A ground crew unloads baggage from a Delta Air Lines flight at Salt Lake City International Airport on July 5. (Angus Mordant/Bloomberg News)

Up, up, and away . . . NOT! Though summer may seem like an ideal time to travel, hot temperatures can make for some tricky calculations needed for operating an aircraft.

While heat rarely becomes an issue, on occasion, flights are delayed or even canceled when temperatures get too high. It happened Wednesday in Salt Lake City, when they soared into the mid-90s.

“First-leg flight ystrdy couldn’t take off at 3pm because *the air was too hot,*” tweeted author James Fallows. “Beyond airline’s operating specs. ‘Happening more often,’ pilot says.”

The Western United States is enduring a brutal heat wave, with excessive heat warnings affecting parts of six states. Death Valley, Calif., has set three consecutive daily record-high temperatures of 127 degrees.

Heat-related flight disruptions are more common for smaller aircraft, but even commercial airliners aren’t immune. The science behind it is simple: As air warms up, it expands, becoming less dense and less supportive of an airplane moving through it.

Though we can’t see it, the atmosphere is layered. Airplanes essentially glide on surfaces of constant density. The aircraft is designed to slice through the air and generate an area of lift around the wing. The more air that passes over the wing, the greater the upward force, and the easier it is for the plane to climb.

But when the mercury soars too high, the air starts to thin out; the density drops, and there is literally less air available at ground level. With less atmosphere passing over the wing, the aircraft can struggle to take off. It takes greater speed to achieve the same upward propulsion, and therefore the distance used on the runway grows. If the room needed to get to a gallop exceeds that of the runway, the flight is automatically canceled.

“You see that on hot summer days, for example at Newark, N.J., and other airports,” Matthias Steiner, director for the Aviation Applications Program at the University Corporation for Atmospheric Research, wrote in an email. “Airlines may have to delay some of the ultra long-distance flights (e.g., going to Hong Kong) until later in the evening when the air has cooled down some and density increases. Otherwise, they sometimes can’t get a fully loaded aircraft off the ground, even with using the maximum length of the runway.”

As Steiner points out, weight is another factor. If the airline is determined to depart on schedule, he explained, they may be forced to offload some passengers. That can end up in people being reaccommodated.

Even when the plane is in the air close to the ground, it behaves more sluggishly when the temperature is too high. With the reduced thrust and engines forced to work harder, every input given by the pilot is just a tad tougher to execute.

It’s the same reason it’s so easy to swim in the Great Salt Lake or the Dead Sea. Because the salt content is so high and the water resultantly so dense, you can float atop that surface with little effort. But in a swimming pool, where the water is much less dense, you have to paddle or flap your arms to generate “lift” and stay afloat. The same is true with the airplane.

Stephen Szulborski, a meteorologist and weather observer at the Detroit Metropolitan Airport, says that it’s rare to have temperatures get high enough to prevent takeoff at a major airport. “That’s usually due to extreme temperatures above 110,” he wrote. But the pilots do have to use more runway when it gets toasty. “Detroit has pretty long runways, though: Our primary one is two miles long.”

Every plane has a different maximum operating temperature, depending on the weight, body and engines of the aircraft. For the Boeing 737, anything above 54 degrees Celsius (129.2 Fahrenheit) is a no-go. Even under ordinary circumstances, the plane can’t legally take off at more than 174,200 pounds. (About 30,000 pounds of that comes from fuel.) For smaller jet airliners, such as the Embraer ERJ 190, the temperature ceiling is lower — varying on an aviation model known as International Standard Atmosphere.

In mountainous environments, such as Salt Lake City, where the air is naturally thinner, airports are even more sensitive to the heat.

“At higher-altitude airports — Tucson, Arizona, for instance — the [Federal Aviation Administration] may have to cancel flights on hot summer days because they can’t take off,” Steiner said. “Moreover, with changing climate conditions and projected increasing temperatures, there will be more days when these kinds of issues will happen.”

Capital Weather Gang’s Jason Samenow contributed to this report.