The first day of summer yesterday brought some of the worst heat the Southwest of the United States has seen in years, forcing flights to be cancelled, straining the power grid and making life miserable for workers toiling in temperatures that reached 49C or higher in some desert cities.

Arizona, Nevada and California saw dramatic temperatures yesterday as researchers say deadly heat waves like this one are going to grow more frequent.

Meteorologists in Phoenix said the temperature topped out at 48C, one that has been matched or surpassed only four other times.

The forecast called for a high of 49C in Phoenix, which it hasn't seen in more than two decades. Death Valley, California, reached 52C and Palm Springs hit 50C, still a degree lower than the same day last year.

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The operator of California's power grid called on people to conserve electricity during peak hours.

Workers at a construction site in a Phoenix suburb huddled under an excavator to find a sliver of a shade during a break. At another building site, men in hard hats and yellow vests laboured and sweated in the heat, downing water to stay hydrated.

The main burn centre in Phoenix warned people to be careful around car interiors and pavements and with their pets. Locals took up the offer from a pet store of free booties to put on to protect pets' paws when they were outside.

Construction workers in Phoenix took shelter from the sun where they could find it, while pets also got a bit of protection from the heat. Photo / Supplied
Construction workers in Phoenix took shelter from the sun where they could find it, while pets also got a bit of protection from the heat. Photo / Supplied

Las Vegas also baked. Visitors tried to stay inside air-conditioned casinos, and some tourists lugged packs of bottled water around the Strip.

Phoenix has hit 49C only three times in recorded history. The record high was 50C on June 26, 1990.

The weather comes as new research found that nearly one in three people now experience 20 days a year when the heat reaches deadly levels.

The study of nearly 2000 deadly heat waves worldwide since 1980 was published on Tuesday in the journal Nature Climate Change.

In the Southwest US, the heat has caused several problems this week.

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In addition to grounding more than 40 flights of smaller planes, airlines have been taking other measures on larger jets to reduce their weight.

American Airlines spokesman Ross Feinstein said the carrier began limiting sales on some flights to prevent the planes from exceeding maximum weight for safe takeoff in the hot conditions.