The National Weather Service (NWS) defines a heat wave as a period of abnormally hot weather that generally lasts more than two days. Such periods can occur with or without high humidity, and the agency notes their potential to cover a large area and expose many people to hazardous heat. Its guidance describes heat as taxing on the body.
WBGT and activity conditions
The NWS's Heat Forecast Tools guidance identifies WBGT as a particularly effective indicator of heat stress for active populations, including athletes and outdoor workers. WBGT uses temperature, humidity, wind, solar radiation and other weather parameters. According to the agency, it can inform modifications to activity during exercise or outdoor work.
The NWS also notes that the American College of Sports Medicine bases its guidelines for the intensity of sport practices on WBGT.
HeatRisk and upcoming heat
The NWS HeatRisk forecast assigns both a color and a numeric value to forecast heat at a specific location. These express a level of heat concern, and the forecast also identifies groups potentially most at risk at that level. Recommendations for heat protection accompany the forecast, which the agency describes as useful for planning for upcoming heat and its associated potential risk.
A daily HeatRisk value is calculated for each location using the high-resolution NWS national gridded forecast database. The forecast period extends from the current date through seven days into the future. The NWS labels HeatRisk experimental and states that it is not supported 24/7.
Heat watch and warning names
A National Weather Service announcement set March 4, 2025, as the implementation date for changes to its heat watch and warning names. The agency said the purpose was to make heat forecasts easier to understand ahead of the summer season. An official Service Change Notification for the heat watch and heat warning name changes had been released on September 5, 2024.
In explaining the changes, the announcement described the preceding year as the hottest on record for the contiguous United States and cited increasingly frequent heat waves. It also noted that the NWS had taken similar steps for cold weather, consolidating and renaming those products.
The announced replacement names were Extreme Heat Watches for Excessive Heat Watches, and Extreme Heat Warnings for Excessive Heat Warnings. The NWS said renaming these products would enable clear and timely decision support.
Scheduling strenuous activity
In its guidance for a heat wave, the NWS recommends slowing down and reducing, eliminating or rescheduling strenuous activities until the coolest time of the day. It also recommends checking weather forecasts ahead of time as preparation for the heat.
For people it identifies as particularly vulnerable, NWS guidance recommends staying in the coolest available place, which it says is not necessarily indoors. Its adult examples include older adults, especially those with pre-existing conditions, taking certain medications, living alone or with limited mobility, as well as people with chronic medical conditions and pregnant women.
The same guidance recommends lightweight, loose-fitting, light-colored clothing to reflect heat and sunlight, and minimizing direct exposure to the sun. It also states that sunburn reduces the body's ability to dissipate heat.