It’s 100 degrees F outside and the wind is blowing pretty hard. You’re in a short sleeved shirt and shorts just standing in the bright sun.
Does it feel cooler than 100 degrees because of the evaporative effect or does it feel warmer than 100 degrees because of the ‘blast furnace’ effect?
Or does it feel more or less the same as it would if there was no wind at all, e.g. 100 degrees?
Depends how hard the wind is blowing. Some years ago I was riding my motorcycle at slow speeds in 100+ temps. My motorcycle has full fairing and a large windscreen, and my full-face helmet has a windblocking panel across the bottom of the chin bar. Here’s what I remember:
-with the visor of my helmet completely down, air exchange near my face was minimal, and my face got uncomfortably hot.
-with the visor of my helmet cracked open just a smidge, there was a little bit of air exchange, and my face felt quite a bit cooler.
-with the visor wide open, I got the “blast furnace” effect, and my face was uncomfortably hot again.
My unscientific answer:
-sweat cools you off by evaporating, and in the process it creates a high-humidity microclimate right next to your skin that inhibits further evaporation. If there’s no breeze at all, you will soon exist inside a microclimate with 100% relative humidity, at which point sweating won’t be optimally effective.
-A very slight breeze continually strips away that microclimate, introducing new dry air without convecting in a bunch of heat. Sweating works really well under these conditions.
-A strong wind won’t make sweating much more effective, but it will tend to heat your skin just by virtue of being hotter than you are.
I expect this is why people in hot, dry desert climates (north Africa, middle east) tend to cover themselves almost completely with loose layers of lightweight fabric rather than showing a lot of bare skin: the loose layers take the prevailing wind and dial it down to just a trickle of fresh air next to your skin.
Thanks Machine Elf. Great explanation.