What is a heat dome? The US heatwave explained 

An intense heatwave is set to blanket much of the central and eastern United States this week as a “heat dome” settles over the region, bringing days of oppressively high temperatures and humidity ahead of the Fourth of July weekend and FIFA World Cup matches in several US cities.

Forecasters say in some places it could feel as hot as 46 degrees Celsius (115 degrees Fahrenheit). Dozens of temperature records could be broken, according to the National Weather Service (NWS), which called the conditions “dangerous”. More than 60 million people are currently under heat alerts.

At the centre of this week’s forecast is a weather phenomenon known as a heat dome. What is it, and why does it make heatwaves even more intense and unbearable?

What is a heat dome?

A heat dome is a large area of high pressure, formed when warm air flows northward, that acts like a lid over the atmosphere, trapping hot air close to the ground.

As the air sinks, it compresses and warms even more. At the same time, the pressure system helps prevent cooler air and storms from moving in, allowing heat to build at the surface and remain trapped there. With few clouds and little wind, the sun has more direct access to the ground, creating a heat feedback loop.

Heat domes are linked to prolonged heatwaves that can last for days.

How long will it last?

The heat dome is already building and is expected to strengthen over the coming days, spreading from the central US towards the east coast, with dangerous heat lasting several days into early July.

The hottest conditions are expected Thursday and Friday, according to the NWS, and are set to continue through the Fourth of July weekend, which marks the 250th anniversary year of the US, and forecasters say some areas across the Great Plains, the southeast, and the mid-Atlantic are likely to remain unusually hot into next weekend.

What will the highest temperatures be?

Many places are expected to see daytime temperatures in the high 30s Celsius (low 100s Fahrenheit), but humidity will make it feel much hotter. In parts of the central and eastern US, the heat index – a measure of how hot it feels when humidity is factored in – could climb between 40C and 46C (100F and 115F).

“That’s heat that’s impactful to anyone,” said NWS meteorologist Bryan Putnam. “It’s not just older adults or younger children or people who are spending a ton of time outdoors, maybe straining themselves a little more than normal. This is heat that really could impact everyone, especially with people outdoors going into the holiday weekend.”

The nights won’t bring much relief either, with temperatures expected to stay in the 20s Celsius (70s Fahrenheit) overnight, creating potentially miserable sleep conditions for those without air conditioning and making it harder for people to cool down.

“Even after the sun goes down, it’s still going to be very hot,” said AccuWeather senior meteorologist Alan Reppert. “We’re at a pattern that’s really going to be hot during the good portion of the afternoon and even into the evening hours.”

Which parts of the US will be hit the hardest?

The most dangerous conditions are expected in a broad corridor stretching from the Great Lakes to the East Coast, where several cities could experience their hottest day of the year so far. New York City, Philadelphia, Washington, Baltimore, Chicago, Indianapolis, Detroit and St Louis are all expected to be affected, with temperatures also soaring farther south in Dallas, Little Rock and Memphis.

Several of those cities are also hosting FIFA World Cup events. In Philadelphia, organisers have already changed Fan Festival hours to start later in the day.

Cities across the US are rolling out emergency measures as temperatures climb.

Chicago said it would open cooling centres and send city workers to check on vulnerable residents.

In New York City, Mayor Zohran Mamdani’s office announced what it called an “unprecedented” response to the heat, including hydration vans and pop-up cooling stations equipped with misting fans and cooling towels.

Washington, DC, where temperatures are expected to exceed 38C (100F) from Thursday through Saturday, the heat will coincide with Fourth of July celebrations, including what organisers say will be the largest fireworks display ever held on the National Mall.

What are some ways to stay cool?

The NWS says people should stay hydrated, avoid strenuous outdoor activity during the hottest part of the day and seek air conditioning or cooling centres where possible. If you’re spending time outside, wear loose, lightweight clothing and stay near shady areas.

Experts say one of the biggest risks during a prolonged heatwave is that the body doesn’t have time to cool down overnight, which can make the effects of the heat build up from one day to the next. They also recommend drinking water before you feel thirsty and limiting alcohol, which can increase the risk of dehydration.

“If somebody realises that they’re hot, but they’re not sweating, or if they begin to feel a little bit dizzy, those are some signs that they really need to take a break, get inside, find some cooling, and drink plenty of water,”  said Geoff Cornish, assistant chief video meteorologist for the weather forecasting company AccuWeather. “And if they really begin to experience significant symptoms, they need to seek medical attention right away.”