Heat dome affects at least 62 million people in the US South

Excessive temperature watches expected to last until Fourth of July holiday next week

Carlos Rodriguez drinks water during a break from digging fence post holes on June 27 in Houston, Texas. AP

A prolonged heatwave kept its grip on the US South on Tuesday as temperatures rising well above 38ºC and oppressive humidity were felt across a wide area of the region through the holiday weekend.

Meanwhile, in Chicago, children, the elderly and people with respiratory diseases were being cautioned to stay indoors because of polluted air.

About 62 million Americans in central Arizona, across Texas and the Deep South and into Florida's panhandle were under excessive heat watches, warnings and advisories that were expected to last until the Fourth of July, the National Weather Service said.

“There may be more danger than a typical heat event due to the longevity of elevated record high night-time lows and elevated heat index readings during the day,” the NWS said in an advisory.

“It is essential to have a way to cool down and interrupt your heat exposure.”

Heat index temperatures were forecast to reach more than 43ºC in Dallas and New Orleans, and 41ºC in Mobile, Alabama on Tuesday, the service said.

It urged people across the region to stay out of the sun and drink plenty of fluids.

“This is a health threat,” New Orleans Mayor LaToya Cantrell told a media briefing on Tuesday.

Ms Cantrell said the city had opened cooling centres.

“This is unprecedented,” she said. “We are living in unprecedented times. We are going to do everything necessary to meet the needs of the most vulnerable.”

The stationary high pressure system across the South that is trapping the heat and humidity, known as a heat dome, has been lingering for the past few weeks.

In several upper Midwestern cities, air-quality alerts were in effect as a cloud of hazy smoke from wildfires in Canada hovered above the region.

Chicago, the third-largest city in the US, had the worst air quality of any large city in the world because of the smoke, according to IQAir, a website that tracks pollution.

The growing frequency and intensity of severe weather across the US is symptomatic of human-driven climate change, scientists say.

As of midday on Tuesday, no widespread power cuts were reported in the South.

But about 110,000 homes and businesses remained without electricity in Arkansas, Tennessee and Oklahoma after strong storms over the weekend took down power lines, according to PowerOutage.us.