I read this study:
The resume shows this:
They had eight volunteers complete a series of cycling time-to-exhaustion tests lasting roughly half an hour, in warm conditions (86 F/30 C, 50 percent humidity) with varying degrees of artificial solar radiation.
The solar radiation levels were 800 watts per square meter (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.
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.
-- Here it becomes interesting. Could we prove this with the help of moxy ? that the bloodflow to the skin changes ? --
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!