From the Gulf Coast to the Great Lakes, over 100 million Americans are under heat alerts as temperatures soar across the country. The National Weather Service is warning over a “dangerous” combination of heat and humidity that will affect most of the lower 48 states this week, with few signs of the heat going away after that.

Because of climate change, heatwaves are happening more frequently, are more intense, and are lasting even longer than they did in the past. They are not only a nuisance but can also be very dangerous, leading to illness and death. Everyone can be affected, but older adults and the very young are the ones facing the most risks

The excessive heat in the US happens because of an intense and expansive zone of high pressure, usually referred to as a heat dome. The dome, which was centered over the Southwest during the weekend, has shifted east on Monday and is expected to shuffle toward Nashville by Wednesday – before going west again later in the week.

Daily high-temperature records are already being set across several cities. North Platte, Nebraska, reached 108 Fahrenheit (42.2 Celsius), breaking a previous record of 103 F (39.4C). Nashville, Tennessee, hit 97 F (36.1C), tying with the previous record, while Jackson, Kentucky, reached 94 F (34.4C), exceeding the mark of 91F (32.7C) from 2000.

The heatwave is even more dangerous because it’s happening in June, close to the longest days of the year. The National Weather Service (NWS) measures temperatures in the shade and the heat index, an estimate of temperature and humidity effect, is obtained using the reading from the shade. This means it will feel even hotter in the direct sun.

“Not only folks who are susceptible to heat-related illnesses, but really just about anybody that’s going to be outside for an extended period of time is at risk for heat-related illnesses,” NWS meteorologist Matt Beitscher told CNN. “These values are sort of your baseline. Then they get worse from there if you’re talking about the impact that the sun has.”

Along with the extreme heatwave, wildfires occurred in Arizona, New Mexico, California, and Colorado. “Critical” fire weather conditions were registered from the Southwest to the southern and central Rockies to the High Plains, NWS said. The highest fire danger is currently in the states of Colorado, New Mexico, Arizona, and Utah.

Not just in the US

An extreme heat event is also happening in Spain and southern France, with temperatures reaching highs normally not recorded until July or August. In France, temperatures have exceeded 35C close to the Mediterranean and would rise even further as a mass of hot air moves northwards, with the Rhone valley already registering 39C.

In neighboring Spain, temperatures so far this month are the hottest registered in at least 20 years, with 40C recorded in Seville and Cordoba and 42C in the Guardiana valley. AEMET, the state meteorological agency, said the country is facing “unusually high temperatures for June.” This is the earliest heatwave ever recorded since 1981, the agency said.

Heat is being pumped north of Europe around an area of high pressure, similar to the heat dome registered in the US. In France, it’s expected to peak Thursday and Friday, while in Spain it would peak Thursday. Spain is also dealing with a record drought, which coupled with the heat is putting most of the country into extreme levels of fire danger.