“It’s just amazing how long it went without any kind of break,” Brady said. “There was no end to it. And you ask yourself: How can people live with so much constant heat?”

Cooling centers, hydration stations and other heat relief measures in Phoenix were scheduled to close on Sept. 30 but remained open until Oct. 7 because of the fall heat wave, the Arizona Republic reported. The Maricopa Association of Governments also said it will maintain its Heat Relief Network map, which identifies cooling, water and donation locations, through Oct. 31.

Outside of Phoenix, Nevada and New Mexico also saw record temperatures in October. According to the National Weather Service, temperatures in Las Vegas have reached 100 degrees six times so far in October, a feat never seen before in the city in the 87 years since records began.

In New Mexico, the Albuquerque branch of the National Weather Service said Oct. 5 that temperatures across the state at that time — which ranged from the mid-80s to 95 degrees — were “more typical of mid-August.”

According to the National Weather Service, extreme heat causes more deaths across the country each year than any other weather event.

In Maricopa County, where Phoenix is ​​located, there have been 389 confirmed heat-related deaths so far this year and 292 more are under investigation, county records show. Last year, the county reported a record 645 heat-related deaths. It's too early to know how this year will compare, officials said.

Part of the problem for Phoenix, like many large metropolitan areas, is that it tends to experience warmer temperatures than more rural parts of its state due to a phenomenon called the “urban heat island effect.”

Densely populated urban areas have buildings, roads and other man-made structures that absorb and retain more heat than natural landscapes, making cities like Phoenix particularly vulnerable to extreme heat.

In a study published in July, Brady and her colleagues at Climate Central analyzed the urban heat island effect in 65 large U.S. cities, home to about 15% of the country's population. They found that temperatures in Phoenix were on average about 7.4 degrees higher due to the city's built environment.

The researchers also found that disadvantaged and underserved communities bear a disproportionate burden of urban heat, putting them at even greater risk when temperatures soar.

The phenomenon is not limited to inner cities, Brady added.

“If you have a suburban area with a lot of sidewalk, a lot of buildings and no trees, it's going to be a lot warmer there too,” she said.

In addition to its threat to human health, intense and persistent heat has far-reaching impacts on the planet. More months of summer-like conditions means longer wildfire seasons, said John Mejia, an associate research professor at the Desert Research Institute in Nevada. And warm temperatures in early spring and later fall also disrupt the natural flow of Earth's water cycle, altering the availability of water circulating between oceans, soil and atmosphere.

“Ecosystems rely heavily on pause and have these cycles of warm and cold, warm and cold,” Mejia said.

Last month, the National Oceanic and Atmospheric Administration announced that the planet's global surface temperatures in August were more than 2 degrees Fahrenheit above the 20th century average, making it the warmest August on record and extending a 15-month streak.

“If you average all temperatures around the globe, temperatures were above records for 15 months,” Mejia said. “That’s pretty much what we expect because we’re increasing base temperatures due to global warming.”

Still, the spate of heat records shattered in recent weeks has stunned many experts, as October rarely sees such high temperatures for days on end.

“We’re pretty perplexed,” Mejia said, “because this is really unusual.”