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When Is It Too Hot To Run Outside? + 8 Tips For Running In The Heat

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For runners who live in some parts of the United States and worldwide, the temperatures can get so hot in the summertime that it is seemingly impossible to run outdoors, if not downright unsafe.

Is running in 100-degree weather safe? Is running in 90-degree weather safe?

So, exactly when is it too hot to run outside? In this article, we will discuss how hot is too hot to run outdoors, as well as tips for running in the heat.

We will cover: 

  • When Is It Too Hot To Run Outside?
  • The Risks of Running In the Heat
  • 8 Tips for Running In the Heat

Let’s dive in! 

A thermometer reading 110 degrees F.

When Is It Too Hot To Run Outside?

Many runners live in areas where the temperatures can get in the upper 90s, if not over 100 degrees Fahrenheit, in the summertime.

Then, deciding whether running in 100-degree weather or even running in 90-degree weather is safe becomes a nearly daily debate.

So, how hot is too hot to run outdoors?

The answer is likely not as simple as you might think. There isn’t a single cutoff temperature that is universally agreed upon as being too hot to run outside.

Moreover, when discussing running in the heat, there is more than just temperature to consider, as humidity plays a major role in the resultant “real feel“ or how hot it actually feels outside.

We will discuss the heat index, which is a measure that takes into consideration both the air temperature and the humidity, later on.

However, if you’re looking for a general guideline for how hot is too hot to run outside, according to the Road Runners Club of America (RRCA), you should avoid running outside if the heat is above 98.6 degrees Fahrenheit and the humidity is above 70-80%.

A hazy sky.

The Risks of Running In the Heat

The reason that it could be dangerous to run in the heat is that the core temperature of the body rises too high, increasing the risk of heat illnesses like heat exhaustion and heat stroke.

Your body does have natural thermoregulatory mechanisms to help cool down your core body temperature when you exercise, such as sweating and increasing circulation to the surface of your skin to move blood further away from the core of your body.

However, these mechanisms are often insufficient for running in extremely hot and humid conditions.

When deciding how hot is too hot to run outdoors, you need to pay attention to not only the air temperature on the thermometer but also the humidity.

Humidity makes running in hot temperatures even more taxing on the body because when the moisture content of the air is higher, it feels hotter.

The Heat Index combines the air temperature with the relative humidity, so the metric measures the apparent temperature or the “real feel” outside. 

A runner sweating from running in the heat.

The humidity doesn’t really cause any appreciable increase in the Heat Index or how hot it actually feels outside at or below 40% humidity.

However, above 40%, humidity rapidly increases the Heat Index.

Essentially, the reason that it is pretty difficult to assign a particular air temperature and humidity value to when it is too hot to run outdoors is that the thermal strain of running in the heat increases exponentially—rather than linearly—with an increase in humidity.

For example, when it’s 88° F (31° C) out with 40% humidity, it will still feel like 88° F, but when it’s 88° F with 70% humidity, it will feel like 100° F (38° C). 

When the humidity rises to 85% at the same temperature, the Heat Index jumps to 110° F (43° C).

For this reason, it’s not easy to calculate the heat index on your own.

Two runners, hands on knees, tired from running in the heat.

However, there are plenty of online calculators, such as the one here, that can help you determine the real feel of the heat out there.

The reason that paying attention to the heat index is particularly important is that the body is much less effective at cooling down your core temperature when running in the heat and humidity versus running in dry heat.

When you run in the humidity, the moisture content in the air prevents sweat from evaporating readily

Essentially, because the moisture content of the humid air is so high, sweat will not evaporate readily, which means that the heat energy stays trapped in your body without getting released.

In this way, running in the humidity causes heat to build up in your body, which in turn, increases the risk of heat illnesses like heat exhaustion and heat stroke.

8 Tips for Running In the Heat

If you are going to be running in 90-degree weather or running in 100-degree weather, it is important to take some precautions to help keep your body safe when running in the heat.

Here are some tips for hot weather running:

A weekly forecast.

#1: Be Patient

Most studies have found that it takes about two weeks for your body to acclimate to running in the heat. 

During this acclimatization period, be patient with yourself and adjust your workouts as needed.

#2: Look at the Hourly Forecast

It is important to pay attention to the combination of air temperature and humidity (the Heat Index) when planning the best time of day to run on hot days. 

Although running in the early morning before the sun rises often will offer cooler temperatures, the humidity is often highest in the morning. 

Running in the middle of the day should usually be avoided because the air temperature will be at its highest; plus, the sun will be most direct. However, the humidity is usually lowest at the peak hours of the day. 

Therefore, if you are able to run on a very shady trail, you may actually find that the heat index and “real feel” when you are running in the shade is better than when running on a humid summer morning on the open road.

Oftentimes, the evening is the best time to run in the summer because the sun has set, so the air temperature begins to drop, and the humidity tends to be lower.

A person drinking a bottle of water on a shaded trail.

#3: Hydrate

Not enough emphasis can be placed on the importance of hydrating properly when running in the heat. Dehydration significantly increases the risk of heat illnesses such as heat exhaustion and heat stroke.

Depending on your sweat rate and the environmental conditions, aim to drink at least 4-8 fluid ounces of water and/or electrolyte-infused sports drink every 15-20 minutes during your run. 

#4: Run By Effort

Although there is certainly training value in paying attention to your pace, when running in the heat, it is often a wiser and safer decision to run by effort.

This means that you should guide your pace and workout specifications based on how intense or difficult the running feels.

The reason that it is important to shift to running by effort rather than pace when running in 90 or 100-degree weather is that when you’re paying attention to pace rather than effort, you’re more likely to ignore heat illness cues from your body. 

Plus, in terms of your training, you are less likely to hit specific paces in the heat anyway. 

A person running on a shaded trail.

#5: Find the Shade 

Dark-colored asphalt radiates heat, and running in direct sunlight is like running in an oven.

Look for trails or bike paths that are covered by the natural canopy of the trees. 

The pavement will remain cooler on these sorts of bike paths, and if you choose natural trails with soil or wood chips, there will be less heat radiating from the surface as these materials will not absorb as much heat energy as black asphalt.

#6: Choose the Right Clothes

Wear lightweight, breathable fabrics for your hot-weather runs and as little clothing as you’re comfortable wearing (or is appropriate!). 

Also, choose light color fabrics as dark colors absorb heat from the sun. 

You should also try to wear a visor, as this type of hat will help shield your face from direct sunlight while still permitting heat to escape from the top of your head.

People running on treadmills.

#7: Cool Down With Water

Cool water can lower your body temperature, so you can soak a bandana in ice water and tie it around your neck before you head out for your run.

Or, bring an extra bottle of water to pour over your head or fill up at a water fountain to douse yourself halfway through your run.

#8: Run Indoors

When it seems like it is too hot to run outside, you should take your workout indoors and either hop on the treadmill or choose a form of cross-training such as aqua jogging and replicate your planned workout.

It is always better to be safe than put your body at risk of heat illness, or struggle through a miserably hot workout.

To keep yourself well-hydrated in the heat and humidity, it is imperative to drink sufficient water and electrolyte-filled sports drinks. If you don’t like the store brands, you can try and make your own with our guide: Homemade Sports Drinks: 5 Recipes To Fuel Your Next Workout.

A person drinking from a bottle of water.
Photo of author
Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, as well as a NASM-Certified Nutrition Coach and UESCA-certified running, endurance nutrition, and triathlon coach. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

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