Extreme heat indices above 105°F to hit 80 million people in U.S.

Over 20% of the U.S.' population — 80 million people — are expected to face an air temperature or heat index above 105° Fahrenheit this weekend as a record-breaking heat wave persists over most of the South, the National Weather Service (NWS) warns.

Why it matters: The extreme temperatures, which have been exacerbated by human-caused climate change, will come after several days of excessive heat and will be an immediate risk to public health.

• Extreme heat events are the top annual weather-related killer in the U.S.
• These can also strain power grids as people turn to air conditioning for cooling and ramp up energy demand.

Threat level: Heat index is what the temperature feels like to the human body when relative humidity is combined with the air temperature, and indices 103°F or above can lead to dangerous heat disorders.

• "Dozens" of temperature records could break across the Southern U.S., including overnight highs, the NWS said.
• Abnormally high nighttime temperatures are particularly dangerous when they occur amid prolonged stretches of hot weather, as they prevent people from cooling off and increase the risk of heat-related hospitalizations and deaths.
• "Take the heat seriously and avoid extended time outdoors," the NWS advised. "Temperatures and heat indices will reach levels that would pose a health risk, and be potentially deadly, to anyone without effective cooling and/or adequate hydration."

State of play: About 115 million people in over a dozen states from California to Florida were under heat alerts on Thursday morning.

• As Phoenix on Wednesday experienced its 20th straight day with a temperature of over 110°F, the Maricopa County Department of Public Health reported 18 heat-related deaths had been confirmed in the city and another 69 were under investigation this year. The true death tolls and destruction are often not immediately apparent.
• "With a high temperature of 119° and a low temperature of 97°, a new all-time record daily average temperature of 108° has been established at Phoenix Sky Harbor Airport," the NWS' Phoenix office tweeted Wednesday evening. This previous record from 1990 was 106.5°F.

Meanwhile, Austin saw its 10th straight day of temperatures at or above 105°F for the first time in recorded history.

What's next: The threatening heat is forecast to continue over the Southwest through "at least" July 28 and may expand into other parts of the country, while the Midwest is expected to experience excessive temperatures starting next week, the NWS said.

• The NWS' Climate Prediction Center projects above-average temperatures across most of the U.S. toward the end of July into early August.

The big picture: The effect of human-caused climate change and global warming, which are making heat events more likely and more severe, are colliding this year with an El Niño weather pattern.

• Historic temperatures are simultaneously occurring in several parts of the world, as Europe and parts of Asia logged record highs this week as well.
• Global temperatures are hitting unprecedented highs, too, this year amid climate change and global warming.
• Elevated temperatures are also contributing to Canada's worst fire season on record, in which at least 27.1 million acres have burned across the country so far.

More from Axios:

• In photos: Extreme heat strikes multiple continents
• Too much heat, too few "chief heat officers"
• What this summer's weather reveals about climate change

Editor's note: This article has been updated to include details of heat alerts, Phoenix's new historic temperatures and with further context on heat indices.