A dangerous heatwave is sweeping across the eastern United States, with temperatures expected to hit 38C (100F) and stay there for several days. The National Weather Service has warned that the heat index – a measure of how hot it feels based on temperature and humidity – could reach as high as 115F in cities such as Washington DC, Philadelphia and New York over the 4 July weekend. This level of rare and long-duration heat, with little or no overnight relief, affects anyone without effective cooling and adequate hydration.
Heatwaves are prolonged periods of excessively hot weather, often accompanied by high humidity. They occur when a high-pressure system traps warm air in a region, preventing the usual cooling from winds or clouds. In this case, stifling heat that had been blasting the Midwest intensified into the Northeast and mid-Atlantic states. The extreme heat is overlapping with major events: President Donald Trump hosting a celebration for America's 250th birthday, multiple World Cup matches played outdoors, and even pop star Taylor Swift's wedding at Madison Square Garden.
“An explainer on the dangerous US heatwave and why extreme heat is so deadly.”
The background to this heatwave includes an unprecedented spell of early summer heat across Europe, with record highs across the continent. While the US East Coast has seen similar extremes – New York could reach 100F in Central Park for the first time in 12 years, and Washington DC may record four consecutive days of 100F heat, a feat only achieved twice before – the current event stands out for its duration and the number of people affected.
For UK readers, this matters because it shows just how dangerous extreme heat can be, even in wealthy countries. The UK has experienced its own record-breaking heatwaves in recent years, and the same underlying factors – climate change, urban heat islands, and lack of preparedness – contribute to the risk. The National Weather Service has stressed that this heat affects everyone, but particularly those without access to air conditioning or who must work outdoors.
Q: What is a heat index? A heat index combines air temperature and relative humidity to determine how hot it actually feels to the human body. When humidity is high, sweat does not evaporate as quickly, making the body feel hotter. In this heatwave, the heat index could reach 115F, even though the air temperature is around 100F.
Q: Why is a heatwave so dangerous? Heatwaves can cause heat exhaustion and heat stroke, which can be fatal. The very young, the elderly, and people with chronic illnesses are most at risk. When temperatures do not drop significantly at night, the body has no chance to recover, increasing the danger.
Q: How can people stay safe during a heatwave? Stay inside in air conditioning if possible, drink plenty of water, and avoid strenuous outdoor activity during the hottest part of the day. Check on vulnerable neighbours and never leave children or pets in parked cars. In this case, New York City Mayor Zohran Mamdani urged all New Yorkers to stay inside and cool.
What happens next? The heat is forecast to continue through the 4 July weekend. President Trump has said he will give a “really long speech” at outdoor celebrations despite the heat. The National Weather Service continues to issue extreme heat warnings from the Midwest to the Northeast. Officials are monitoring conditions at outdoor events, and some rehearsals have been scaled back for safety.