Heat Exhaustion vs. Heat Stroke
These are not the same condition, and confusing them can be fatal. Heat exhaustion is your body losing its fight to stay cool. Heat stroke is your body's cooling system shutting down entirely. The response for each is completely different.
Heat exhaustion shows up as heavy sweating, weakness, nausea, headache, dizziness, and pale or clammy skin. The critical detail: the person is still sweating. Their body is still trying. You can treat this on the trail by moving to shade, removing excess clothing, applying cool water to the skin, and sipping electrolytes slowly. Most people recover within 30 minutes if you act quickly.
Heat stroke is a medical emergency. Sweating stops. Skin becomes hot and dry. The person is confused, may slur their speech, or lose consciousness. Core body temperature has exceeded 104°F and the brain is being damaged. Call 911 immediately. Cool the person aggressively with any water you have (pour it on their head, neck, armpits, and groin). Do not wait to see if they improve.
The critical difference: If the person is still sweating, you likely have time. If they have stopped sweating and seem confused, you are dealing with a life-threatening emergency. Every minute counts.
Heat exhaustion progresses to heat stroke faster than most hikers expect, sometimes in as little as 10 to 15 minutes without intervention. The transition is not always obvious because the person experiencing it may not recognize their own confusion.
Timing Your Hike
The simplest way to reduce heat risk is to avoid the hottest part of the day entirely. In summer, temperatures peak between 10 AM and 3 PM, with UV radiation at its strongest around solar noon (roughly 1 PM in most U.S. time zones during daylight saving time).
We recommend two windows for hot-weather hiking:
- Dawn starts: Begin at or before sunrise. You gain 3 to 5 hours of cooler hiking before the heat builds. This is the best option for longer trails.
- Late afternoon: Start at 4 PM or later. Temperatures drop faster than most people realize once the sun angle decreases. Carry a headlamp in case you finish after dark.
Terrain matters as much as time. South-facing slopes receive direct sun for most of the day and can be 10 to 15°F hotter than north-facing slopes on the same mountain. Canyon bottoms and narrow valleys trap heat because air cannot circulate. If your route includes exposed ridgeline, plan to be there during the cooler hours.
Check both the temperature and the heat index before you go. A 92°F day with 60% humidity has a heat index of 105°F. That humidity prevents your sweat from evaporating, which is your body's primary cooling mechanism. When sweat cannot evaporate, your core temperature rises regardless of how much water you drink.
Hydration Math
In temperatures above 90°F, your body loses roughly 1 to 1.5 liters of sweat per hour during moderate hiking. You need to replace most of that. The general rule: carry and drink approximately 1 liter per hour of active hiking in heat.
For a 4-hour summer hike, that means 4 liters minimum (just over 1 gallon). That weighs about 8.8 pounds. This is non-negotiable weight.
Pre-Hydration
Drink 16 to 20 ounces of water about 2 hours before the trailhead. Not right before (you will just urinate it out), but early enough for your body to absorb and distribute it. Starting a hot-weather hike already dehydrated is setting yourself up for trouble by mile 2.
Electrolytes Are Essential
Drinking plain water alone in extreme heat can actually make things worse. You lose sodium, potassium, and magnesium through sweat. Replacing water without replacing electrolytes dilutes your blood sodium levels, a condition called hyponatremia. Symptoms mimic heat exhaustion (nausea, headache, confusion), making it easy to misdiagnose.
We recommend adding electrolyte tablets or powder to at least half your water. Look for products with 300 mg or more of sodium per serving. Sports drinks work in a pinch, but most contain too much sugar and too little sodium for serious exertion.
The urine test: Clear to pale yellow means you are hydrated. Dark yellow to amber means you are already behind and need to increase intake immediately. By the time you feel thirsty, you have already lost 1 to 2% of your body weight in water, enough to impair performance and judgment.
What to Wear in Heat
Your instinct is to wear as little clothing as possible. That instinct is wrong. Exposed skin in direct sun absorbs radiation and burns, both of which raise your core temperature. The right clothing actually keeps you cooler than bare skin.
The ideal hot-weather hiking outfit:
- Light-colored, loose-fitting, UPF-rated long-sleeve shirt. Light colors reflect solar radiation. Loose fit allows airflow between fabric and skin. UPF 50+ blocks 98% of UV rays. We recommend shirts with mesh ventilation panels at the back and underarms.
- Lightweight hiking pants or convertible pants. Nylon or polyester blend. Loose enough for airflow. Zip-off legs give you the option to switch to shorts in shaded sections.
- Wide-brim hat (3-inch brim minimum). Protects your face, ears, and neck. A sun cap with a removable neck drape works well too. Baseball caps leave your ears and neck exposed.
- Moisture-wicking socks. Merino wool or synthetic. Wet cotton socks in heat cause blisters faster than almost anything else.
Sunscreen on all exposed skin. SPF 50 or higher, broad-spectrum. Apply 15 minutes before sun exposure and reapply every 2 hours, or immediately after heavy sweating. Most people apply only 25 to 50% of the recommended amount. Use more than you think you need. A full-body application takes about 1 ounce (a shot glass worth).
Cooling Strategies on the Trail
Active cooling is the difference between finishing your hike strong and being evacuated by search and rescue. These techniques work because they target the areas where blood flows closest to the skin's surface.
- Wet bandana on your neck: The carotid arteries run through your neck. Cooling this area drops your perceived temperature significantly. Soak a bandana or buff at every water source.
- Dip your hat in streams: Evaporative cooling from a wet hat can lower your head temperature by several degrees. Works especially well in dry climates where evaporation is fast.
- Pour water on your wrists: Major blood vessels run close to the surface at your wrists, inner elbows, and behind your knees. A small amount of water on these pulse points cools blood flowing to your core.
- Rest in shade every 30 to 45 minutes: Find shade, sit down, remove your pack, and drink. Even 5 minutes of shaded rest drops your core temperature measurably. Do not skip this because you feel fine. Heat illness sneaks up.
Know your water sources. Before the hike, identify every stream, spring, and reliable water source on your route. These are both hydration resupply and cooling stations. Carry a water filter so you can refill from natural sources. Running out of water in 100°F heat is a true emergency.
If someone in your group starts showing signs of heat exhaustion, stop immediately. Move them to shade, lay them down with feet slightly elevated, remove excess clothing, and apply cool water to their neck, armpits, and groin. Have them sip (not gulp) electrolyte solution. Do not resume hiking until they have fully recovered, which takes at least 30 minutes.
Recommended Gear
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Frequently Asked Questions
What temperature is too hot for hiking?
Above 95°F, heat-related illness risk rises sharply for most hikers. Above 105°F, we recommend avoiding exposed trails entirely. Temperature alone does not tell the full story. Humidity is equally important because it determines whether your sweat can evaporate. Check the heat index before you go. A 92°F day with 60% humidity feels like 105°F to your body.
How do I recognize heat stroke vs heat exhaustion?
Heat exhaustion: heavy sweating, pale and clammy skin, weakness, nausea, headache. The person is still sweating and still coherent. Heat stroke: confusion, slurred speech, hot and dry skin (sweating stops), rapid pulse, possible loss of consciousness. Heat stroke is a medical emergency requiring immediate 911 and aggressive cooling.
How much water should I drink when hiking in heat?
Plan for roughly 1 liter per hour of active hiking when temperatures exceed 90°F. For a 4-hour hike, that means at least 4 liters (about 1 gallon). Pre-hydrate with 16 to 20 ounces about 2 hours before starting. Add electrolyte tablets to at least half your water to replace sodium and potassium lost through sweat.
Should I wear long sleeves when hiking in hot weather?
Yes, if the fabric is right. A lightweight, loose-fitting, UPF-rated long-sleeve shirt keeps you cooler than bare skin in direct sun. The fabric blocks UV radiation, reducing sunburn and heat absorption, while allowing airflow. Look for shirts with mesh ventilation panels and light colors that reflect rather than absorb solar heat.
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