On Sunday, thousands of people will run the Rotterdam Marathon. Me too. Rain or shine. Runners often claim that there is no such thing as bad weather, only bad clothing. However, temperature, sun, wind, and humidity significantly affect athletic performance and the associated risks. The risks of prolonged exercise in high temperatures or high humidity are greater than during average weather. A few years ago, we already wrote some climate reports about the weather during long sporting events such as the marathon and the Nijmegen Four Days. And about how the so-called WBGT (wet-bulb globe temperature), a variant of the wet-bulb temperature, can be a relevant measure for endurance athletes. With next Sunday in mind, it is high time to dig up those old reports from the archive and supplement them with recent data.
Running, not sprinting
To avoid confusion about the phenomenon of runner: you sprint to catch the train or bus, you run (or jog) for pleasure, and walking, well, that is walking. Running takes a considerable amount of energy, and heat is released in the process. To prevent overheating, the body tries to dissipate that heat to the environment (among other things, by sweating). Wind accelerates cooling. The warmer and more humid the outside air, the more effort the body has to make to get rid of that heat. More sweating, up to more than one liter per hour!
WBGT measures the weathers influence on outdoor athletes
A quantity that describes the weathers influence on outdoor athletes is the “Wet-Bulb Globe Temperature” (WBGT). This incorporates temperature and humidity, as well as wind speed and solar radiation. Humidity increases WBGT. So it can happen that a not-so-warm but humid (and therefore very muggy) day has just as (dangerously) high a WBGT as a hot day with low humidity. In some countries, WBGT is already used as an official measure to assess heat-related risks (KNMI is also actively involved in this). Ideal WBGT values for marathon runners lie between 5 and 13 degrees. Above that, performance, especially for non-elite runners, quickly declines (slower times, more dropouts).
Weather change on Sunday during the marathon?
After weeks of very dry air from the continent, the wind will shift in the coming days, bringing in more humid air. We see this very concretely reflected in the gradual increase of the (dew point) temperature (image 1). When the warmer humid air reaches its peak, whether or not it will bring rain was not very precisely known in the past few days. The plume shows that we will likely be at the upper end of ideal regarding WBGT next Sunday. Temperature and dew point will be quite close together on Sunday, so the relative humidity will be significantly higher than in recent times. However, higher WBGT temperatures cannot be completely ruled out. So, runners, follow the weather reports and adjust your drinking plan as necessary!