PHOENIX — The Sandra Day O'Connor Federal Courthouse here is a visual triumph of modern architecture. Designed by the acclaimed architect Richard Meier, it is a sleek steel and glass structure of impressive dimensions and design whose undulating roof resembles, from a distance, an approaching jagged wave — a sort of apparition in a city in the desert.

But inside, a great expanse of the courthouse is hot, even for Arizona, where "hot" is not employed lightly.

In Arizona summers, the midday temperature rarely dips below 100 degrees, and it does not change much at night. Life in these months is best lived indoors, and in most public places air-conditioners are set so low that, after a few minutes, visitors might wish they had brought along a light jacket.

Not in the towering atrium of the courthouse. Baristas in the food kiosk wear shorts and tank tops. Deputy federal marshals who provide security during high-profile proceedings — like the civil-rights trial of Sheriff Joe Arpaio of Maricopa County, which ended Thursday — change their posts every hour so everyone gets some time inside the air-conditioned courtrooms.

Court officers at the security station by the building's entrance have special permission to take off the polyester jackets and ties that are part of their uniform and work in short-sleeve white shirts.

"We're probably the only courthouse in the country where, in the summertime, they work in shirts, then in the winter, they have gloves and jackets," said David Gonzales, U.S. marshal for the District of Arizona, whose office is in the courthouse.

There is no air conditioner in the atrium, and no heating either. If it is cold outside — and it does get cold here, by desert-living standards — it is also cold inside.

The building uses a process known as adiabatic cooling to regulate the temperature in the open space. As rising hot air is allowed to escape, outside air is sucked in just under the roof, 115 feet above ground. Then, water nozzles on the sixth and top floor mist the air, which cools as it absorbs the moisture and begins to drop, joining overflow air from the air-conditioned balconies along the way.

People who work or have spent much time in the courthouse say the constant misting makes the air inside the atrium feel heavy and humid, so humid that sometimes it is more comfortable to be in the adjoining outdoor plaza.

"At least there's a breeze outside," a deputy marshal said one recent afternoon. Another compared the climate in the atrium to Puerto Rico — "minus the beach, unfortunately," he said. (They spoke on the condition of anonymity because they were not authorized to give interviews.)

The courthouse, which cost $123 million, has acquired a handful of nicknames since its opening in 2000. "Greenhouse" is one; "solar oven" is another. The cooling system is supposed to make the temperature in the atrium 15 to 25 degrees cooler than the air outside, but, as Gonzales put it, "it doesn't work."

Mafara Hobson, a spokeswoman for the General Services Administration, the federal agency that manages the building, said, "The atrium was designed as an extension of the outdoor plaza."

The security post by the courthouse entrance has portable cooling units to help alleviate the heat.

Two months ago, the agency installed a similar unit at the food kiosk, Legal Press. It has an external thermometer, which put the temperature in the atrium on Wednesday at 85 degrees at noon.

"I always thought that shorts were inappropriate for a federal courthouse," Jessica Leal, 25, one of the kiosk's baristas, said. "But it's either wearing it, or melting away."

In a blog post titled "Cooking Lawyers Alive," Ron Coleman, a New York lawyer, wrote after a summer visit that the atrium "seems only briefly preferable to the blast furnace outside."

There are courtrooms and offices along the south side of the building on each floor; all are air-conditioned.

Most are windowless, though, depriving people inside of enjoying what many people here agree is the building's greatest feature: its bright interior, a function of the clear glass that is all around.