No less than four once-in-a-millennium storms struck different areas of the United States between July 4th – 8th, a four-day span—that resulted in major flooding in their respective regions, leading to the deaths of at least 129 people, and leaving even more missing. While the statistical probability of such events happening would typically be extremely low, the odds of their occurrence climbs with the temperature—and in turn, the capacity to hold water—of the atmosphere.

In the early morning hours of July 4, the remnants of Tropical Storm Barry combined with a small-scale, low-pressure cyclone that released 6.5 inches (170 millimeters) of rain in just three hours, causing Texas’ Guadalupe River to rapidly rise to 37.52 feet (11.44 meters) at a point where two branches of the river meet near Hunt, Texas; over the course of the storm, some areas would see nearly a foot of rainfall, with the highest amount, 20.33 inches (516.4 millimeters), recorded in an area northwest of the community of Streeter.

The resulting surge that swept down the Guadalupe tore through riverside communities, resulting in over 237 people having to be rescued from floodwaters by search and rescue crews. Tragically, at least 120 people lost their lives, including 27 campers and counselors who were attending Camp Mystic, with six children still unaccounted for. Overall, roughly 170 people are still missing across Texas Hill Country.

Two days later, on July 6, Tropical Storm Chantalmade landfall in South Carolina, dropping more than 10 inches (250 millimeters) of rain across Alamance, Chatham, and Orange counties, triggering flash floods that caused the state’s Eno River to rise to 25.64 feet (7.82 m)—a record for the river—that ultimately killed six people.

Two days after that (July 8), heavy rains across the Ruidoso area in New Mexico also caused flash flooding, prompting 85 rescues by emergency teams. The Rio Ruidoso itself swelled to over 20 feet (6.1 meters); three people died in the floods, including two children.

And on the same day, a 5.45-inch (138.4 millimeter) downpour in Chicago flooded highways, viaducts and basements, with “5.12 inches of rain in just 90 minutes” being recorded west of the city’s United Center arena, “a staggeringly high rainfall rate,” according to the National Weather Service. Thankfully, the flooding claimed no victims, but emergency crews performed numerous water rescues for Chicagoans trapped by the rapidly-rising waters.

But as the temperature of the atmosphere increases under the strain of global warming, so does its capacity to hold water, meaning that when strong storms form, they have much more water to release than its predecessors.

“It is a mathematical certainty that as the atmosphere holds more water, it can also discharge more water in an instantaneous fashion,” explained hydrometeorologist Dave Gochis, who is the head of prediction services at Airborne Snow Observatories, a California company that measures and models global snow and water resources.U