Person running on a track under intense heat conditions, with a red-orange heat overlay.

Learn About Extreme Heat

Heat is a leading cause of weather-related fatalities each year. People and communities across the country experience heat impacts differently due to a combination of personal, environmental, physiological, and social factors.

Learn more below about extreme heat, including the different heat tools from the National Weather Service, below.

What is Extreme Heat?

There are several ways to define extreme heat:

  • A period of high heat and humidity with temperatures above 90°F for at least two to three days.
  • A temperature that is much hotter and/or humid than average. Because some places are hotter than others, this depends on what’s considered average for a particular location that time of year. For example, while summertime temperatures of 100˚F might be normal for Phoenix, Arizona, they would be considered extreme for Seattle, Washington.

What is a Heat Wave?

A heat wave is a period of abnormally hot weather generally lasting more than two days. Heat waves can occur with or without high humidity. They have potential to cover a large area, exposing a high number of people to hazardous heat. Heat can be very taxing on the body. Learn more about heat-related illnesses.

What is a Heat Dome?

A heat dome is an exceptionally hot air mass that develops when high pressure aloft prevents warm air below from rising, thus trapping the warm air as if it were in a dome. One of the most famous recent heat domes took place in the Pacific Northwest in 2021, which broke temperature records for the region and led to over 1,000 heat-related emergency department visits (CDC MMWR 2021).

What are Some of the Impacts of Heat?

During extremely hot and humid weather, your body’s ability to cool itself is challenged. When the body heats too rapidly to cool itself properly, or when too much fluid or salt is lost through dehydration or sweating, body temperatures rise and you or someone you care about may experience a heat-related illness. Additionally, when humidity is high, it is harder for sweat to evaporate and keep a person cool. Heat, especially when made worse by humidity, can cause heat exhaustion and heat stroke, which can lead to hospitalization or death. Learn more about the health impacts of heat here.

Extreme heat also impacts our infrastructure, including transportation, utilities, clean water, and agriculture. High heat can deteriorate and buckle pavement, warp or buckle railway tracks, and exceed aircraft operational limits. Electricity usage increases as air conditioning and refrigeration units in homes and offices work harder to keep it cooler indoors. Transmission capacity across electric lines is reduced during high temperatures, further straining the electrical grid. Water resources are also strained as conventional power plants require large quantities of water for cooling, people increase water consumption to stay hydrated and cool, and crops require increased water intake. Heat can have negative impacts on crop production, leading to a smaller supply and/or increased cost to farmers and consumers.

Hotter temperatures are expected to reduce agricultural food supply, disrupt subsistence activities, cause stress to livestock, and impact water availability. Extreme heat can strain livestock and limit their production, while changes to the natural timing of seasonal plant growth lead to decreased crop yields. Warming temperatures can also increase the range of pests and non-native species that can harm crops and livestock.

What Is The Difference Between Heat Forecast Tools From The National Weather Service?

Air temperature can become especially hazardous when it exceeds the human body’s normal temperature of 98.6°F. At this point, heat begins to build within the body. Risk is also possible at lower temperatures when performing strenuous activities. 

The National Weather Service (NWS) has three different forecast heat stress tools. Each of these tools (Heat Index, Wet Bulb Globe Temperature, and the experimental HeatRisk) have strengths and limitations and are designed for different audiences. The application of each tool must be for the appropriate audience to ensure effective utilization and heat stress awareness.

Color-coded chart from the National Weather Service showing the Heat Index based on temperature (80°F to 110°F) and relative humidity (40% to 100%). The chart indicates the likelihood of heat disorders with prolonged exposure or strenuous activity. Zones range from yellow (Caution) to dark red (Extreme Danger). For example, at 96°F with 65% humidity, the Heat Index reaches 121°F, falling in the Extreme Danger category. NOAA logo is visible in the bottom right corner.

The Heat Index

The Heat Index tells you how hot it feels by combining temperature and humidity. For example, 96°F combined with 65% humidity feels like 121°F. Heat Index values assume shade and light wind—direct sun can make it feel up to 15°F hotter. The National Weather Service issues alerts when the index exceeds 105–110°F for two or more days. Strong, dry winds can also increase heat stress. Use the chart or calculator above to check current conditions.
Person running on a track under intense heat conditions, with a red-orange heat overlay.

Wet Bulb Globe Temperature (WBGT)

WBGT measures heat stress by factoring in temperature, humidity, wind, and sunlight. It’s especially useful for protecting outdoor workers and athletes. WBGT helps guide safe activity levels—many school sports programs and workplaces use it to adjust schedules.
Map of the eastern United States showing heat risk levels using color-coded zones. Areas with the highest heat risk are marked in dark red and purple, primarily covering parts of the Midwest, Ohio Valley, and Mid-Atlantic regions. Surrounding areas in orange and yellow indicate moderate to lower heat risk. Major cities such as Chicago, St. Louis, Atlanta, and Washington, D.C. are included in the affected regions.

HeatRisk Forecast

HeatRisk is an experimental forecast tool from the National Weather Service that predicts the risk of heat-related impacts over a 24-hour period. It considers how unusual the heat is for the time of year, how long the heat will last—including overnight temperatures—and whether the heat poses an elevated health risk based on CDC data.

Familiarize Yourself With These Terms to Help Identify an Extreme Heat Hazard:

  • Heat Wave – Prolonged period of excessive heat, often combined with excessive humidity.
  • Heat Index – A number in degrees Fahrenheit (F) that tells how hot it feels when relative humidity is added to the air temperature. Exposure to full sunshine can increase the heat index by 15 degrees.
  • Heat Cramps – Muscular pains and spasms due to heavy exertion. Although heat cramps are the least severe, they are an early sign that the body is having trouble coping with the heat.
  • Heat Exhaustion – Typically occurs when people exercise heavily or work in a hot, humid place where body fluids are lost through heavy sweating. Blood flow to the skin increases, causing blood flow to the vital organs to decrease. This results in a form of mild shock. If not treated, the victim’s condition will worsen. Body temperature will keep rising and the victim may suffer heat stroke.
  • Heat Stroke – A life-threatening condition. The victim’s temperature control system, which produces sweating to cool the body, stops working. The body temperature can rise so high that brain damage and death may result if the body is not cooled quickly.
  • Sun Stroke – Another term for heat stroke.
  • Extreme Heat Watch – Be Prepared! An Extreme Heat Watch is issued when conditions are favorable for an extreme heat event but its occurrence and timing is still uncertain. Plan to suspend all major outdoor activities if a warning is issued. If you do not have air conditioning, locate the nearest cooling shelter or discuss staying with nearby family or friends who have air conditioning.
  • Extreme Heat Warning – Take Action! An Extreme Heat Warning is issued when extremely dangerous heat conditions are expected or occurring.  Avoid outdoor activities, especially during the heat of the day. If you must be outside, be sure to drink plenty of water and take frequent breaks in the shade. Stay indoors in an air-conditioned space as much as possible, including overnight. Check on family and neighbors. 
  • Adaptability – The ability of a person to take measures to reduce exposure and sensitivity; for example, avoiding outdoor activities during the day or wearing personal protective equipment (PPE) that is designed to mitigate heat buildup. When exposure is not preventable, adaptability can help reduce the impact of heat.
  • Exposure – The extent to which an individual is exposed to extreme heat. Going outside on a hot, humid day and working in direct sunlight constitutes high exposure, while reduced exposure includes avoidance of these activities. Some exposure is not preventable.
  • Sensitivity – Inherent characteristics of a person that make them disproportionately affected by heat, such as pre-existing conditions, age, or occupation. To understand how to protect these groups, see Populations of Concern.

Extreme Heat Forecast Information

Map of the eastern United States showing heat risk levels using color-coded zones. Areas with the highest heat risk are marked in dark red and purple, primarily covering parts of the Midwest, Ohio Valley, and Mid-Atlantic regions. Surrounding areas in orange and yellow indicate moderate to lower heat risk. Major cities such as Chicago, St. Louis, Atlanta, and Washington, D.C. are included in the affected regions.

These forecasts are organized from longer term outlooks, at the top, to near term forecasts at the bottom.

CDC Publishes 2023 Heat-Related Emergency Department Visits Report 

A picture of the outside of an emergency department

Many regions of the United States experienced record-breaking high temperatures in 2023, with populations exposed to extremely high temperatures for prolonged periods. The Centers for Disease Control and Prevention (CDC) examined emergency department (ED) visits associated with heat-related illness (HRI) from the National Syndromic Surveillance Program and compared daily HRI ED visit rates during the warm-season months (May–September) of 2023 with those during 2018–2022. In the 2023 warm-season months, daily HRI ED visit rates peaked in several regions and remained elevated for a prolonged duration. More males than females sought care in EDs for HRI, especially males aged 18–64 years. CDC issued multiple public health alerts using the Epidemic Information Exchange system to bring attention to increases in ED utilization for HRI.

View the CDC Heat-Related Emergency Department Visits Report
A collage of different weather images, including palm trees in the wind, a hurricane, a tornado, and drought in a basin.

Compound and Cascading Hazards: Heat’s Other Health Impacts

Extreme heat can also exacerbate and compound other health hazards. Click on the hazards below to learn more:

Heat Impacts on Marine Systems

Extreme heat can also be destructive and deadly for marine systems. In 2023, a marine heat wave caused unprecedented heat stress conditions in the Caribbean Basin, including waters surrounding Florida and in the Gulf of Mexico. A marine heat wave dominated the northeastern Pacific from 2013 to 2020 and upended ecosystems across a huge swath of the Pacific Ocean, causing fishery collapses and fishery disaster declarations up and down the coast. NOAA launched an experimental marine heatwave forecasting tool in 2023, providing a future-focused look at how temperatures are changing. Scientists believe climate change may make marine heat waves more common and global forecasts can provide up to a year’s advance notice of marine heatwaves.