National Weather Service United States Department of Commerce

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; check out the heat related illnesses that can occur.

Extreme heat also impacts our infrastructure - from transportation to utilities to clean water and agriculture. High heat can deteriorate and buckle pavement, warp or buckle railway tracks, and exceed certain types of aircraft operational limits. Electricity usage increases as air conditioning and refrigeration units in homes and offices work harder to keep indoors cooler. 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 and crops may need increased water consumption, and people increase water consumption to stay hydrated and cool. Heat can have lasting impacts as crops may be damaged, reducing production which leads to short supply and or increased cost to the farmers and consumers.

Stay Informed: Monitor local radio and television (including NOAA Weather Radio), internet and social media for information and updates.

+ How to Stay Safe During Excessive Heat Events

+ Safety Tips for Parents and Caregivers

Children and Heat Vulnerability (Source: The Impacts Of Climate Change On Human Health In The United States: A Scientific Assessment)

  • Newborns, infants, and young children are particularly vulnerable to heat-related illness and death. Their bodies are less able to adapt to heat than are adults.
  • Children under four years of age experience higher hospital admissions for respiratory illnesses during heat waves.
  • The effects are more severe on children because their bodies warm at a faster rate than adults.

Protect and prepare: Keep children cool by having them drink plenty of water, take lots of breaks, wear light-colored and lightweight clothing, and limit playing outdoors to cooler times of the day. Make sure fluids are not very cold or high in sugar/sweetener content.

Heat Safety in Vehicles

Even on mild days in the 70s, studies have shown that the temperature inside a parked vehicle can rapidly rise to a dangerous level for children, pets and even adults. Leaving the windows slightly open does not significantly decrease the heating rate. A dark dashboard or car seat can quickly reach temperatures in the range of 180°F to over 200°F. These objects heat the adjacent air by conduction and convection and also give off long wave radiation, which then heats the air trapped inside a vehicle. Click here to learn more and follow these tips to ensure childrens’ safety.

  • Touch a child's safety seat and safety belt before using it to ensure it's not too hot before securing a child
  • Never leave a child unattended in a vehicle, even with the windows down, even for just a minute
  • Teach children not to play in, on, or around cars. They could accidentally trap themselves in a hot vehicle.
  • Always lock car doors and trunks--even at home--and keep keys out of children's reach.
  • Always make sure children have left the car when you reach your destination. Don't leave sleeping infants in the car.

+ Community Interventions


If you are a public official, please download the Excessive Heat Event Guidebook for best practices during heat waves and to learn about options that communities can use to develop their own mitigation plans to prepare for and respond to extreme heat. The Guidebook was developed by the Environmental Protection Agency (EPA) in 2006, in collaboration with the National Oceanic and Atmospheric Administration (NOAA), the CDC, and the Department of Homeland Security (DHS). Municipal officials in both the U.S. and Canada provided vital information you can use to help the public cope with excessive heat. The guidebook highlights best practices that have been employed to save lives during excessive heat events in different urban areas and provides a menu of options that officials can use to respond to these events in their communities. It can be found on the EPA Heat Islands webpage.

  • Assess locations with vulnerable populations, such as nursing homes and public housing
  • Staff additional emergency medical personnel to address the anticipated increase in demand
  • Shift and expand homeless intervention services to cover daytime hours
  • Open cooling centers to offer relief for people without air conditioning and urge the public to use them.
  • Provide toll-free numbers and Website addresses such as this one for heat exposure symptoms and responses
  • Open hotlines to report concerns about individuals who may be at risk
  • Coordinate broadcasts of excessive heat events response information in newspapers and on television and radio.

+ Heat Safety for Outdoor Workers

Outdoor workers can be at a higher risk to the effects of excessive heat. The Wet Bulb Globe Temperature (WBGT) is a good measure of heat stress during outdoor activities; learn more here. See Occupational Safety and Health Administration (OSHA) resources and recommended practices when working under hot conditions.

  • Drink water often
  • Rest and cool down in the shade during breaks
  • Gradually increase workload and allow more frequent breaks for new workers or workers who have been away for a week or more
  • Know symptoms, prevention, and emergency response to prevent heat-related illness and death  
  • Check weather forecasts ahead of time to be better prepared.