The US is currently experiencing its first big heat wave of 2025 thanks to a powerful ‘heat dome’ covering large parts of the country.
The powerful ‘heat dome’ is currently covering vast swathes of the country, bringing with it stifling temperatures and oppressive humidity.
National Weather Service has issued warnings of severe temperature impacts, with more than 150 million Americans under the highest-level extreme heat alerts.
Governor Kathy Hochul declared a state of emergency across 32 Empire State counties in response to the rising temperatures. Philadelphia health officials have also declared a heat health emergency, urging people to look out for each other.
Climate change has been exacerbating heat waves, and the problem isn’t going away any time soon. An increasingly hot planet — due largely to burning fossil fuels such as coal, oil and natural gas — means already hot regions are getting worse.
Heat doesn’t just mean canceled events. It also poses a health risk, especially for children, older people and those with certain health conditions.
Here are some tips to stay safe in the heatwave.
The answer depends on more than the temperature. The most detailed measurement is called the wet bulb globe temperature (WBGT), which includes temperature, humidity, cloud cover and wind.
The heat index, which measures temperature and humidity, is less descriptive but easier to find on weather apps. Both explain why a shaded soccer field on a 90 degree F day (32 degree C) in arid Phoenix may be less risky than an exposed park on an 80 degree F (27 degree C) day in soupy Little Rock.
Just based on heat index, NOAA has a chart that calculates how dangerous prolonged exposure can be.
For example, a day where temperatures reach 96 degree F (36 degrees C) and 45% humidity would fall into the “danger” category for prolonged exposure or strenuous activity.
The WBGT threshold isn’t exact, but recent research suggests that even some young, healthy people can’t endure hours of exposure to high heat and humidity.
Overnight lows can be a particularly dangerous part of a heat wave, said Ashley Ward, director of the Heat Policy Innovation Hub at Duke University.
“Your body needs a reprieve,” she said. “You don’t get that overnight, we start the next day at a deficit.” Heat can worsen labor productivity and lead to more visits to the emergency room.
