WASHINGTON - Excessive heat is the No. 1 weather killer in the United States, and it's at its most dangerous when it doesn't cool down at night.

The heat wave over California, Nevada, Arizona and New Mexico has temperatures hitting triple digits, with little relief at night. Hot weather is also baking the rest of the far West, including Washington, Oregon, Idaho, and parts of Utah and Montana.

Question: What's so disturbing about this current heat wave?

Answer: It's unrelenting stubbornness. There is no relief at night. Phoenix set a record for highest nighttime temperature: 91. Las Vegas has gone three days without getting below 90, according to readings at the airport.

"Nighttime heat is especially bad," said Eli Jacks, chief of fire and public weather services at the National Weather Service. "Not to get below 90 is crazy."

Q: What's so dangerous about that?

A: If you aren't in an air-conditioned place, "your body never has a chance to recover" at night, Jacks said. Normally the "feels-like" index - which factors in temperature and humidity - has to get to 80 degrees or below for your body to recover from the daytime heat, Jacks said.

The lack of nighttime cooling is more dangerous than the 117 degree all-time record in Las Vegas, experts said.

Q: How do heat waves compare to other weather killers?

A: In recent years, heat has been more deadly than other weather extremes in the United States.

On average, heat waves are killing about 117 people a year, according to figures from the National Oceanic and Atmospheric Administration. However, those numbers are incomplete and based only on reports during periods of extreme heat. The much more comprehensive numbers from the Centers for Disease Control and Prevention indicate that on average 658 people die each year from too much heat.

Q: Who is most at risk?

A: The elderly and children. The elderly make up 36 percent of heat deaths in the past decade, according to the CDC. And of all the excessive heat deaths, 69 percent were men. Also on average, 37 children left in car seats die from heat each year, according to a study at San Francisco State University.

Q: What can you do to stay safe?

A: Drink lots of water; the dry heat in the Southwest evaporates sweat so quickly that people don't notice they are perspiring and get dehydrated more quickly, Jacks said. Stay in the shade and out of the heat between 10 a.m. and 4 p.m. Use sunblock of SPF 15 or higher. Wear light-colored clothing and light clothing. Reduce use of caffeine and alcohol, which tend to dehydrate, and slow down.

Q: So what's causing all this?

A: Part of it is normal summer heat spurts, but what's happening now is "a really big kink in the jet stream, about as big as you can see anytime, covering the whole western U.S.," said Ken Kunkel, a professor of atmospheric sciences at North Carolina State University.

Q: When will it end?

A: The extreme heat should continue for about a week, but it won't set records, James said.

On StarNet: View the five-day forecast for Tucson and Southern Arizona at azstarnet.com/app/weather