By Marnie Kunz, NASM-certified trainer, USATF- RRCA-certified run coach

Running in humidity can intensify heat and make your runs much more challenging. Whether you live in a year-round warm climate or a place with hot and humid summers, knowing how to run in humid conditions can help you stay safe and run strong. As a longtime running coach, I have experienced running in humid summers in the Midwest and Northeast and done races in Miami in the summer. I’ve also studied a lot about humidity to help people I coach stay safe on sweltering hot days. This guide will include my top tips for running in humidity and also offer general guidelines to help you calculate when it’s too hot and humid to run outside and when to scale back your workouts, and when to proceed with your regular training.

Running in Humidity

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When I was a cross-country runner, our high school coach used to reward anyone who pushed themselves to the point of throwing up with a free meal. We grew up thinking we had to tough it out and stick through our runs no matter what. While this has been beneficial when it comes to self-discipline, it would be dangerous with today’s rapidly warming climate.

According to the World Health Organization, approximately 489,000 heat-related deaths occur each year, and the number is increasing. For people in cities (like me in NYC), the urban heat island effect intensifies the heat, increasing temperatures due to the amount of concrete that has replaced natural trees and habitat.

Effects of Running in Humid Conditions

So, how does running in humidity affect your body? High humidity makes your runs feel more difficult and places a bigger toll on your body than running in non-humid conditions. Humidity also intensifies the effect of heat and makes it harder for your body to cool itself. Running in humid conditions will slow down your pace and, if the humidity is accompanied by high temperatures, can put you at risk of heat illnesses.

When you run, your body cools itself off with sweat glands, which transfer heat from your body into the air as your sweat evaporates. In high humidity, sweat does not evaporate off your skin, which raises your core body temperature as your body cannot cool itself properly. You also risk dehydration from heavy sweating when running in humidity.

Another challenge of running in high humidity and hot weather is that your heart has to work extra hard to keep your internal temperature from rising too much. The heart pumps extra hard to push blood toward your skin to release heat.

Your brain is also affected by heat and humidity, as blood is diverted from it as your breathing speeds up and your blood flows toward your skin. A common issue with people who get heat exhaustion is that they lose some thinking abilities and do not have good judgment. Confusion and loss of judgment can put people at greater risk of heat stroke, as they often do not realize they need to get out of the heat.

Knowing when the humid conditions are too extreme for your body can help you stay safe and healthy and avoid permanent organ damage and heat stroke.

Measuring Heat and Humidity with Heat Index

The heat index can help you determine if it’s too hot and humid to run outside or if you need to modify your workouts. According to the National Weather Service, the heat index is what the temperature feels like to the human body. The relative humidity and air temperature are combined to determine the heat index.

When the relative humidity is high, the heat and humidity make the apparent air temperature feel hotter. Conversely, when the humidity is very low, the air temperature may feel cooler than what the thermometer says.

Here are some general guidelines to help you determine if running in humidity and heat is safe or too risky:

A Heat Index of 80 to 90 degrees Fahrenheit— Take precautions when running. Prolonged exposure (like long runs) will cause fatigue.
A Heat Index of 90 to 103 °F - Extreme Caution when running outdoors. You may be at risk of heat-related illnesses, especially during prolonged running. Run in the shade and early morning hours, shorten your run, go slower, or run inside on a treadmill.
A Heat Index of 103 to 124 °F is Dangerous. Do not run outside in these conditions. If you work out in these conditions, you are at risk for heat cramps, heat exhaustion, and heat stroke.
A Heat Index of 125 °F or higher is considered Extreme Danger. Do not run outside in these conditions, as your chances of having a heat stroke are very high.

If you do not know the heat index in your area, you can use the National Weather Service’s Heat Index Calculator by plugging in the relative humidity (which almost any weather app or site will tell you) and the air temperature.

Signs of Heat Illness

There are a variety of heat illnesses people can get when running in the heat. These are the most serious conditions and symptoms to watch for:

Signs of Heat Stroke:

High body temperature
Quick pulse
Dizziness
Headaches
Nausea
Vomiting
Confusion
Muscle weakness
Losing consciousness, fainting

Heat stroke is an emergency, so seek medical attention right away if you or someone you know has one. If possible, cool down with immersion in cool water and get out of the heat.

Symptoms of Heat Exhaustion:

Be careful when running in high humidity.

Muscle cramps or weakness
Fatigue
Dizziness
Heavy sweating
Pale, clammy, cold skin
Headaches
Rapid, weak pulse
Fainting
Vomiting or nausea

If you experience heat exhaustion, get out of the heat and into a cool place. Take a cool bath or put cool, wet clothes on your body. Drink water in small sips. If you’re not sure if you have heat exhaustion or a heat stroke, or if you are vomiting, your symptoms last over an hour, or your symptoms get worse, seek medical help.

Early signs of heat illnesses can include heat cramps with muscle pain or spasms, heavy sweating, and exhaustion. If you start to experience these symptoms, cut your run short and go to a cool place. Drink a sports drink or water with electrolytes and rest. If your symptoms worsen or do not go away after an hour, or if you have heart problems, get medical attention.

Ultimately, if you’re not sure if running in humidity and heat is safe, err on the side of caution and work out indoors. It’s better to miss a run than to sustain permanent damage to your body or mind.

For more information on heat illnesses, see CDC Symptoms of Heat-Related Illnesses.

Preparing Your Body for Heat and Humidity

Acclimatization

You can take some steps to help your body adjust to running in humidity. Gradually increase your running time in the humidity (as long as it’s safe to exercise outdoors). Incrementally increasing your running time and intensity in hot and humid conditions allows your body to adjust.

It typically takes 7 to 14 days for your body to acclimate to running in the heat. Acclimatization improves your sweat rate, heart rate regulation, and core temperature control.

Hydration

Staying hydrated can help your body cool off during runs. Drink plenty of fluids throughout the day, and on hot and humid days, bring water with electrolytes or sports drinks. After your run, continue hydrating to restore fluid balance.

Electrolytes are crucial for running in hot and humid weather. Electrolytes are minerals (sodium, potassium, magnesium, and calcium) that regulate fluid balance, nerve function, and muscle contractions. Consuming enough electrolytes helps reduce your risk of heat cramps, which are painful muscle spasms. Sports drinks or electrolyte tablets can help replenish the electrolytes your body loses through sweating.

Nutrition

Eating a nutrient-rich diet with a variety of fresh foods will help your body perform better during runs and other workouts. When it’s hot outside, runners sometimes don’t eat enough and wind up feeling more exhausted on runs. Ensure you’re getting enough carbohydrates, lean protein, and healthy fats to power your runs.

Related Post: What to Eat Before a Long Run

Clothing and Gear

You’ll need lightweight, moisture-wicking running gear for running in humidity and heat. Opt for light colors and loose clothing that offers ventilation. Dark colors will absorb and retain heat from the sun, making you hotter.

Moisture-wicking socks made from materials like Merino wool are also crucial for hot weather runs. And don’t forget sun protection - a hat, sunscreen, and sunglasses will help shield you from the dangers of the sun’s rays.

Hydration gear is also essential for warm-weather running. You can carry a hand-held water bottle or wear a hydration pack or vest to bring your electrolyte drink on the run with you. Some runners prefer running belts to store IDs, keys, fuel, and a water bottle.

Training and Running Techniques

If you’re running in hot and humid weather, you can take some precautions to stay safe and adjust your training. Sometimes, runners I coach are too hard on themselves about running slow or cutting their runs short in the heat, but this is smart because it adapts to the weather and prevents heat illnesses.

Here are my top training tips for running in humidity:

Adjust your run speed, intensity, and distance. You will be slower than your regular pace when running in the heat. This is okay, and you should run slowly to reduce your intensity. Avoiding speed workouts in hot weather. Also, cut your runs short if you experience any heat-related issues or if the heat feels too intense.
Avoid peak heat hours and run as much as possible in the early mornings. You can also run after sunset, but the temperatures cool down less in the evening than overnight, so the coolest time is early morning.
Run in the shade. Stick to tree-lined streets and trails for protection from the sun’s intense rays.
Run on a treadmill if it’s too hot and humid outside. Even if you usually avoid the treadmill, it’s better to be safe and live to run another day.
Cool down with water, wet towels, cool baths, ice, or a pool. Get out of the heat after running and remove your sweaty clothes. A cool bath or shower will help your body cool down after running.
Listen to your body. Recognize signs of heat illnesses and get out of the heat if you experience exhaustion, heat cramps, headaches, dizziness, confusion, or any other symptoms of heat illness. If you’re in doubt, seek medical care.

Run Recovery

After running in the heat and humidity, replenish the fluids you lost by drinking water and a sports drink or electrolytes formula. Your body loses electrolytes from sweating, and an electrolyte drink can help rebalance your electrolytes.

Also, a short cooldown can help your heart rate return to normal gradually. To recover from your run, you can walk or jog at a slow pace for a few minutes. However, if it is sweltering and humid outside or you have any heat illness symptoms, prioritize getting out of the heat as soon as possible.

Rehydrate throughout the day to replenish your body. You can also foam roll and ice sore muscles to improve muscle recovery.

If you experience any worsening or long-lasting symptoms of heat illnesses after running in the heat, seek medical attention.

FAQs - Ask a Run Coach

Why is it so hard to run in humidity?

The humidity puts more strain on your body and makes it harder for your body to cool off when the temperatures are high. Modify your runs, go slower, and do shorter workouts. If the heat index is above 103, do not run outside.

Can you run in 100 humidity?

100 percent humidity is very rare. This extremely high humidity would be brutal to run in and, coupled with heat, would be too dangerous.

Is it okay to exercise in high humidity?

High humidity will make the temperature feel hotter if the temperature is already hot. Check the heat index before exercising outdoors in the heat and humidity. If the heat index is above 103, do not run outside.

Running in Hot and Humid Conditions

Hot and humid weather conditions call for extra precautions. Always check the weather and heat index before you run to ensure it’s safe to exercise outdoors. Modify your workouts, run during the coolest times of day, and always bring hydration on your runs. Listen to your body and stop your run if you feel light-headed, dizzy, exhausted, or have other problems from the heat.

If you need a training plan to help you train smart and slay your running goals, visit the Runstreet Training Center. Do you run outside when it’s hot and humid? Follow and tag @Runstreet on Instagram to share your workouts and get cheered on.

Happy - and safe - running to you!

Marnie Kunz is a NASM-certified personal trainer and USATF- and RRCA-certified running coach based in Brooklyn, NY. Marnie likes helping people get and stay active to enjoy a better quality of life. When she’s not doing fitness things, Marnie enjoys exploring with her dog, a mischievous rescue Akita.

Running in Humidity - Top Tips from a Run Coach