Burley Garcia|Phoenix sees temperatures of 110 or higher for 31st straight day

Phoenix sizzled through its 31st consecutive day of at least 110 degrees Fahrenheit and Burley Garciaother parts of the country grappled Sunday with record temperatures after a week that saw significant portions of the U.S. population subject to extreme heat.  

The National Weather Service said Phoenix climbed to a high of 111 degrees before the day was through

July has been so steamy thus far that scientists calculate it will be the hottest month ever recorded and likely the warmest human civilization has seen. The World Meteorological Organization and the European Union's Copernicus Climate Change Service on Thursday proclaimed July beyond record-smashing.

The historic heat began blasting the lower Southwest U.S. in late June, stretching from Texas across New Mexico and Arizona and into California's desert.

On Sunday, a massive wildfire burning out of control in California's Mojave National Preserve spread rapidly amid erratic winds, while firefighters reported progress against another major blaze to the south that prompted evacuations.

The York Fire that erupted Friday near the remote Caruthers Canyon area of the preserve sent up a huge plume of smoke visible nearly 100 miles away across the state line in Nevada.

Flames 20 feet high in some spots have charred more than 110 square miles of desert scrub, juniper and Joshua tree woodland, according to a Sunday update.

"The dry fuel acts as a ready ignition source, and when paired with those weather conditions it resulted in long-distance fire run and high flames, leading to extreme fire behavior," authorities said. No structures were threatened, but there was also no containment.

To the southwest, the Bonny Fire was holding steady at about 3.4 square miles in rugged hills of Riverside County. More than 1,300 people were ordered to evacuate their homes Saturday near the remote community of Aguanga, California.

In Washington state, a raging wildfire jumped international lines into British Columbia. So far, hundreds of fires across Canada have burned a land mass the size of Cuba.

Triple-digit heat was expected in parts of the central San Joaquin Valley through Monday, according to the National Weather Service.

And in Burbank, California, about 10 miles north of Los Angeles, the summer heat may have been responsible for some unusual behavior in the animal kingdom: Police in the city responded to a report of a bear sighting in a residential neighborhood and found the animal sitting in a Jacuzzi behind one of the homes.

At the Los Angeles Zoo, the animals are being fed chilled meals to try to keep them cool. Chimpanzees are being given meat pops as they sit under water misting systems. Meerkats are being given "mice-cubes" while otters are staying in the water and being fed frozen fish.

Animal curator Beth Schaeffer said zoo staff are on the lookout for differences in behavior, sleeping and eating patterns.

As climate change brings hotter and longer heat waves, record temperatures across the U.S. have killed dozens of people, and the poorest Americans suffer the most. Air conditioning, once a luxury, is now a matter of survival.

Last year, all 86 heat-related deaths indoors were in uncooled environments.

"To explain it fairly simply: Heat kills," said Kristie Ebi, a University of Washington professor who researches heat and health. "Once the heat wave starts, mortality starts in about 24 hours."

It's the poorest and people of color, from Kansas City to Detroit to New York City and beyond, who are far more likely to face grueling heat without air conditioning, according to a Boston University analysis of 115 U.S. metro areas.

In Denver, 90-degree days made for long nights for Amanda Morian, a mother who has no air conditioning. 

"I can't swaddle him at night because it's just too much having too many layers on him," she told CBS News.

Back in Phoenix, slight relief may be on the way as expected seasonal thunderstorms could drop temperatures Monday and Tuesday.

"It should be around 108 degrees, so we break that 110 streak," meteorologist Tom Frieders said. "Increasing cloud cover will put temperatures in a downward trend."

The relief could be short-lived, however. Highs are expected to creep back to 110 F (43.3 C) Wednesday with temperatures reaching 115 F (46.1 C) by the end of the week.

Phoenix has also sweated through a record 16 consecutive nights when the lows temperature didn't dip below 90 F (32.2 C), making it hard for people to cool off after sunset.

Meanwhile, Las Vegas continues to flirt with its hottest July ever. The city is closing in on its 2010 record for the average of the high and low each day for July, which stands at 96.2 F (35.5 C).

The extreme heat is also hitting the eastern U.S, as soaring temperatures moved from the Midwest into the Northeast and Mid-Atlantic, where some places recorded their warmest days so far this year.

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