By CLARICE SILBER and ANGIE WANG ,  Associated Press

PHOENIX (AP) — The first day of summer was expected to bring some of the worst heat the Southwest U.S. has seen in years, forcing flights to be canceled, straining the power grid and making life miserable for workers toiling in temperatures that could reach 120 degrees in Phoenix.
Arizona is seeing some of the most dramatic temperatures Tuesday, but the heat wave is being felt across Nevada and California, too. It comes as researchers say deadly heat waves like this one are going to grow more frequent.
The forecast called for a high of 120 degrees (49 degrees Celsius) in Phoenix, 119 in Palm Springs, California, and 126 in Death Valley. The extreme heat led the operator of California's power grid to call on people to conserve electricity during peak hours.
At a downtown Phoenix construction site, men in hard hats and yellow vests labored and sweated in the morning heat and downed water to stay hydrated. 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.
"We anticipate the weather, so we keep everyone hydrated, we keep everyone watered down," Russell said.
Landscaper Juan Guadalupe scaled a spindly palm tree more than 50 feet tall in Phoenix, using a chain saw to hack the branches. He planned to drink at least 2 gallons of water and quit his day at 2 p.m. — before the really ugly temperatures arrive.
He didn't mind being tethered to a tall tree while operating a chain saw because he occasionally catches a cool breeze.
"Down here, it's hot," Guadalupesaid.
Phoenix has hit 120 only three times in recorded history — the last time 22 years ago. The record high was 122 degrees on June 26, 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 nearly 2,000 deadly heat waves worldwide since 1980 was published Monday in the journal Nature Climate Change.
In the Southwest U.S., the heat has caused several problems this week.
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.
The main burn center in Phoenix has issued a warning to people to be careful around car interiors and pavement and with their pets.
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.