In a nutshell 💡
- The thresholds you see everywhere—27, 30, or 35 °C—come from the same federal document or no identifiable source at all. These are safety guidelines, not biological laws.
- What really matters is the combination of humidity, wind, and solar radiation. At 35 °C in dry, breezy air, your sweat still evaporates. At 27 °C in humid, still air, it no longer does its job.
- Water loss measured during a walk in a hot environment reaches 1.4 liters per hour, compared to 0.8 in moderate conditions. Your endurance—not just your comfort—depends on it.
- The sign that distinguishes heatstroke from simple fatigue is neurological, not skin-related. Skin that remains damp does not rule out anything.
- Dense forest cover raises the ambient air temperature by 2 to 4 °C. Choosing your route is a more powerful factor than any piece of equipment.
27 degrees. 30 degrees. 35 degrees.
These are the three thresholds you’ll find if you’re looking for the temperature at which it becomes unreasonable to hike. Three reputable sources, three different figures, and none that explains where their number comes from. The first one comes up most often—so much so that it’s copied verbatim by a good half of the French-language sources available.
The problem isn’t that any of these figures is wrong. The problem is that a thermometer alone cannot answer the question. A hike at 33 °C under dense canopy, with a bit of a breeze, is nothing like the same hike at 28 °C in a humid, still valley bottom. The second scenario is the more dangerous of the two, and it’s the one that no threshold warns you about.
This article won’t tell you to start early—you already know that. It gives you the three variables that really matter: what the heat actually takes out of a hiker, how to choose a route that works for you, and how to cool off during the hike rather than once you get back.
Why aren’t the 27, 30, or 35 °C thresholds enough to make a decision?
The most widely cited guideline in France comes from the medical committee of the French Hiking Federation. It states that thermoregulation begins to falter around 27 °C and recommends considering canceling the hike starting at 30 °C, especially on a route with prolonged exposure to the sun. This text is reproduced verbatim by several federal and departmental websites, which gives it the appearance of a broad consensus when in fact it comes from a single source.
This recommendation is useful. It was written for leaders who are supervising a group and need to make quick decisions without instruments. In this context, a simple number is better than an equation. However, it does not describe a physiological tipping point, and the scientific literature does not identify any dry-air temperature threshold at which evaporation would cease to function.
What really matters: humidity, wind, and radiation
Your body releases excess heat by evaporating sweat. This evaporation does not depend on the air temperature, but on the difference between the humidity of your skin and that of the surrounding air. When the air is already saturated with water vapor, sweat runs down your skin instead of evaporating, and it no longer cools you down at all.
Add to that the wind, which stirs up the saturated air clinging to your skin, and direct sunlight, which heats you up regardless of the ambient temperature. These three factors carry more weight than the number displayed on your watch.
What the heat really costs a hiker
The challenge of hiking is twofold. Your muscular effort already generates a lot of heat, and on top of that, you have to contend with the heat from the environment. The two combine, and you pay the price in water loss.
In a simulated hike in hot conditions, measured fluid loss reached 1.4 liters per hour, compared to 0.8 liters in moderate conditions. Over the course of the test, this amounts to 2.27 liters lost instead of 1.28. These figures describe specific experimental conditions, not a universal rule: there is considerable variation among individuals, and some lose more than 2 liters per hour.
The practical consequences are immediate. Fatigue sets in sooner, concentration wanes, and estimates of duration become inaccurate. A six-hour run in mild weather can take eight or nine hours when temperatures rise. It is this discrepancy—more than the heat itself—that turns a pleasant day into a difficult run.
To learn more: How does our body regulate its temperature in the summer?
How do you know when it’s time to stop?
Dehydration sets in without warning, because thirst only appears once a fluid deficit has already developed. Next come headaches, dizziness, nausea, and cramps. At this stage, a break in the shade, sipping water in small sips, and a significant slowdown are almost always enough.
The sign that changes everything is neurological
Exercise-induced heatstroke is something else entirely. It is defined by a core body temperature above 40°C combined with neurological symptoms: confusion, incoherent behavior, disorientation, nonsensical speech, and sometimes seizures. It is this last symptom—and this alone—that should alert you.
A widespread misconception is that the skin becomes dry and hot to the touch. International guidelines are clear on this point: during exercise-induced heatstroke, sweating often remains profuse. Damp skin therefore does not rule out the condition at all, and waiting for it to dry before taking action means wasting precious minutes.
What to Do in an Emergency. If you notice confusion, unusual behavior, or loss of consciousness, call 15 or 112 and begin cooling the person immediately, without waiting for emergency responders or taking their temperature. Remove any unnecessary clothing, move the person out of direct sunlight, and douse them with water while providing ventilation. Immersion in cool water is the gold standard when possible. Do not give anything to drink to a confused or drowsy person: there is a real risk of choking. A cooling towel dampened and placed on the back of the neck and forearms is a helpful aid while waiting, but never a treatment in itself.
A must-read: What to Do in Case of Heat Stroke?
| What you observe | Heat exhaustion | Heat stroke |
|---|---|---|
| State of consciousness | Alert, tired, complaining | Confused, incoherent, disoriented |
| Skin | Moist | Moist or dry: not a definitive sign |
| What provides relief | Shade, water, rest | Nothing on site: medical emergency |
| What you should do | Stop, take a long break, resume slowly | Call 15 or 112, cool the person immediately |
What time should you leave when it's very hot?
This is the simplest strategy—and the one that pays off the most. Setting out between 5 a.m. and 7 a.m. allows you to tackle most of the elevation gain in the cool of the morning and be in the shade before 1 p.m. The hours between noon and 4 p.m. are spent in the shade, taking a long break rather than hiking.
Here’s a nuance that few guides mention: in cities and on dark surfaces, the peak temperature isn’t at noon but around 5 p.m., when the asphalt releases the heat it has absorbed. If your route begins or ends with an urban section, plan your schedule around this factor rather than the sun’s position.
Check the weather forecast the day before, and don’t just look at the high temperature: check the expected relative humidity and wind conditions. These factors will tell you whether a day forecast to reach 31 °C will be comfortable or grueling.
“There’s no specific temperature at which you should stop riding, and that’s precisely what makes this a dangerous topic. What matters most is the humidity, the available shade, and the time of departure—three factors, none of which are shown on a thermometer.”
— Édouard Castaignet, Co-founder of G-Heat
Choose a route that works for you
Not all routes are created equal in the summer, and the difference between two nearby options far exceeds what gear can compensate for.
Forests lower temperatures by several degrees—and this has been measured
Across Europe, the air temperature under forest cover is, on average, 2.1 °C lower than in open areas during the summer. During extreme heat waves, microclimate studies observe a difference of 3 to 4 °C under dense canopy cover, and the effect increases with the density of the foliage. This isn’t just a feeling—it’s a documented difference.
So opt for the forest floor, deep valley bottoms, and riverbanks. North-facing slopes receive significantly less direct sunlight, especially in the afternoon, and retain residual moisture that cools the air. Conversely, avoid bare ridges, scree slopes, and open plateaus, where the rock reflects heat and it stagnates.
In the mountains, keep in mind that ultraviolet radiation increases by about 10 to 12 percent for every thousand meters of elevation gain, under comparable atmospheric conditions. High-altitude winds give a false impression of coolness while your skin is actually burning, making this a classic pitfall.
Hydration in Extreme Heat: How Much, When, and What
Recommended fluid intake varies from one-third to three times the standard amount depending on the source, and this makes sense: sweat output ranges from 0.5 to over 2 liters per hour depending on the individual, pace, and air conditions. Therefore, no single figure can apply to everyone.
A reasonable guideline is to aim for at least 1.5 liters for a short outing, and 2.5 to 3 liters for a summer day—more if there are no designated water stations along the route. Increase this amount if you sweat heavily, if the air is dry, or if you’ll be out for a long time.
How you drink is just as important as how much you drink. Take a few sips every fifteen to twenty minutes, without waiting until you’re thirsty: a large amount swallowed all at once passes through your system too quickly to be absorbed. During long workouts, a drink containing a little sodium helps prevent cramps caused by mineral loss, and lightly salted snacks serve the same purpose. Avoid alcohol and sugary drinks, as they worsen the deficiency.
Dressing for extreme heat: loose-fitting first, light-colored next
On this subject, sources openly contradict one another. Most recommend light-colored clothing. Another cites the dark clothing worn by Bedouins to support the opposite view. Both positions are based on some truth, and the issue deserves to be clearly addressed.
What the study on dark clothing really says
The study in question is by Shkolnik and his colleagues, published in Nature in 1980. It does not conclude that black keeps you cool. It shows that in a loose-fitting, thick, and ventilated garment, the extra heat absorbed by the dark fabric is dissipated by convection before it reaches the skin, thanks to the air space between the garment and the body. The experiment involved a single volunteer and sessions lasting about 30 minutes.
In other words, it’s the cut that makes the difference, not the color. With a tight-fitting T-shirt, lacking an air gap or ventilation, the physics of heat absorption take over, and light colors become preferable again. The key recommendation can therefore be summed up in three words: loose-fitting, full-coverage, breathable—and light-colored if the garment fits snugly against the body.
For your head, a cooling, UV-protective hat with a wide brim protects your scalp, the back of your neck, and your ears—areas where a traditional baseball cap leaves the back of your neck exposed. For the torso, the cooling, UV-protective T-shirt is available in men’s and women’s cuts, featuring JadeCool™ technology. Complete the look with Category 3 or 4 sunglasses. To understand what a protection factor really means, see our article on UPF 50+ fabric against ultraviolet rays and our guide on which fabric to choose to stay cool in the summer.
Stay Cool While Walking, Not Just After
This is the aspect most often overlooked in available guides, even though it affects immediate comfort and your ability to complete the hike. Staying cool while walking isn’t a luxury—it’s what allows you to stick to your planned route instead of cutting it short.
The mechanism—and its limitations
A hydroactive fabric works through evaporation. The water it contains evaporates, drawing latent heat from the skin and the surrounding air—approximately 2.4 megajoules per kilogram of water. The fabric does not produce cold; it acts as a reservoir and an exchange surface.
The published order of magnitude is clear: a fabric combining water transport and conduction results in a skin temperature that is 4.1 °C lower than that obtained with conventional cotton, under a given experimental protocol. This refers to skin temperature, not core temperature, and this distinction matters.
The limitation is just as well documented. For evaporative vests tested at 35 °C, increasing relative humidity from 20% to 40% at a constant air velocity reduces cooling capacity by 28%. The more saturated the air is, the less evaporation occurs, and the garment simply becomes damp.
In dry, breezy conditions, dampen your gear at every water source and let evaporation do its work: that’s when performance is at its peak. In humid, windless conditions, the same approach is much less effective, so you’ll need to adjust your strategy: walk in the shade, take longer breaks, and slow down your pace. The EVO cooling sleeves, featuring PhaseCool™ technology, target the forearms—a highly vascularized area where heat exchange is efficient—and the nape of the neck deserves the same treatment. We discuss these two areas in detail in our articles on protecting your head from the sun and on what an anti-UV sleeve is really worth.
To learn more about the areas to prioritize, check out our article on the body areas to focus on for cooling, as well as our explanation of how cooling fabrics work. To help you decide between the two types of gear, our comparison of UV protection vs. cooling settles the matter. The entire range is available in the cooling apparel, cooling T-shirts, and UV-protective gear collections.
Lighten Your Load, Slow Down, and Know When to Take a Break
A backpack that’s too heavy accelerates body temperature rise and dehydration. The generally accepted guideline is not to exceed 10% of your body weight—about seven kilograms for a 70-year-old. Beyond that, fatigue sets in faster and your safety margins shrink.
Deliberately slow down, break the hike into segments, and take frequent breaks in the shade—ideally every forty-five minutes to an hour. Reduce elevation gain rather than distance: it’s the ascent that generates the most metabolic heat. A sunshade or a lightweight umbrella—often mocked—actually lowers the perceived temperature by blocking direct sunlight during breaks. Following the same logic, opt for gear that weighs just a few dozen grams rather than bulky accessories: a cooling neck wrap weighs just a few dozen grams, fits in a pocket, and can be re-chilled at the first water source.
See also: Essential Gear for Your Summer Hikes, which details the checklist to pack in your backpack. To choose a trail suited to your level, see also which hike is right for you.
Why choose G-Heat for walking in extreme heat?
- We lay out the limits of our technologies. You’ve just read that an evaporative fabric loses nearly 30% of its effectiveness when the air becomes humid, and that an advertised UPF rating degrades when the fabric is stretched. These two facts aren’t meant to boost sales—they’re meant to help you make an informed decision.
- Three different mechanisms rather than just one. Depending on the product line, the cooling effect comes from evaporation, contact, or phase change. This is what allows our products to work in both dry and humid conditions—where evaporation alone falls short.
- A product line organized by body zone. Head, neck, torso, forearms: you equip what matters for your activity, without buying what you won’t use out in the field.
- Gear that doesn’t hinder your stride. Lightweight, no cumbersome activation, compatible with a backpack and trekking poles. An accessory you take off after ten minutes doesn’t protect anyone.
- A brand born from a thermoregulation disorder. G-Heat has been around since 2017 because one of its co-founders lived with a circulatory disorder on a daily basis. The issue of the body no longer being able to regulate itself is at the heart of the brand—not an afterthought.
Conclusion: What to Consider Before Setting Out
The right question isn’t what temperature you should avoid, but under what conditions your body will still be able to dissipate heat. Three variables are enough to make that call: humidity, wind, and exposure to sunlight along your chosen route. A thermometer alone will never tell you that.
From there, everything becomes simpler. You adjust your schedule, choose a shaded route, bring enough to drink regularly, wear loose-fitting clothing, and cool off while walking rather than waiting until you get home. And on the day when all three variables are unfavorable, you postpone your hike without a second thought.
Find the entire range in the hiking collection, as well as among the cooling hats, cooling arm sleeves, cooling towels, and cooling accessories.
Whatever the weather. G-Heat, in all weather conditions.
FAQ: Hiking and Extreme Heat
At what temperature should you avoid hiking?
There is no established physiological threshold. French federal guidelines indicate that discomfort begins around 27 °C and recommend considering canceling the hike once the temperature reaches 30 °C in the shade, especially on a route with prolonged sun exposure. Keep in mind, however, that this figure alone is not enough: at 35 °C in dry, breezy air, sweat evaporation still works, whereas at 28 °C in humid, windless air, it’s already overwhelmed.
How much water should you bring on a hike in hot weather?
Allow at least 1.5 liters for a short outing and 2.5 to 3 liters for a summer day hike, and more if there are no water stations. Sweat output varies from 0.5 to over 2 liters per hour depending on the person, pace, and weather: adjust your intake rather than sticking to a fixed amount. Take a few sips every fifteen to twenty minutes without waiting until you’re thirsty.
What are the signs of heatstroke while hiking?
The defining sign is neurological: confusion, disorientation, incoherent behavior, and sometimes seizures, with a core body temperature above 40 °C. Contrary to popular belief, sweating is often profuse: clammy skin does not rule out the diagnosis. Stop immediately, move to shade, cool the person down right away, and call 15 or 112.
Is it better to wear light-colored or dark-colored clothing when it’s very hot?
It’s primarily a matter of fit. The often-cited study on the dark clothing worn by Bedouins focused on loose-fitting, airy robes, where the air space dissipates absorbed heat before it reaches the skin. With tight-fitting clothing, this effect disappears, and light colors become preferable again. Therefore, opt for loose-fitting, covering, and breathable clothing—and choose light colors if the fabric clings to the body.
Do cooling garments really work while hiking?
Yes, with one important caveat. Evaporative fabrics lower skin temperature by a few degrees; one published study found a 4.1 °C difference compared to cotton under a specific protocol. However, their effectiveness drops when the air is humid: in tests conducted on vests at 35 °C, increasing relative humidity from 20% to 40% reduced cooling capacity by 28%. In humid conditions, rely more on shade and slowing down.
Sources and References
[1] “Extreme Heat, Heat Waves”, Santé publique France
[2] “Hiking Guide: Managing Heat Waves Effectively”, Medical Commission of the French Hiking Federation
[3] Gambichler T. et al., “Influence of wetness on the ultraviolet protection factor of textiles”, Photodermatology, Photoimmunology and Photomedicine, 2002
[4] “EN 13758-2, Textiles: Protection against solar ultraviolet radiation”, European Committee for Standardization
[5] “ForestTemp: Microclimate under forest canopy in Europe”, Global Change Biology, 2021
[6] Shkolnik A., Taylor C.R., Finch V., Borut A., “Why do Bedouins wear black robes in hot deserts?”, Nature, 1980
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