The National Weather Service (NWS) and local officials urged people to stay indoors, avoid strenuous activities and stay hydrated.

But that wasn't stopping Phoenix resident, Trevor Doi, 34, who says he has acclimated to the weather. Doi spoke to Reuters after finishing a hike. At the entrance of the trail, warning signs can be seen alerting hikers about the dangers of strenuous activity during extreme heat. "I like it cause it's quiet out. There's not a lot going on, and you can just have this whole place to yourself, and it's not too hot-- for an Arizonan," Doi said.

While some choose to be outside during active heat alerts, others have no choice. Daryl Johnson, 34, drinks a lot of water and takes plenty of breaks while working in landscaping. "So I've been working outside in the heat for about the past three summers, and I don't think you ever actually acclimate to it. I think you just kind of know it's there and continue to go."

The U.S. heat wave follows conditions in Europe this week that have sparked wildfires and set record temperatures in the kind of weather event that scientists say will become more frequent with climate change.