeconomic Pathway (SSP) 2-4.5), a high-emissions future (SSP 3-7.0) and a very high-emissions future (SSP 5-8.5).²⁵ For this graph, we used LOCA2 data over Washington, DC from 2025-2099. Access LOCA2 datasets and learn more about the methodology. - https://loca.ucsd.edu/

For more information on how to prepare and stay safe before, during, and after extreme heat, visit the American Red Cross Extreme Heat Safety information site.