Safety
Hiking in the heat
Three things put hikers down on a hot day, and two of them look alike while needing opposite treatment. The fixes are mostly a start time and a turnaround time. Here's the plan, and how to tell the three apart if it goes wrong.
- 1–2 qt
- Water an hour of hot hiking can cost you
- 15–20°F
- Hotter in the sun than the shade temperature on the sign
- 10am–4pm
- The window the Park Service tells you to be off the trail
- 19%
- Of Grand Canyon heat patients had suspected hyponatremia
Grand Canyon's figure for hikers in summer heat.
Which is the temperature the forecast gave you.
Hike before it and after it, and rest in shade during.
Too much water, not too little. Heat exhaustion accounted for 23%.
The temperature in the forecast is not the temperature on the trail. That number is a shade reading from a weather station, and you're going somewhere with no shade. Grand Canyon tells hikers that the sun temperature runs 15 to 20 degrees above the posted shade temperature, and the National Weather Service says standing in full sun can add as much as 15°F to the heat index. So the 88 degrees is accurate. It's just measured somewhere you won't be standing.
That catches hikers in one of three ways, and the three are separate problems. Heat exhaustion is the common one, and rest and shade usually fix it. Heat stroke is rare and kills. Hyponatremia, which is drinking so much plain water that you dilute the salt in your blood, is the one almost nobody plans for, and at Grand Canyon it accounted for nearly one heat patient in five between 2004 and 2009, close behind heat exhaustion. All three can open with a headache and nausea. What you do next is different for each, and for the last two it's close to opposite: hyponatremia needs fluids withheld, and heat stroke needs aggressive cooling and no fluids either.
Heat is mostly a scheduling problem, which is what makes it fixable. Almost everything that works gets decided at home the night before, or in the dark at a trailhead at five in the morning. By noon on an exposed slope you've run out of options.
Planning the day around the heat
None of this costs money or weighs anything, and it all matters more than the gear you bring.
A week out
Let your body catch up
Acclimatization is real, and it isn't quick. The Wilderness Medical Society's 2024 guidelines put full benefit at roughly one to two hours a day of exertion in the heat for at least seven days. Fly from a cool climate to a hot one on Friday, hike on Saturday, and you're not acclimatized. Plan a smaller day than you think you can do.
The night before
Look up the heat index, not the temperature
Humidity decides how much of your sweat can evaporate, and the heat index folds that in. Then look at the route: how much shade it has, where the water is, how high it goes. Washington Trails Association's rule of thumb is that every 1,000 feet of elevation gain buys 3.5 to 5°F, which is often the difference between two trailheads an hour apart.
The night before
Pick a turnaround time and say it out loud
A time on the clock, not a landmark. Heat problems compound on the way back, when you're emptier and it's hotter, and on any canyon hike the descent is the easy half. Grand Canyon puts it bluntly: in the inner canyon, self-rescue is required, because help may not arrive quickly. Pick the hour while you're comfortable and unhurried, then don't argue with yourself about it later.
Before first light
Start stupidly early
If you only change one thing, change this. The Park Service asks hikers to be on the trail before 10 a.m. and after 4 p.m. Walking at 5 a.m. has nothing to do with being hardcore. It puts the hot hours somewhere other than an exposed slope. WTA frames the same idea as ten miles by ten o'clock.
By 10 a.m.
Be off the exposed ground
Not finished necessarily, but past the part with no shade and no water. If you're still climbing a bare south-facing slope at eleven, the plan has already failed. Turn around. Don't speed up.
1–6 p.m.
Sit it out
The siesta is normal practice in hot countries and it works on trail too. WTA puts the optimal window at 1 to 6 p.m. Find shade, eat something salty, top off water if there's any, and wait. Trying to make up time in these hours is what gets people into trouble.
After 4 p.m.
Move again, with a light in your pack
The back half of the day is cooler and usually quieter. The trade is that you're hiking closer to dark, so a headlamp stops being optional, including on a hike you expected to finish by lunch.
Why the heat wins
Your body has essentially one tool for shedding heat during exercise, and that's sweat. Sweat does nothing sitting on your skin. It cools you as it evaporates, which is why humidity matters so much: when the air is already wet, evaporation slows down and the same air temperature gets far more dangerous. That's what the heat index is measuring, and it's why 88°F in Virginia in July can be harder work than 95°F in the high desert.
The ground is working against you too. Rock and sand soak up sun all morning and radiate it back at you all afternoon, from below, where no hat helps. That's the gap between the shade temperature you were quoted and the temperature you're actually standing in.
The other problem is what leaves with the sweat. Salt goes out along with the water. Replace the water and ignore the salt for long enough and the sodium in your blood falls, which is hyponatremia. It gets careful hikers because the thing that causes it sounds responsible. The published research on exercise-associated hyponatremia points at overhydration as the primary cause, and it pointedly refuses to give you a safe hourly number. Healthy kidneys clear somewhere around one to one and a half liters an hour, but plenty of people who end up hyponatremic never drank that much, because a long hot day suppresses the hormone that lets you pass the surplus. That same literature calls fixed intake targets inappropriate and potentially dangerous, and points at thirst instead. Drinking on a schedule because you read that you should, while eating nothing, will do it.
So water and salt travel together. Eat something salty every time you stop to drink. Grand Canyon's own examples are pretzels, chips and cheese crackers, which is about as unglamorous as advice gets. On a hot day the food is doing half the hydration work.
Three ways a hot day goes wrong
Read this before you need it. Untreated heat exhaustion can progress to heat stroke; hyponatremia is a separate problem that opens with the same early symptoms. The row that decides your next move is the one about their head.
| Heat exhaustion | Heat stroke | Hyponatremia (too much water) | |
|---|---|---|---|
| How often it happens | The common one. 23% of the Grand Canyon cases. | Rare, and the one that kills. Grand Canyon sees two or three a year. | 19% of the same cases. Nearly as many as heat exhaustion. |
| What they feel | Heavy sweating, weakness, dizziness, cramps, nausea, headache. | Throbbing headache, nausea, dizziness, then collapse. | Headache, nausea, vomiting, a general sense of being unwell. |
| Skin and temperature | Cool, pale, clammy. Still sweating hard. | Hot and red, dry or damp. Core temperature above 103°F. | Unremarkable. They're not especially hot. |
| Their head | Clear. They know where they are and answer you normally. | Confusion, slurred speech, strange behavior, unconsciousness. | Clear at first. Confusion and seizures if it gets worse. |
| The tell | Shade and rest visibly help within the hour. | Altered mental status. It's what the Park Service uses to separate the two. | Hours of steady drinking with almost no salty food, and they haven't needed to ration water. |
| What to do | Stop. Shade, loosen clothing, wet them down, sips of water plus salty food. No clear improvement within an hour means call for help. | Call 911 now. Cool hard: soak, fan, immerse in the coldest water available. Do not give fluids. | Stop the water. Salty food only if they are fully alert. Any confusion or seizure is a 911 call. |
Symptom lists follow the National Weather Service. The Grand Canyon proportions are paramedic assessments of the 474 nonfatal cases in a study of 480 heat-illness cases handled by park EMS from 2004 to 2009; a patient could be assessed with more than one, so the figures don't add to 100. Field cooling guidance follows the Wilderness Medical Society's 2024 heat illness guidelines.
What works, and what only sounds like it does
Hot-weather advice is full of half-remembered rules, and several of them are cold-weather rules being applied to the opposite problem.
Works
- Starting before dawn. It's the only change that moves the hike itself out of the dangerous hours.
- Eating something salty every time you drink. You sweat out both, so you have to replace both.
- Getting wet and staying wet. Grand Canyon's advice is to soak your clothing and body at every water source, so evaporation keeps working for you.
- Loose, light-colored, long-sleeved clothing and a real hat. Covering skin beats exposing it once the sun is the problem.
- Cotton, in dry heat. It holds sweat against you instead of wicking it away, which is exactly what you want when the air is doing the drying.
- Trading the trailhead for a higher or shadier one. Roughly 3.5 to 5°F per 1,000 feet is a big lever for a short drive.
- Reading the heat index rather than the temperature, then adding for full sun.
Sounds right, isn't
- "Drink as much water as you can." This is the direct cause of hyponatremia. Balance the water with food, and drink when you are thirsty instead of on a stopwatch.
- Treating an electrolyte drink or a salt tablet as a substitute for eating. Salty food is what the land managers and the hyponatremia literature both point at.
- "Cotton kills." True when the danger is hypothermia. Wrong when the danger is heat, which is why Grand Canyon's own heat guidance asks for loose-fitting cotton clothing.
- Planning against the forecast temperature. The sun adds up to 15°F to what your body experiences, and the ground adds more.
- Pushing on because the car isn't far. On a canyon hike the distance you have left is the uphill part, and the hottest.
- Assuming a helicopter solves it. Grand Canyon warns that rescue can be delayed by staffing and call volume, and that helicopters fly badly in extreme heat.
The hot-day checklist
Nothing here is exotic. The failures tend to be ordinary: a late start, a route with no shade, and a pack with plenty of water and nothing to eat.
Before you leave the house
- Check the heat index for the trailhead, not just the temperature
- Look up how much shade the route actually hasA north-facing forest trail and a bare ridge at the same temperature are not the same hike.
- Find out where the water is, and whether it's running this month
- Set a turnaround time before you start walking
- Tell someone the route and when to worry
- Consider swapping to a higher or shadier trailEvery 1,000 feet is worth roughly 3.5 to 5°F.
In the pack
- More water than the distance suggests, plus a way to treat moreAn hour of hot hiking can cost one to two quarts.
- Salty food you'll actually eatPretzels, chips and cheese crackers are the Park Service's own examples.
- A brimmed hat and a loose long-sleeved shirt
- Something you're happy to soak: a bandana, a sun shirt, your hat
- Sunscreen, and the discipline to reapply it
- A headlamp, even on a hike you expect to finish by lunchHot-day plans routinely push the finish into the evening.
- A way to call for help where there is no phone signal, if you are going remote
On the trail
- Eat every time you drink
- Keep your shirt and hat wet whenever you pass water
- Rest in shade during the hot hours instead of pushing through them
- Watch the people you're with, not just yourselfConfusion is the sign that matters, and nobody spots it in themselves.
- Turn around at the time you agreed, whatever the summit is doing
If someone goes down
In order. The one people skip is checking whether the person is making sense. This is field first aid, not a substitute for medical care or a wilderness medicine course.
- 01
Stop, and get them into shade
Any shade: a boulder, a juniper, a tarp held up by two people. Get them off hot ground and lying down. Loosen or remove tight clothing.
- 02
Check whether they are making sense
Ask something with a real answer: where are we, what were we doing, what's your friend's name. A lucid person with heat exhaustion answers normally and is just miserable.
Confusion, slurred speech or odd behavior changes everything that follows.
- 03
If they are confused, call for help now
Don't wait to see if rest fixes it, and don't start walking them out. Heat stroke and severe hyponatremia both progress quickly, and both need a hospital. Grand Canyon's instruction is that somebody calls or goes for help while the cooling carries on, so this step and the next one happen together.
- 04
Cool them, and do it seriously
Water over the skin, clothing soaked, vigorous fanning. If there's a creek or a lake, immersion in the coldest water available is the most effective field cooling there is, and it's what the Wilderness Medical Society recommends for heat stroke. It also says to start cooling on scene, before evacuation, wherever that's possible.
- 05
Fluids only if they are fully alert
Sips of water with salty food for someone who is lucid and improving. Nothing by mouth for anyone confused or drowsy. That's the rule whether the problem turns out to be heat stroke or too much water.
- 06
Give it the hour before you decide
Grand Canyon's guidance for heat exhaustion is to stop hiking and rest in shade for 30 to 45 minutes with water and electrolytes. Somebody who's clearly better after that can go on, carefully and downhill. If they're not better within an hour, or they're vomiting, they need help rather than another attempt.
The words on the warning signs
- Heat index
- What the air temperature feels like once humidity is accounted for, because humid air stops sweat evaporating. The National Weather Service treats a heat index of 103°F and up as Danger, where heat exhaustion is likely and heat stroke possible with exertion. The chart assumes shade, and full sun can add up to 15°F.
- Sun temperature
- The temperature where you're actually standing, as opposed to the shaded reading a weather station reports. Grand Canyon puts the gap at 15 to 20°F.
- Evaporative cooling
- The only meaningful way your body sheds heat while you exercise. Sweat only cools you once it evaporates, which is why still, humid air is so much more dangerous than a dry breeze at the same temperature.
- Acclimatization
- The physiological adjustment to working in heat: you start sweating sooner and lose less salt doing it. It takes repeated exposure rather than one hot day, roughly one to two hours of daily exertion in the heat across at least a week.
- Hyponatremia
- Blood sodium diluted below the normal range, typically by drinking large volumes of plain water while sweating out salt. In its exercise-associated form it starts as headache, nausea and vomiting, and can progress to confusion and seizures. The treatment is to stop drinking, not to drink more.
- Exertional heat stroke
- Heat illness that has reached the point of altered mental status with a core temperature usually above 104°F. That figure is the Wilderness Medical Society's, and Yosemite's rangers use it too; the Weather Service's public guidance draws its line lower, at 103°F. Neither is a number you can measure on a trail, which is why mental status does the work. It's a medical emergency, and the field priority is immediate, aggressive cooling while help is on the way.
Common questions
How hot is too hot to hike?
There's no single cutoff, because humidity, shade, altitude and how acclimatized you are all move the line. A useful proxy is the National Weather Service heat index: from about 103°F it's classed as Danger, where heat exhaustion is likely and heat stroke possible with prolonged activity. Land managers get more direct in extreme conditions. During an excessive heat warning, Grand Canyon simply recommends not hiking below the rim at all.
How much water should I carry for a hot hike?
Plan around the loss rate rather than a fixed bottle count: Grand Canyon tells hikers they can lose one to two quarts an hour in summer heat, and Death Valley suggests at least a gallon a day. Then figure out where you can refill, because carrying a full day's worth is heavy and often impractical. The volume on its own isn't enough, though. Water without salty food is what causes hyponatremia.
How do I tell heat exhaustion from heat stroke?
By their head, not their skin. Someone with heat exhaustion is sweating, pale, clammy and thoroughly miserable, but lucid, and they answer questions normally. Heat stroke brings altered mental status: confusion, slurred speech, strange behavior or collapse, usually with hot skin and a core temperature above 103°F. Don't lean on the old rule that heat stroke means dry skin: the Weather Service's own description of heat-stroke skin is "hot, red, dry or damp", so someone who is still sweating is not ruled out. Treat confusion as the emergency sign.
Can you really drink too much water hiking?
Yes, and it isn't a fringe concern. In a study of heat-illness cases handled by Grand Canyon EMS between 2004 and 2009, paramedics logged suspected hyponatremia in about 19% of the nonfatal cases, not far behind heat exhaustion at 23%. It comes from drinking large amounts of plain water while sweating out salt and eating little. The fix is prevention: eat salty food alongside the water, and let thirst do some of the steering rather than drinking to a schedule.
Is cotton good or bad for hiking in the heat?
Good, in dry heat, which surprises people who learned "cotton kills" as a universal rule. That rule is about hypothermia, where cotton holds moisture and chills you. In hot, dry conditions holding moisture is the point, because the damp fabric keeps cooling you as it dries. Grand Canyon's heat guidance specifically asks for loose-fitting cotton clothing, and Washington Trails Association makes the same case: in hot weather, moisture retention is exactly what you want. In cold or wet conditions the original warning still stands.
How long does it take to get used to hiking in heat?
About a week of real exposure. The Wilderness Medical Society's 2024 guidelines describe roughly one to two hours a day of exertion in the heat over at least seven days as achieving the full benefit. There's no way to compress this into the day before a trip, so if you're traveling from a cool climate into a hot one, plan a shorter and easier first few days than your fitness suggests.
Sources
Facts on this page were checked against these. Conditions, fees and rules change — check the official page before you go.
- Hike Smart — Grand Canyon National Park, National Park Service · checked August 4, 2026
- Summer Safety: Avoid Hiking During Excessive Heat Warnings — Grand Canyon National Park, National Park Service · checked August 4, 2026
- Beat the Heat: Prevent Heat-Related Illnesses — National Park Service · checked August 4, 2026
- Heat Stroke on a Hot Summer Day — Yosemite National Park, National Park Service · checked August 4, 2026
- Safety (Death Valley) — Death Valley National Park, National Park Service · checked August 4, 2026
- Heat Related Illnesses — National Weather Service · checked August 4, 2026
- What is the Heat Index? — National Weather Service · checked August 4, 2026
- Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Heat Illness: 2024 Update — Wilderness & Environmental Medicine (SAGE) · checked August 4, 2026
- Exertional Heat-Related Illnesses at the Grand Canyon National Park, 2004–2009 — Wilderness & Environmental Medicine, via PubMed Central · checked August 4, 2026
- Exercise-Associated Hyponatremia in the Grand Canyon — PubMed Central · checked August 4, 2026
- Exercise-Associated Hyponatremia — PubMed Central · checked August 4, 2026
- Summer, Sun and Sweat: A Guide to Keeping it Cool — Washington Trails Association · checked August 4, 2026
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