Exercising outdoors in summer can range from merely uncomfortable to genuinely dangerous, and the difference usually comes down to one number. Many runners who train through the hottest months skip the plain temperature forecast and check the wet bulb temperature instead before nearly every run. That figure matters more than the actual air temperature, and even more than the app's "feels like" reading, because it is what actually gauges how safe it is to exercise outside. When that number climbs too high, workouts get delayed, shortened, or scrapped altogether.
Heat Alone Doesn't Tell the Whole Story
It isn't just perception, the body genuinely runs slower in the heat. The heart has to pump extra blood to the skin to cool the body down, while also sending blood to the muscles to keep the workout going. That double demand is why hot-weather training feels so much harder and why heart rate climbs so quickly. But air temperature alone doesn't determine how hot the body actually gets, humidity plays just as large a role. Sweat is the body's primary cooling mechanism, and it works far better in dry air than in humid air. On a muggy day, sweat doesn't evaporate as easily, so it doesn't cool the skin as effectively. That's the reason a dry heat never feels as brutal as the same temperature on a humid day. Wet bulb temperature is useful precisely because it folds heat and humidity into a single reading. As an example, a wet bulb temperature of 82 degrees, commonly flagged as high risk for heat illness, can occur when it's 100 degrees with 10 percent humidity, or just 80 degrees with 70 percent humidity.
What Wet Bulb Globe Temperature Actually Measures
Rather than just glancing at the day's temperature forecast, the better way to figure out how the heat will affect the body is to look at a metric called wet bulb globe temperature, or WBGT. The concept behind it is that wrapping the bulb of an old-fashioned thermometer in wet fabric produces a reading that shows how much the surrounding air can actually cool something down. The real-world process is more involved than that and typically uses three separate thermometers, but that's the underlying idea. WBGT measures how well evaporation can cool a wet surface, in addition to factoring in the temperature itself. That matters because sweating, one of the body's main cooling mechanisms, doesn't work well once humidity gets too high, since humidity interferes with the evaporation that normally cools skin. WBGT accounts for that lost cooling effect. The good news is that no thermometer or wet cloth is required to estimate it, temperature and humidity can simply be looked up on a chart, or read straight from a forecast in a weather app such as Carrot Weather, which displays WBGT hour by hour throughout the day. Interestingly, humidity is often higher during the cooler parts of the day, so the wet bulb reading doesn't always shift much between morning and afternoon. Evening runs are still generally preferred when possible, since the sun sits below the horizon and isn't beating down directly, but the day's wet bulb reading is what ultimately decides whether a run should be shortened or skipped entirely.
The Official Cutoffs for Cancelling a Race
U.S. Soccer maintains a map that splits the country into three regions, each with a different threshold for cancelling practice, on the assumption that someone living in Texas can tolerate more heat than someone living in Minnesota. Similar guidelines exist for organizers of road races such as marathons. If the WBGT climbs above 82 degrees, the race should be cancelled outright. Above 73 degrees, "extreme caution and slower pace" are strongly advised. To picture how an 82-degree WBGT actually forms, it can happen in 75-degree weather with 90 percent humidity, in 84-degree weather with 50 percent humidity, or in 100-degree weather with just 10 percent humidity.
Guidelines for Athletes Not Yet Used to the Heat
For people who aren't acclimated to exercising in the heat, the American College of Sports Medicine lays out a general set of recommendations. If the WBGT is above 65 degrees Fahrenheit, extra rest and close attention to hydration are advised. Above 72 degrees, extra rest should be paired with a shorter workout duration. Above 78 degrees, the advice adds a third layer, extra rest, limited duration, and reduced intensity, meaning no pushing for a faster pace. Once the WBGT crosses 82 degrees, rest time should roughly match work time, all of the previous precautions apply, and there should be a sharp watch for signs of heat illness, since the risk at this level is considered high. Above 86 degrees, the recommendation is simply to stop and head home.
A Bit More Leeway for Heat-Acclimatized Athletes
People who have been safely exercising in the heat for several weeks or more get a somewhat relaxed set of cutoffs, since their bodies have adapted to cool themselves more efficiently. For this group, above 72 degrees WBGT, normal exercise can continue, with extra attention paid to hydration. Above 82 degrees, intense or prolonged exercise should be planned with discretion, and anyone at high risk should be watched for heat illness symptoms. Above 86 degrees, intense exercise should be limited and exposure to the heat kept short, while still watching for warning signs. Once WBGT crosses 90 degrees, the risk becomes too great even for athletes who are fully heat-acclimatized.
How These Numbers Play Out on an Actual Run
A wet bulb reading between 62 and 72 degrees is generally treated as ordinary summer discomfort, and most outdoor runs happen comfortably within that range. Once the number climbs past 72, extra precautions kick in, direct sunlight gets avoided by choosing shaded trails or an evening slot, and water supply gets bumped up, typically at least half a liter, or 16 ounces, carried in a hydration belt for an hour-long run. When driving to a running route, keeping extra water in the car is common too, in case of returning thirsty, and one popular trick is stashing a water bottle with an ice pack inside an insulated lunchbox to keep it cold. When the wet bulb reading sits in the mid-70s, all of that gets amplified further. On a longer run, the plan often shifts to returning to a car or house after the first 3 to 5 miles for a break in air conditioning. During stretches of routine 10-mile summer runs, that has meant splitting the distance into two 5-mile segments with a cooldown break in the car in between, packing a sandwich to eat during that break while sitting in the air conditioning, and reapplying sunscreen before heading back out. Pace itself turns out to be a major factor too, an easy run at a 72-degree wet bulb reading poses no real problem, but a tougher session, such as a tempo run or a set of hard intervals, needs a readiness to cut things short if the heat starts taking a toll. So far, a run has never gone ahead when the wet bulb temperature hit 80 degrees or higher, on the handful of days it reached that mark, the run simply moved to another day instead. A flexible schedule turns out to be one of the most valuable tools in a summer training plan.
Other Precautions Worth Remembering
All of this advice works alongside the usual precautions for staying safe in extreme heat. Extra fluids and electrolytes should be packed for hydration breaks, and spots with shade or air conditioning should be sought out to cool down. U.S. Soccer's guidelines even suggest a kiddie pool filled with a bag of ice as one option for cooling off. It's equally important to know the signs and symptoms of heat exhaustion and heatstroke, and to know what to do if those symptoms show up, whether in oneself or in a teammate or workout partner. Heatstroke counts as a 911-level emergency, so help should be sought immediately if it's suspected.
