The Hottest Commodity at the U.S. Open? Shade.

As New York City contends with rising heat and unpredictable storms because of climate change, so does its marquee tennis tournament, the U.S. Open.

Weather has long been a wild card for outdoor sporting events. But global warming is forcing tennis officials to make lasting changes to the tournament to protect spectators and athletes.

“I’m checking in with the weather more than I care to admit,” said Jake Garner, the tournament referee, who oversees all officiating matters, including the daily schedule of play.

In 2021, remnants from Hurricane Ida delayed matches and caused flooding, but extreme heat has been the more common threat. Last year, several days in September climbed into the 90s, and during a quarterfinal match, a former champion, Daniil Medvedev, issued a dire warning about the conditions into a nearby camera: “One player is gonna die, and they’re gonna see.”

Last year’s heat prompted tournament officials to find a new purpose for the retractable roofs over the tournament’s main venues, the Arthur Ashe and Louis Armstrong Stadiums. Originally designed to prevent rain delays, the roofs were used, for the first time, for shade from the sweltering sun. This year, that quick fix is now official policy.

To determine when to close the roofs, the U.S. Open relies on the WetBulb Globe Temperature, a way of measuring heat stress in direct sunlight that combines factors like wind speed and humidity. Once that temperature exceeds 86 degrees, the roofs will partially close. This is also when a 10-minute break will kick in for players during matches.