High temperatures pounding parts of the U.S. are forcing airlines to adjust operations in the hardest-hit cities, reducing fuel or baggage – and in some cases shedding passengers – to help aircraft operate.

Allegiant Airlines, a unit of Allegiant Travel Co., said it will delay flights if there’s a threat to passenger safety or comfort as temperatures in the Southwest continue to hit records. In Las Vegas, where Allegiant is based, excessive heat advisories have been extended through Sunday, and temperatures have been above 100 degrees F since June 30, according to National Weather Service data.

High temperatures make the air less dense, reducing engine performance and the amount of lift produced at a given speed. That generally means lower takeoff weights and longer takeoff distances are needed to produce sufficient lift. As a result, carriers are having to reduce pounds on the planes.

On July 17, several Delta Air Lines Inc. passengers voluntarily got off a flight from Las Vegas to Atlanta after aircraft weight issues in the heat caused delays.

“Additional protocols have been put in place to address the operational impacts extreme heat has on aircraft, including loading less fuel to account for weight and balance and schedule refueling along the route when needed,” Delta said in a statement Friday.

Planes are hooked up to external cooling units when they’re at gates, but extreme heat can overcome their ability to work well. Once they leave the gate, in most cases air sucked into the engines is cooled and then circulated in the cabin, a system that’s not ideal when a plane is idled on a tarmac.

American Airlines Group Inc. said it is taking extra steps to make sure it has cooled air on jet bridges hooked to planes. American also performed early maintenance on auxiliary power units that run onboard systems when engines are off, Chief Executive Officer Robert Isom said on a Thursday conference call.

“Though we’ve had a very small number of diversions and delays related to high temperatures, the plan we have in place has allowed us to avoid significant impact,” an American spokeswoman said, adding that the carrier changed its operating plan to avoid weight restrictions.

Delta, American, and United Airlines Holdings Inc. said that airport ramp workers, baggage handlers, and others laboring outside are getting more frequent breaks, additional water supply, access to shaded areas or air conditioning, and cooling towels.

The heat has persisted for weeks and is expected to continue through the weekend for much of the Southwest. In addition to Las Vegas, where the high soared to 116F on July 16, Phoenix hit 119 on July 19 and 20 and has seen temperatures above 100 since June 14.

Extreme temperatures during the summer of 2017 forced the cancellation in Phoenix of some flights, primarily by smaller regional jets and older aircraft.