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Fundamentals of Evaporative Cooling
In hot environments and when exercising vigorously we all sweat cool ourselves evaporatively. It's very efficient, and if there's any kind of breeze available, as on a motorcycle, this works really well. Popsicles and wet neck wraps are great too, if you have those. The lower the humidity, the better a wetted t shirt or a moist wrap around one's neck works.
Evaporation from even a small wet bandana provides tremendous evaporative cooling to one's entire body. The circulation of blood passing by the evaporative wrap (carotid artery, etc) quickly carries the freshly chilled blood cells to every area of one's body. The effect can be so great one can actually become chilled despite it being a very hot day. It’s the same as putting an ice cube on one's wrist or eating ice cream too fast.
Re-wetting a bandana, neck wrap or silk scarf as needed while on the move is simple if you keep a squirt-type water bottle or water bag handy inside a tank bag or pocket. And for longer days during very hot and dry conditions consider covering yourself entirely with something more protectively windproof than mesh. Any traditional leather or textile outer garment with a few zippered vents will let you manage and achieve a comfortable, healthy, moist microclimate between this gear and your skin — just as nomadic peoples living in desert areas have done for centuries by wearing their traditional loose cotton and silk robes. (Those aren't 'colorful native costumes', but rather functionally highly refined clothing that works. Just like your riding gear.)
Mesh gear is fine for short-distances in these conditions, but during longer exposures the risks of inadvertent dehydration are significant. The tipping point between a healthy fluid balance and heat-stoke can occur quite suddenly, and the consequences may quickly become extremely serious. Even life-threatening in some instances.
Otherwise unexplainable single-vehicle motorcycle accidents sometimes happen on clear hot days because with no warning a rider may simply faint. One suddenly feels very sleepy and a moment later they go unconscious and ride into the ditch and crash at full highway speed, on an otherwise empty road. (Have you ever seen a photograph of a row of soldiers standing stiffly at attention on some parade ground somewhere, on a very hot day, and one of them has suddenly fainted and is lying sprawled on the ground between the others?) If you don’t pay enough attention to your body's cooling needs and fluid requirements on hot days that could happen to you when riding. The danger is real.
Mil-spec 'big H' history...
Silk was so important that its trade probably changed the world history more than any other technology. The famous 'Silk Road' connected Europe to the Far East during the middle ages, and this brought together global knowledge that advanced civilization and helped begin the renaissance.
Strategic military considerations, not demand for fashionable garments, caused the establishment of this trade route. During the middle ages front-line soldiers lived, traveled and fought wearing coarse tunics and outer uniforms. Silk scarves allowed soldiers to close their protective battle (and outdoor survival) garments tightly around their necks without chafing or discomfort, so they could fight better and travel farther. This was a true battlefield advantage.
As recently as World War I this remained so. Early planes and cars were all 'open cockpit'. Soldiers still lived and campaigned outdoors for weeks at a time, and this meant wearing heavy gear. After the war civilian pilots, race car drivers, motorcyclists and movie idols (Douglas Fairbanks, Errol Flynn...) further enshrined silk scarves as garb synonymous with bravery, courage and endurance.
Then airplanes and cars became enclosed and military dress adapted. Soft, slippery scarves became unnecessary. Silk's swashbuckler history and function was not only forgotten, it became a laughable cliché. Something only worn by old men.
The last vestiges of the silk scarf's centuries-long military role evolved into the fashion of men wearing neckties with their sport coats, blazers and business suits. Silk scarves had become decorative neckwear. (Now you know where neckties come from.)
That's sort of a sad ending for such an important habiliment -- except once again riders have rediscovered how great these scarves work when worn with modern riding gear closed tightly around one's neck. Once you've tried wearing one, you'll never look back. Make some history.