RangerGress wrote:
Over the past week I've been back wondering about this same issue. So I went poking around Google and, to my very great surprise, I stumbled upon this thread, started by one of my earliest posts here at Slowtwitch. But even this thread never really resulted in the kind of science based hard answers I was looking for. Altho some folks in the thread pointed out that people in desert environments wear loose fitting clothes, there is near consensus from those that train and race in the heat that shirtless is cooler. My guess is that the more clothes in the desert anecdote doesn't account for the fact that the racer is creating most of his heat and needs to dump it. Whereas the dude in the desert isn't so much creating heat as trying to shield himself from radiant heat. Humidity is a big issue too. I could imagine that when humidity is pretty high, a shirt could help evaporative heat losses because of the high surface area of high tech shirts. Certainly with no shirt you don't get much evaporative heat loss because in high humidity your sweat just drips off. But convection is a big deal and a shirt is certainly going to reduce convective heat losses.
What we need is some studies that look at optimum solutions for different temps, sunlight intensity, and humidity.
Quick note on this one.
From high 90's F to 100+ degrees and you're rivaling to superseding skin temperatures while exercising (depending). Therefore you begin going from Convective loss to the environment to neutral to convective heat gain from the environment, which is the benefit of wearing multiple loose layers. It's basically insulation against gaining Convective heat.
Solar radiation is very dependent on Air mass which is affected by Elevation angle and altitude.
In the summer, there isn't too much difference in elevation angle based on location on the earth. Going from Sea level to 5,000 feet results in a larger gain in radiation than from London To Cairo. Both however are insignificant compared to the changing of seasons which can easily reduce solar radiation by 25 to 50% depending on location. However, season makes a large difference as it results in a large Solar Elevation angle difference.
Long story short, during summer Egypt has less solar radiation than Denver. ~1050w/m2 vs. 1030w/m2 vs. 1000w/m2 for London. Egypt winter is ~850 w/m2
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