The World Cup opens this week against a backdrop of familiar North American summer hazards - extreme heat, heavy humidity and thunderstorms that can appear with little notice. Seasonal outlooks point to temperatures above normal across much of the United States, while moisture moving north from the Gulf of Mexico adds the potential for convective storms and severe weather during the tournament's opening weeks.
Although forecasts cannot specify conditions for individual matches that far in advance, experts warn there are tangible weather-related risks for a tournament staged across Canada, Mexico and the United States. Those warnings focus less on simple air temperature than on wet-bulb globe temperature - a composite measure that blends heat, humidity, sunlight and wind to estimate the degree of heat stress on the human body.
World Weather Attribution has cautioned that roughly a quarter of the matches could occur under conditions that exceed recommended safety limits for heat stress.
Internal heat generation and the limits of cooling
Physiology researchers emphasize that elite athletes produce substantial internal heat during play, which complicates the body's ability to stay cool in hot or humid settings. Chris Minson, a professor of physiology and co-director of the Exercise and Environmental Physiology Labs at the University of Oregon, notes that a large majority of the energy used during exercise ends up as heat.
"Seventy-five percent of all the energy that we utilise during exercise gets converted to heat," Minson said. "Only about 25% goes to actually doing the exercise." In conditions of high temperature, direct sun or elevated humidity, the body's normal heat-dissipation mechanisms begin to falter. Sweat cools only when it evaporates, which becomes problematic as humidity rises.
"One of the hardest things for us is when the humidity is very high," Minson added. The venues most likely to face elevated humidity levels include Houston, Miami, Dallas and Monterrey.
Climate change and elevated match-day heat risks
Recent research from Climate Central finds climate change has increased the chance that temperatures will reach levels capable of impairing player performance at 97 of the 104 tournament matches. The analysis identifies the June 26 group-stage encounter between Uruguay and Spain in Guadalajara as the single match with the largest projected increase in heat-related performance risk. There, researchers estimate a 70% likelihood of performance-impairing heat - 37 percentage points higher than it would have been absent climate change.
Biological sciences professor Ryan Calsbeek of Dartmouth College, who studies how body type interacts with climate in athletic performance, said heat and humidity can alter both player welfare and how the game is played.
"Higher temperature, higher humidity is likely to slow games down," Calsbeek said. "When athletes have to perform for a very long time, they’re just not going to be able to balance the explosive power of their fast-twitch efforts with the more aerobic long-term efforts of a 90-plus minute game in the heat and humidity."
Nearly half of the matches face at least a 50% chance of temperatures exceeding 28 degrees Celsius (82.4 Fahrenheit), a threshold associated with declines in sprinting, total distance covered and recovery time.
Calsbeek also highlighted altitude as a factor in player response, noting Mexico City's elevation of roughly 2,240 meters (7,350 ft) could influence performance for participants who arrive from lower elevations without adequate time to acclimatize. Mexico City is scheduled to host five matches.
Organizational responses, limits and recommendations
FIFA has announced that every match will include a three-minute hydration break in each half. Tournament scheduling decisions, the organization says, considered elements such as average temperatures, travel logistics, rest days, medical planning and the availability of cooling infrastructure.
Several stadiums slated to host matches are equipped with retractable roofs or climate-control systems, and tournament regulations permit matches to be delayed, suspended, rescheduled or relocated when health, safety or security concerns - including severe weather - warrant such action.
Minson urged more prescriptive measures tied to wet-bulb globe temperature readings. He recommended mandating interventions when the wet-bulb globe temperature reaches 26C and said tournament organizers should consider postponing matches when readings approach 28C to 30C. He also proposed a set of field-level responses when conditions demand them: six-minute cooling breaks, shaded cooling areas, emergency ice baths and extended halftime intervals.
"If you have a player who seems to be having some delirium or not thinking straight, or collapses on the field, you need to cool them down immediately," Minson said.
For event organizers, FIFA's World Cup remains a major logistical undertaking that must balance venue capabilities, scheduling and medical preparedness. For players, coaches and sports scientists, the tournament will also be a real-time examination of how football copes with higher temperatures and more volatile summer weather.