How Much Does Sunlight Slow You Down?

We all know that hot weather makes you slower. And we also know that there’s more to heat than just the air temperature—humidity makes a big difference. But there are other factors too, like wind and sunlight, that affect how hot a given day feels.

The scale that environmental physiologists like to use is the “wet-bulb globe temperature” (WBGT) which is a blended scale that combines the ordinary air temperature with the “wet-bulb” temperature (measured with a thermometer whose bulb is wrapped in a wet cloth, which depends on the air temperature, humidity, and wind), and the “globe thermometer” temperature (which measures solar radiation).

If you want to know whether your time is going to be slower than normal, or if there’s a risk of heat exhaustion, WBGT is a better predictor than temperature and humidity alone.

But WBGT is only a rough approximation, calculated by adding 70 per cent of the wet-bulb temperature, 20 per cent of the globe temperature, and 10 per cent of the air temperature. While previous studies have measured the effects of temperature, humidity, and wind on endurance performance, no one has actually measured the effects of solar radiation.

That’s what a new study in the European Journal of Applied Physiology, by researchers at Himeji Dokkyo and Hyogo universities in Japan, set out to do.

They had eight volunteers complete a series of cycling time-to-exhaustion tests lasting roughly half an hour, in warm conditions (30 C, 50 per cent humidity) with varying degrees of artificial solar radiation.

The solar radiation levels were 800 watts per square metre (corresponding to noon under a clear sky at the latitude of Japan or Britain), 500 (thin clouds in summer), 250 (thick clouds in summer), and 0.

Here are the average results for the four conditions:

It’s clear that solar radiation makes a big difference, at least when it’s already hot enough that the subjects are dealing with overheating.

What’s interesting is how the solar radiation did (and didn’t) affect the physiological variables they measured. Core temperature, sweat loss, heart rate, and several other parameters were unaffected.

What did change significantly was skin temperature, which was higher when radiation was higher. It’s possible that this had a physiological effect, due to increased blood flow to the skin (though they weren’t able to detect this); or it may contribute to a greater perception of body temperature. The data doesn’t give any clear answers here.

Still, there’s a pretty important practical message to take from this study: looking at the thermometer, and even checking the humidity, doesn’t give you the whole picture when you’re considering how hot a run is likely to be. Check the cloud cover!

(As a more winter-appropriate postscript, this topic caught my interest partly because I recently wrote an article about the supposed differences between “damp cold” and “dry cold.” The evidence suggests that, unlike in hot weather, humidity actually makes little or no difference to how cold you feel—instead, the apparent difference may reflect sunny versus cloudy days.)


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