How to Calculate the Effect of Humidity on Running Performance

How many times over the last few months have you stepped outside, from the comfort of your home, into what feels like a sauna.

The air feels so thick, you can almost feel yourself sweating before you even take a step.

Running in hot, humid weather does not ever appear to get any easier either. Even heat training for running doesn’t make it feel any more comfortable, although we do get used to running in the heat over time.

But what is the other option? Waiting until the humidity drops later in the day, but the temperature has soared?

Remember, that we found out runners are more likely to suffer from heat stroke in a 5k than they are running a marathon.

Now:

Is it better to run in extreme humidity or heat?

We all know humidity makes it feel harder, but just how much does heat and humidity affect running performance?

Today we are going to look into what the research has found about how dew point affects running performance and how you can best prepare for running in extreme heat and humidity. We have a chart showing you the heat effect of a high dew point on running and show you how to use our temperature pace calculator to adjust your expectations for your races this summer.

Before we begin, does this sound familiar?

“It’s a hot day for a run, but don’t worry…it’s a dry heat!” 

Ever heard that from a running buddy?

You might be wondering how much humidity and dew point really impacts your running ability, especially when you take temperature out of the equation.

Today, it’s common knowledge that dry heat tends to be easier to run in, but that wasn’t always the case.

World-renowned running coach Arthur Lydiard, the pioneer of, among other things, the long run and tapering, went to his grave claiming that “wet heat” (muggy, humid days) were far easier for a runner to train in than dry heat.

Thanks to scientific research, we’re certain now that is not the case.

Why Runners Struggle in Humid Weather

Still, studies on the effects of heat and humidity can be tricky because of the effects of acclimatization: when you aren’t used to hot, humid conditions, you don’t handle the physiological stress nearly as well, and your performance suffers.

To get around this problem and uncover the fundamental physiological effects of humidity and heat on performance, a group of researchers led by J. Sen Gupta studied a small group of male soldiers in India, who would be naturally acclimated to hot, humid conditions.1

Why is breathing harder and heart rate higher in high humidity?

Gupta et al. studied oxygen consumption and heart rate during intensive cycling in comfortable, hot & dry, and hot & humid conditions.

Notably, the study was designed so that the wet bulb globe temperature—a measurement of temperature, humidity, and sunlight that more accurately gauges the apparent temperature felt by your body—was equivalent in the hot and dry versus the hot and humid conditions.

“Hot and dry” was 115° F with 30% relative humidity, while “hot and humid” was 104° F and 60% humidity.

Both of these produced a wet bulb globe temperature of 93° F, hot enough for most road races in the United States to fly a black weather warning flag and cancel the event.

Researchers found that, even when the wet bulb globe temperature was equivalent, hot and humid conditions were more taxing than hot and dry conditions.

Now:

In dry heat, the peak oxygen consumption the soldiers were able to achieve was 5.7% lower than their maximal oxygen consumption in comfortable conditions; when exercising in hot and humid conditions, this performance decrement jumped to 12%.

Heart rates during exercise showed a similar pattern.

For a given power output on the exercise bike (akin to running at a given pace on a treadmill), hot and humid conditions elicited a higher heart rate than hot and dry, which in turn resulted in a higher heart rate than comfortable temperatures.

How does humidity and temperature affect marathon runners?

The compounding effects of heat and humidity can be observed outside the laboratory too.

A 2012 study by Nour El Helou and other researchers at the Biomedical Research Institute of Sports Epidemiology in France analyzed the performance of almost 1.8 million marathon finishers at major-city marathons over a ten-year period, trying to find correlations between marathon completion time and local weather conditions on the day of the race.2

El Helou et al. identified temperature as the strongest environmental factor associated with marathon times.

As you’d expect, hotter temperatures result in slower marathon times, but humidity had a strong impact too.

Here’s where it gets really interesting:

Dew point, on the other hand, only had a weak association with performance levels.

This is convenient, as it allows us to use heat index (which factors in only temperature and relative humidity) as a strong predictor of running performance.

At What Temperature Does High Humidity Make it Harder to Run?

El Helou et al.’s data also show that the body’s response to increasingly hotter temperatures is non-linear, meaning your performance will get vastly worse moving from 70 to 90° F as compared to 50 and 70° F.

Partially because of this, it’s tremendously difficult to predict exactly how much a given temperature and humidity combination will affect you.

According to research by Tim Noakes, distance running performance is impaired markedly when core temperature approaches a critical threshold

Once your body approaches this point, your nervous system will intentionally induce fatigue, slowing you down to protect itself from excessive heat buildup.3

When will your core temperature reach critical threshold?

Unfortunately, it is not that easy to give you an answer to this one.

How long it takes to approach this critical temperature is a function of the ambient temperature, the humidity, how fast you are running, your sweat rate, your body mass, your height, and a whole host of other factors.

Add to this the fact that individual heat tolerance is highly variable in its own right, and you can see why it’s not so easy to create a rule of thumb that works for everybody!

Now:

For a marathon, El Helou et al.’s data can be used to predict about how much you’ll slow down based on the temperature you’ll encounter, but even this model does not take humidity into account.

How Can Runners Figure Out How Much Hot, Humid Weather Will Affect Their Performance?

The best approach is to track your own personal performance as a function of the heat index, which combines the effects of ambient temperature and humidity.

Even this is not perfect, as it does not factor in radiant heat from sunlight, nor does it acknowledge that hot and humid conditions will impair your performance more than hot and dry conditions, even at equivalent relative temperatures.

Gupta et al.’s work suggests your performance could be slowed by up to twice as much in hot and very humid conditions compared with hot and dry conditions at the same heat index value.

Keeping all this in mind will help you set more realistic training and racing goals when you are faced with oppressive heat and humidity this summer.

When using Dew Point as a guideline, use this chart to calculate how much to adjust your pace

Although El Helou et al. found that dew point had weak association with performance levels, many runners find it very helpful to adjust expectations for running perofmrnace based on dew point. Dew point is often considered a more accurate way of measuring the humidity and comfort of air, which for runners, makes a big difference.

But firstly, where do you find dew point?

Dew point can be found on any weather reporting app or website, and will give a number in degrees that runners can use to adjust expected paces for an upcoming race or workout.

Once you have found the dew point, use this as a guide to running safely and effectively in high heat and humidity.

Dew Point in °F (°C)	Performance Adjustment	Easy Running	Hard Running
Below 55 °F (12°C)	0%	Unaffected	Unaffected
55°F (13°C) to 60 °F (15°C)	1%	Unaffected	Slightly difficult
60°F (16°C) to 65 °F (18°C)	2-3%	Slightly difficult	Difficult
65°F (18°C) to 70°F (21°C)	3-5%	Difficult	Very difficult
70°F (21°C) to 75°F (23°C)	5-8%	Difficult	Very difficult
75°F (23°C) to 80°F (25°C)	12-15%	Very difficult	Not recommended
Above 80°F (25°C)	Just run	Not recommended	Not recommended

Remember, that although it may seem counter intuitive, we found runners are more likely to suffer from heat stroke in a 5k than in longer events like a marathon.

The more time your body has to prepare for running in high heat and humidity, the better you are going to feel running in extreme heat. Take a read of our previous article on how to adapt to running in the summer heat for more information, or if you are an older runner, we have an article on running in the heat for masters.