What is the heat dome causing record-breaking temperatures in the US?

Millions of people in California and the US South-west are experiencing record high temperatures due to a "heat dome" this week.

About 30 million people in northern California, south through Arizona and east into Texas are under excessive heat warnings and heat advisories issued by the National Weather Service until Saturday.

Similar heat domes have scorched the US in the past: the same region suffered under weeks of extremely hot weather last summer.

The city of Phoenix in the US state of Arizona set a heat record, with 19 straight days of temperatures above 40°C, beating the previous record set in 1974.

What is a heat dome?

A heat dome occurs when high-pressure atmospheric conditions trap heat over an area. These are combined with high winds caused by La Nina, according to the National Oceanic and Atmospheric Administration.

This high pressure also causes air to sink. As the air sinks it warms, leading to higher temperatures. And since the air cannot escape it creates a dome, with temperatures getting hotter and hotter.

The area trapped under the dome can stretch across several countries and linger for weeks.

Most records are set during heat domes, which can result in dangerously high temperatures.

Who suffers most in a heat dome?

Officials have urged residents across the region to stay indoors, drink plenty of fluids and, if able, to check on neighbours and loved ones.

Elderly people and those who do not have access to spaces to keep cool are most at risk during a heat dome. The high temperatures can be deadly if people do not hydrate or find ways to cool down.

Cities often adopt heat-beating measures, such as opening cooling centres and setting up sprinkler systems.

The impact of constant heat may also make people more prone to illness.

Where have heat domes taken place?

More than 2,000 people in India were killed in 2015 during a period of intense heat that lasted several weeks.

The country has experienced other heatwaves in the years since, leading to more than 6,500 deaths.

Excessive heat has also been linked to the country's hindered economic development.

One of the deadliest heatwaves occurred in 2003, when searing conditions resulted in about 30,000 casualties across Europe between July and August.

More recently, a heat dome settled over France last year, leading to intense high temperatures that were felt across Europe.

A record-breaking summer in Europe led to more than 61,000 heat-related deaths in 2022.

Is climate change a factor behind heat domes?

Climate change is a driver in the increasing frequency of heatwaves.

A 2023 study found that Afghanistan and parts of Central America are both areas susceptible to high-risk heatwaves.

Forecasters say it was difficult to link the record-breaking heat experienced by the US South-west in recent years to human-induced climate change, but such extremes are becoming more frequent because of global warming.