The first day of summer brought some of the worst heat the south-west US has seen in years, forcing flights to be canceled, straining the power grid and making life miserable for workers toiling in temperatures that reached 120F or higher in some desert cities.

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

Meteorologists in Phoenix said Tuesday evening the temperature topped out at 119F, which has only been matched or surpassed four other times.

The forecast called for a high of 120F – (nearly 49C) – in Phoenix, which the city hasn’t seen in more than two decades. Death Valley, California, reached 125F and Palm Springs hit 121F, still a degree lower than the same day last year.

At a downtown Phoenix construction site, men in hard hats and yellow vests labored and sweat in the morning heat and downed water to stay hydrated. The project superintendent, Tommy Russell, says his company has held weekly safety meetings to prepare for the heat, and he will send all his workers home if it hits 120 degrees.

“We anticipate the weather, so we keep everyone hydrated, we keep everyone watered down,” Russell said.

Las Vegas also was baking. Out-of-town visitors tried to stay inside air-conditioned casinos as much as possible, and some tourists lugged packs of bottled water around the Strip. Others went to a bar where the temperature is set at 23F (-5C), and glasses, walls and seats are sculpted from ice.

Tonya and Lavonda Williams traveled to Sin City from Orlando, Florida, to get out of town and see the Backstreet Boys in concert. Walking on the Las Vegas Strip in 112F was too much to handle, even for people accustomed to high temperatures.

“This is like the oven door is open,” Lavonda Williams said as the sisters walked from a pedestrian bridge into The Palazzo casino-resort.

“It’s too hot to even drink alcohol,” Tonya Williams added.

Juan Guadalupe, a landscaper, scaled a spindly palm tree more than 50ft tall in Phoenix, using a chain saw to hack the branches. He planned to drink at least two gallons of water and quit his day at 2pm – before the really ugly temperatures arrive.

He didn’t mind being tethered to a tall tree while operating a chainsaw because he occasionally catches a cool breeze.

A Salvation Army hydration station in Phoenix, Arizona. Photograph: Ross D. Franklin/AP

With the cooling and hydration stations in full swing across the region, hundreds flocked to Grace Lutheran Church in Phoenix for water, meals, snacks and refuge.

“We have homeless people come from a long way to sit here,” said longtime volunteer Moses Elder. “There are other spots where you can go get cold water and sit down and cool off, but there are few places you can lay down and get something to eat.”

The church houses about 180 people every day during the summer and typically goes through about 25 cases of water and 50 pounds of ice a day, Elder said.

Phoenix has hit 120 only three times in recorded history – the last time 22 years ago. The record high was 122 degrees on 26 June 1990.

The city reached 118 on Monday, which the National Weather Service says is rare. In fact, temperatures at that mark or higher have only been recorded 15 times since record-keeping started in 1896.

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 almost 2,000 deadly heat waves worldwide since 1980 was published Monday in the journal Nature Climate Change.

In the south-west US, the heat has already caused several problems.

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

The main burn center in Phoenix has issued a warning to people to be careful around car interiors and pavement and with their pets.

Four-year-old dog Chase wears a pair of booties that protect paws from the hot pavement. Photograph: Angie Wang/AP

People showed up at a PetSmart store in Tempe on Tuesday to receive free booties to put on their pets’ paws so they don’t burn on concrete and pavement.

Shelby Barnes, 48, picked up three sets of the booties for her dogs. She says she will yell at people who walk their dogs without them in the afternoon heat.

“If you can’t put your foot on the sidewalk, neither can they,” she said.