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We might be just seven months into 2025, but it’s already a safe bet that this year is going to be one of the hottest ever. According to NASA and Europe’s Copernicus Climate Change Service, June 2025 was the third-warmest on record, trailing only 2024 and 2023. And although it’s impossible to predict the future, NOAA suggests that the year as a whole has a 95 percent chance of being one of the four warmest years ever recorded. For the world as a whole, that means more widespread heat illness and more extreme weather. For hikers in particular, that means some tough choices as we decide when it’s worth braving the sun to tick a few trail miles and when it’s just too hot to hike.
If you’re hoping for a simple answer, prepare to be disappointed. There’s no single temperature where hiking suddenly becomes unsafe. In the U.S., some areas like Phoenix, Arizona, close trailheads when the National Weather Service issues an extreme heat warning. The criteria for that can be complex and vary by region, but in most of the country, the NWS issues a warning when the maximum heat index climbs to 105°F for multiple days and the low temperature is 75°F or higher. Heat illness can strike at temperatures far lower than that, but some very hardy, very experienced, and very careful folks still hike safely when the temperature climbs higher.
Extreme heat is the single deadliest kind of weather event on earth. Although it’s difficult to say exactly how many people it kills—many victims die from heart attack, stroke, or other health events brought on by the extra physiological strain that high temps provoke—the United Nations estimates that nearly a half-million people around the world pass away from excessive heat every year. Because we hikers willingly expose ourselves to the elements, we’re especially prone. Before braving the heat, consider these factors.
How much shade does your chosen route have?
Whether or not you’re exposed directly to the sun can make the difference between a tolerable hike and a scorching one. According to the University of New Hampshire, shade can lower air temperature by as much as 15 to 20°F. Depending on their aspect, vegetation, and topography, trails even relatively near each other can feel radically different. Look for deep canyons, thick forests, and (in the northern hemisphere) north-facing slopes. Topo maps (like the ones on Outside’s Gaia GPS) and apps like Shademap can help you figure out in advance where you’re likely to be able to stay out of the sun.
What kind of access to water is there?
Whether you’re a chugger or a camel, you likely need more water for your summer hikes than you think. A well-known rule suggests hikers should carry a gallon per person per day. In reality, many people will consume much more than that in summer temps. Dr. Tom Myers, a Grand Canyon guide and physician who’s made a career out of treating patients at the Big Ditch, suggests that a liter per hour is a good baseline at 100°F, which works out to more than two gallons for a full day on the trail.
If you don’t plan on toting all of that H2O in your pack, you’ll need to plan where you’re going to get it. If your route takes you along a creek, you may be all set. However, if you’re depending on tanks, small streams, and other seasonal water sources, make sure to get the latest beta from a ranger or a hiker who’s fresh off the trail. (And don’t forget to top off on salty snacks. Hyponatremia is a serious and underestimated threat in high temperatures.)
How acclimated are you?
The weather isn’t the only factor in determining whether you can hike safely in the heat. How well-accustomed you are to high temps can have a big impact on how you tolerate them. It’s not just psychological: Exposure to heat encourages your body to produce more heat shock proteins, which protect cells from the damage they might incur under heat stress. That, in turn, triggers a cascade of adaptations. You sweat more, cooling your body; your blood also circulates more efficiently, lowering your heart rate. If you regularly train in the kind of hot conditions that you expect to experience on the trail, then you might thrive in the heat more than if you’ve arrived from a moderate climate.
How humid is it?
There’s a limit to the level of heat that humans can survive, and it’s not all about degrees. Research suggests that a healthy person can only survive about six hours at a wet bulb temperature—a measurement that uses a thermometer covered in a wet cloth to simulate heat loss through sweating—of 95°F.
That’s really, really hot. At 50 percent humidity, that would equal a dry-bulb temperature of about 113°F, a combination that would have most hikers instinctively tapping out. But the principle behind it is still important: The more moist the air is, the less you’ll be able to cool yourself off by sweating, and the more cautious you should be.
Hike at night.
Itching for a hike but not willing to brave the sun? Take a tip from the desert’s native wildlife and head out before sunrise or after sunset. Learn how to do it safely with this advice.