To run safely in the heat, run early in the morning or later in the evening, slow your pace and judge effort rather than speed, drink regularly before and during longer runs, use electrolytes when you sweat heavily, and wear light, breathable clothing with a hat and sunscreen. Give your body one to two weeks to acclimatize to hot weather with shorter, easier runs. Most importantly, know the warning signs of heat illness: stop and cool down if you feel dizzy, nauseated or develop a headache, and call emergency services for confusion, collapse or very hot skin, which may signal heat stroke.
Health note: This article is general information, not medical advice. Heat stroke is a medical emergency. People with heart, kidney or other medical conditions, those taking certain medications, pregnant people and older adults should talk to a doctor about exercising in hot weather.
Hot weather running tips
| Tip | Why it helps |
|---|---|
| Run early or late | Avoids the hottest part of the day and strongest sun |
| Slow down | Heat raises heart rate and effort at any pace |
| Hydrate before and during | Replaces fluid lost through sweat |
| Use electrolytes on long runs | Replaces sodium lost in sweat |
| Wear light, wicking clothing | Helps sweat evaporate and keeps you cooler |
| Hat, sunglasses and sunscreen | Protects from sun exposure |
| Choose shaded routes | Reduces radiant heat from sun and pavement |
| Acclimatize over 1 to 2 weeks | Allows the body to adapt to heat |
How heat affects running
When you run, your muscles generate heat. Your body cools itself mainly by sending blood to the skin and producing sweat that evaporates. In hot weather, the difference between your body and the air is smaller, so cooling is less efficient. Humidity makes it worse because sweat evaporates more slowly. As a result, heart rate rises at the same pace, effort feels harder and performance usually drops. Research on marathon results shows that finishing times tend to slow as temperatures rise above cool conditions, so adjusting expectations in the heat is normal, not a sign of poor fitness.
Temperature alone does not tell the whole story. Humidity, sun, wind and radiant heat from pavement all matter. Many sports organizations use the wet bulb globe temperature, or WBGT, which combines these factors, to decide when to modify or cancel events. For everyday runners, checking the heat index or feels-like temperature and humidity is a practical guide.
Adjust your pace and plans
Run by effort rather than pace on hot days. Expect to be slower, sometimes significantly, and use heart rate or perceived effort to keep easy runs easy. Move hard workouts to cooler times of day or indoors, shorten intervals or extend recoveries, and consider breaking long runs into loops that pass water sources or your home. On very hot days, a treadmill or indoor track can be a sensible alternative.
Hydration and electrolytes
Start runs well hydrated by drinking regularly through the day. For runs under about an hour in moderate heat, drinking before and after may be enough, though carrying water is wise on hot days. For longer runs, carry fluids or plan routes with water fountains, and drink to thirst. Sports drinks or electrolyte tablets help replace sodium lost in sweat, especially for heavy or salty sweaters. Avoid drinking far more than you sweat, which can lead to low blood sodium, known as hyponatremia. Weighing yourself before and after runs can help estimate how much fluid you lose.
Acclimatize to the heat
Your body adapts to heat over about 10 to 14 days of regular exposure. Adaptations include earlier and greater sweating, increased blood plasma volume, lower heart rate at a given effort and lower core temperature. Start with shorter, easier runs in the heat and gradually increase duration and intensity. If you are traveling to a hot race, arriving early or training in warm conditions beforehand can help. Acclimatization fades after a few weeks without heat exposure.
What to wear
Choose light-colored, loose-fitting, moisture-wicking clothing that helps sweat evaporate. Avoid cotton, which holds moisture. A lightweight hat or visor shades your face, and sunglasses reduce glare. Apply sweat-resistant sunscreen to exposed skin. Some runners use cooling towels, ice in a hat or bandana, or pour water over their head and neck during runs to help cool down.
Heat illness warning signs
Heat illness ranges from mild to life-threatening. Heat cramps are painful muscle spasms during or after exercise. Heat exhaustion can cause heavy sweating, dizziness, weakness, nausea, headache, fast heartbeat and pale, clammy skin. If you notice these signs, stop running, move to shade or air conditioning, drink fluids and cool down with water or wet towels. Seek medical help if symptoms do not improve quickly.
Exertional heat stroke is a medical emergency in which core body temperature rises to dangerous levels, typically above about 104°F (40°C). Signs include confusion, irritability, slurred speech, loss of coordination, fainting, collapse and seizures. Skin may be hot and either dry or still sweaty, so continued sweating does not rule out heat stroke. Call emergency services immediately. Rapid cooling, ideally by cold water immersion under medical supervision, is the most effective treatment, so start cooling with water, ice and shade while waiting for help.
Racing in hot weather
Hot conditions on race day call for a different plan. Adjust your goal before the start, since trying to hit a time set for cool weather can lead to a painful slowdown or heat illness. Many runners slow their target pace and focus on even effort instead. Start conservatively for the first few miles, and take water at every aid station, both to drink and to pour over your head and neck. Use sponges, ice and cooling stations if the race provides them. Wear a light hat and sunglasses, and apply sunscreen before the start. Stay in shade before the race and avoid a long warm-up in the sun. Race organizers may change start times, shorten courses or issue heat flags in extreme conditions, so read their communications and follow instructions from medical staff. If you feel dizzy, confused or unwell, stop at the nearest aid or medical station. Finishing another day is always better than risking heat stroke.
How to prepare for a hot weather run
- Check the forecast, including heat index and humidity.
- Plan your timing for early morning or evening.
- Choose a shaded route with access to water.
- Hydrate beforehand and carry fluids for longer runs.
- Dress in light, wicking clothing with a hat and sunscreen.
- Run by effort, not pace.
- Tell someone your route and carry a phone.
- Stop early if you feel unwell.
Common mistakes to avoid
- Trying to hit normal paces in hot, humid conditions.
- Running at midday in peak heat and sun.
- Ignoring early warning signs such as dizziness and nausea.
- Overdrinking water without electrolytes on long runs.
- Skipping acclimatization when temperatures rise suddenly.
- Wearing dark, heavy or cotton clothing.
Frequently asked questions
How hot is too hot to run?
It depends on heat, humidity, sun and your fitness; on high heat index days, move indoors or cut back.
How much water should I drink when running in the heat?
Drink to thirst, start hydrated and add electrolytes on longer or very sweaty runs.
How long does it take to acclimatize to heat?
About 10 to 14 days of regular, gradual exposure for most people.
