Research: Running in Warm, Humid Weather is Harder Than Hot and Dry

Running in the summer is tough. Heat and humidity can do a number on you.

Don’t believe me? Read this post about my experience running in the Florida heat.

But any discussion of summer weather invites the question, “Which is worse: heat or humidity?” There are always those who will say that dry heat isn’t as bad as humidity.

Well, it turns out that warm, humid weather is worse for exercise than hot, dry weather. At least that’s a logical conclusion to draw from a research study about the differences of exercising in warm, humid climates and hot, dry climates.

In their 2020 article in the Journal of Athletic Training, Vanos and Grundstein explored the impact of varying type of weather on athletes. Keep reading for a breakdown of their research and its implications for you as a runner.

Context for Understanding the Research Article

This research is based on the premise that heat makes exercise more stressful. It doesn’t get into the particulars of how heat makes exercise more stressful, other than to assert that athletes generate heat through exercise, they need to get rid of that body heat, and some weather conditions make it harder to do so.

The study focuses on one particular measurement of weather conditions – wet-bulb globe temperatures (WBGT) – because it is a single number that attempts to combine multiple weather measurements into one statistic. This is in turn used by organizations like the American College of Sports Medicine (ACSM) and National Athletic Trainers’ Association (NATA) to provide recommendations on when weather conditions are unsafe for strenuous activity.

This single statistic can be computed from four underlying statistics: air temperature, humidity, radiant heating, and wind speed. Each of these can have an impact on how an athlete dissipates body heat, and so the researchers are looking at whether different combinations of these parameters can result in the same WBGT but have significant differences in how much heat an athlete can dissipate.

This matters because recommendations based on WBGT are uniform across all climates. But if in different environments, the heat stress induced at a given WBGT varies, it makes sense to them create more specific recommendations for these different climates.

The methods of this study are based on math and statistics. They started with a dataset comprised of fifteen years worth of climate data, measured in August, and spread across 217 locations in the United States. This data had been gathered and calculated in a previous study. They then isolated the individual readings that exceeded safety thresholds – 32.3C WGBT – which occurred 5,505 times. There were additional observations at lower safety thresholds (30.1C and 27.9C), but they focused on the highest threshold.

Each of these wet bulb globe temperature readings was comprised of a unique combination of the four weather factors (air temp, humidity, radiant heating, and wind speed). Based on these four factors, they used some other fancy math to model how much heat an athlete could dissipate in those conditions. This math was based on some assumptions about what kinds of activity a football lineman would engage in, as well as the clothing and equipment they would wear during practice.

Their Research Question and Findings

The title of the article is “Variations in Athlete Heat-Loss Potential Between Hot-Dry and Warm-Humid Environments at Equivalent Wet-Bulb Globe Temperature Thresholds.” In plain English, they looked at the difference between hot, dry weather and warm, humid weather, and how it could potentially impact an athlete’s ability to dissipate heat.

Specifically, their research questions were:

How does maximum potential heat loss differ in a hot, dry environment verse a warm, humid environment under equivalent wet bulb globe temperature values?

What are the main avenues of heat exchange (i.e. convective, radiative, or evaporative) in each environmental type during stressful environmental conditions?

Their hypothesis was that, given equivalent wet bulb globe temperature values, the exercising in warm, humid conditions would be more stressful than exercising in hot, dry conditions.

Their first finding was that there are in fact distinct climate patterns across the United States. By looking at the weather factors contributing to a high WBGT, they could define each one as hot and dry or warm and humid. Hot, dry conditions were prevalent in the western half of the United States. Warm, humid conditions were prevalent in the eastern half.

A second finding was that in hot, dry climates, greater amounts of heat could be dissipated through evaporative heat loss than in warm, humid climates. As a result, athletes should be able to dissipate heat more effectively in hot, dry climates than in warm, humid climates – assuming a consistent wet bulb globe temperature.

They made three other minor conclusions:

  1. The difference became magnified as a wet bulb globe temperature increased. Comparing a lower threshold (27.9C to 32.3C), there was a greater different in heat loss between hot, dry and warm, humid climates at 32.3C. In other words, as temperature increases the impact of humidity also increases.
  2. Greater amounts of clothing reduced the difference between the two climates and had a greater impact on the dissipation of heat in warm, humid climates. The study focused on football linemen in practices, so the least clothed option was shorts and t-shirts. The most clothed option was full football gear.
  3. The difference was magnified greatly by an increase in activity speed and wind velocity. They compared two scenarios – where players moved quickly enough to generate 0.3 m/s of wind velocity and 0.7 m/s of wind velocity. In the faster scenario, they could dissipate much more heat.

Limitations of the Study

While this study is interesting – and has some potential implications for runners – it also has some limitations to keep in mind.

Above all, the greatest limitation is probably that this is purely a mathematical paper produced through modeling and statistics. Many of the assumptions in the paper are based on other studies that actually collected experimental data, but this study did not actually measure heat generation or dissipation by athletes taking part in actual activities. It would be great to see some testing of this with actual athletes in actual, read world conditions.

For us as runners, another limitation is that this is based off research into football players. There are going to be some things that generalize across sports. But there are assumptions in here about the clothing football players where and the speeds at which they move. Those things will differ from runners – and that could potentially change things.

In particular, I’d love to see how this varies with the speeds at which marathon runners move. At what point is a runner moving quickly enough to generate the air velocity required to make a significant difference in heat dissipation? How is this different between 4-hour marathoners and 3-hour marathoners? And what about elites, chugging along at close to 2-hours for a marathon?

It’s also important to remember that this is based on theoretical levels of heat dissipation – not on actual impacts to performance. While it’s fairly plausible that there is a link between the two, it’s not a given. Again, this is a place where empirical evidence of actual athletes performing in different conditions would be helpful.

What This Means for You As a Runner

As a runner, this brings to mind a few thoughts.

First, this confirms a generally held notion that humidity is worse than heat. Given “equivalent” conditions, it’s relatively easier to exercise in hot, dry conditions than warm, humid conditions. One piece of advice for summer running is to run late in the evening – when the temperature is slightly higher but the humidity is lower – and this supports that suggestion. By contrast, pre-dawn usually has the highest humidity, albeit with lower air temps.

Second, I thought the findings about clothing was interesting. What – and how much – to wear during the summer is another age old question. Some people suggest light clothing helps, others suggest you run with no shirt. I’d be willing to bet that in most cases, less clothing is better – but I’d be interested in knowing whether a tank top (with exposed shoulders/arms) is significantly different from shirt-less (or in a sports bra). Or maybe they’re both “light” enough that it doesn’t make a difference.

Third, it’s just a great reminder to pay closer attention to the particular weather conditions on any given day. While I like to use dew point as the single best predictor of what is or isn’t great running weather, it’s important to consider air temp, humidity, wind speed, and sunlight. Each can have a different impact on your running experience.

Check out the full article – Variations in Athlete Heat-Loss Potential Between Hot-Dry and Warm-Humid Environments at Equivalent Wet-Bulb Globe Temperature Thresholds – and let me know what you think in the comments below.

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