Eleven world records have fallen at the Summer Olympics in Rio de Janeiro as of Thursday morning. More records may be broken before the games wrap up on Aug. 21, but the top world marathon times are probably safe. The current world records — 2:02:57 for men and 2:17:42 for women — were set by Dennis Kimetto and Paula Radcliffe at the Berlin and London marathons, respectively. The last runner to set a world record in the marathon at the Olympics was Ethiopian Abebe Bikila, who won the gold medal in 1964 in Japan.

Ahead of the women’s marathon Sunday and the men’s a week later, I examined factors that make for fast marathon times, hunting for clues as to whether a record is likely to fall in Rio. It’s hard to break a world record in any event, but it’s especially difficult in the marathon, a 26.2-mile race where finishing times are heavily influenced by the course and race-day conditions. The weather during the Summer Olympics might be the biggest barrier to a world record — and that’s no different this year.

Optimal weather: cold and dry

Even though it’s technically winter in Brazil, the temperatures may feel summery to many athletes. Heat is like kryptonite for marathon runners, and as of now, the weather forecast for the women’s race predicts a temperature of 67 to 72 degrees Fahrenheit at the 8:30 a.m. start time, with the temperature reaching about 80 degrees by 11 a.m., when racers will be finished. That may sound pleasant for a stroll along the beach, but it’s practically scorching if you’re running all-out for more than two hours. Running generates a lot of body heat, and the faster you go, the more heat you produce. Athletes can acclimate to the heat to some extent — that’s why marathoner Shalane Flanagan trained in a heat and humidity chamber at the U.S. Olympic Training Center — but to perform at their best, they need that body heat to dissipate.

The conventional wisdom is that the optimal temperature for running a fast marathon is 50 degrees Fahrenheit, said Alex Hutchinson, a Runner’s World columnist, but a 2012 study analyzing data from nine years’ worth of finishes at the Paris, London, Berlin, Boston, Chicago and New York City marathons concluded that the fastest median times were posted at a race-day temperature of about 43 degrees Fahrenheit for men and about 44 degrees for women. “Is it going to be hot or just moderately warm in Rio? It doesn’t matter,” Hutchinson said. “If it’s not cold, it’s too hot to run a great marathon time.”

The humidity in Rio (predicted to be around 70 percent at the start of the women’s race Sunday) will only make things worse. As humidity increases, the body’s ability to cool itself via evaporating sweat diminishes. “Once you start dripping, you’re no longer dissipating heat as well as you could,” said Hutchinson, who has also published an analysis of what it would take to run a sub-two-hour marathon — the marathon’s version of the four-minute-mile barrier and a mark that has yet to be broken.

Optimal course layout: flat with few turns

A fast time requires a course with few momentum-crushing turns and little elevation gain or loss. Downhills aren’t as helpful as they might seem, because downhill running hammers the quadriceps, creating muscle damage that can accumulate over a marathon, said Dr. Michael Joyner, a physician and exercise researcher at the Mayo Clinic. And record-breaking times don’t officially count if they’re achieved on a course with too much downward slope. Rules by the sport’s governing body, the International Association of Athletics Federations, specify that world records in road races like the marathon can be set only on courses where the overall decrease in elevation between the start and finish does not exceed 1 meter per kilometer.

In terms of elevation, the pan-flat Rio course appears promising for record-seeking runners. The 42.2-kilometer (26.2-mile) route begins at the Sambódromo, the famous site of Rio Carnival parades, with a 5-kilometer stretch (by Hutchinson’s measurement) that takes runners out to the shore. There, competitors run three 10-kilometer laps on a flat, mostly straight out-and-back segment along the beach, before ending with a 7.2-kilometer route back to the finish, also at the Sambódromo. In his analysis of the course at Sweat Science, Hutchinson counted 25 sharp turns — far more than the Berlin Marathon’s 17 turns, but fewer than the Chicago Marathon’s 31 turns. Those are two famously fast courses, so the turns in Rio probably won’t be the main barrier to record-seekers.

Optimal field: fast, with prizes that run deep

Athletes tend to race more tactically at the Olympics, where glory ends at third place, than they would at other major marathons, where prizes go deeper and bonuses are often awarded for records. “They’re jockeying for medals, not time,” Joyner said. Elite marathoners typically race only a couple of times a year, and runners who find themselves languishing at the games without a chance for a medal may ease up or even quit so they can live to fight another day at one of the fast autumn marathons.

Optimal pacing: even, with pacers helping out

A record-setting run requires careful pacing, but Olympic runners won’t have a benefit of pacers, which are allowed in some other top marathons. Also called rabbits, pacer runners go to the front and set a fast, even pace while allowing racers to draft and expend less effort to maintain their speed early in the race. Pacers eventually drop back and allow the racers to duke it out to the finish, but in the meantime they save those other runners a lot of energy. A runner in the lead without a pacer has to constantly wonder, am I running the right speed? “It takes a lot of cognitive effort to monitor your pace at all times,” Hutchinson said. “You have to constantly inhibit your natural response, which is to slow.” With a pacer in the lead, racers can turn off that decision-making process, rest the mental muscle required to stay on pace, and focus on staying a few feet behind the pacer.

Verdict: a record is unlikely, but don’t rule it out

Despite the factors standing in the way, I’m not foolish enough to set the chances of a world record in Rio at zero. In 2008, nobody would have predicted a fast marathon time amid Beijing’s stifling heat and humidity, and yet Kenyan Sammy Wanjiru took nearly three minutes off the previous men’s Olympic marathon record with his time of 2:06:32. It wasn’t a world record, but it was fast by any standard. Wanjiru didn’t do it alone — a small group of Kenyans and Ethiopians set a fast early pace. A record in Rio won’t be easy, but improbable dreams are the stuff the Olympics are made of.

We’re on the ground in Rio covering the 2016 Summer Olympics. Check out all our coverage here.