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Travelers are encountering thousands of delayed flights Monday as extreme heat from Nebraska to the Carolinas snarls air traffic at major airport hubs.

On Monday, extreme heat will oppress much of the Central and Eastern U.S., with the most dangerous combination of high temperatures and humidity continuing through Wednesday, according to the NWS forecast. From Nebraska to the Carolinas, high temperatures will soar into the upper 90s to low 100s, with heat index values ("feels like" temperatures) surpassing 110-115 degrees. Several major metropolitan areas—including Raleigh, Charlotte and Orlando—will experience “extreme heat risk,” which means dangerous and potentially deadly heat stroke likely with limited exposure.

Extremely high temperatures, especially when combined with humidity, can increase the likelihood and severity of thunderstorms, which can cause the Federal Aviation Administration (FAA) to issue ground delays and stops for affected airports. In addition, extremely high temperatures can force airlines to reduce the weight carried on planes, which in turn can cause flight delays and passengers being bumped to lighten the load. Lightening the plane becomes necessary because it takes more to get the plane aloft. “The warmer the air becomes, the less dense it also becomes, which means a higher groundspeed just to reach the appropriate airspeed [for takeoff],” Melanie Dickson, a lecturer at The Ohio State University’s Center for Aviation Studies, told Forbes. “Because the air is less dense, there are fewer air molecules traveling over the wings so the aircraft has to go faster to achieve the same amount of lift.” Hotter temperatures mean more runway is required for takeoff and landing, while engines produce less thrust. “This means that there is a possibility that the runway isn’t long enough to accommodate the longer takeoff roll—especially at high altitude airports, which already have air that is less dense than at or near sea level.” Dickson said. “If you add heat to that mix, it can grind everything to a halt.” Travelers can expect more turbulence at takeoff and landing “from the hot temperature causing thermals and rising air,” Dr. Bob Thomas, assistant professor of aeronautical science at Embry-Riddle Aeronautical University, told Forbes.

A weather issue in one region often blossoms into a national air travel problem because of how airlines reuse planes and crews for multiple trips during the day. A morning flight delay in Boston can prevent a plane from making it to Dallas for a midday departure, which can in turn bump back the same plane’s afternoon flight out of Chicago. When thousands of morning flights are delayed, it creates a domino effect throughout the rest of the day.

Essentially, aircraft air-cooling systems are designed to work while the plane is in the air, not on the ground. “The air conditioners on airplanes work differently than you’d expect,” Thomas told Forbes. “At the airport gate, a big hose will pump cool air into the airplane, but in the time between leaving the gate and being airborne can get a little warm in the cabin.” According to tarmac rules of the Department of Transportation (DOT), airlines are required to let passengers off the plane after three hours for domestic flights and four hours for international flights. But extremely hot days can mean that passengers sit in sweltering cabins for hours when the plane is stuck on the tarmac, either waiting to takeoff or after landing.

The simplest way for travelers to reduce the risk of a flight disruption is by booking an early flight, when temperatures are cooler. “Especially down in the Southeast or anywhere in the South where there are afternoon thunderstorms every day, it’s always smarter to take the very first flight in the morning,” Dr. Thomas told Forbes. “You avoid the risk of thunderstorms delaying and changing routes.”

74%. That’s the portion of flight delays in the U.S. caused by weather, according to FAA data.

Over 7,000 U.S. Flights Disrupted Thursday—Latest Day Of Heavy Delays (Forbes)