Heat, from a human perspective, is more than just a matter of high temperatures in the shade. Two doctors working in the 1950s watched marines in the heat and humidity of Paris Island, North Carolina. They wrote of “disabling heat” at temperatures in the eighties. Young marines wore fatigues, helmets, cartridge belts, bayonets in scabbards, and boots. They carried full canteens, rifles, entrenching tools, and twenty-two-pound packs. Marching in the sun, running in the sun, low-crawling in the sun, digging foxholes in the sun, they complained of cramps and exhaustion. Young marines occasionally collapsed.
The doctors saw that temperature alone could not predict heatstroke and heat exhaustion as well as what became known as the Wet Bulb Globe Temperature. When a marine recruit suffered in the heat, he also suffered in the humidity and the sun. The Web Bulb Globe Temperature combined the air temperature in the shade, the air temperature in the sun, and something called the “natural wet-bulb temperature.” The air temperature in the sun is measured with a black globe thermometer–a thermometer inside a blackened sphere of copper, left in the sun. The natural wet-bulb temperature comes form a thermometer with its bulb covered by a moistened sock. As the moisture evaporates, it removes heat. Increase the humidity and decrease the rate of evaporation, and in turn increase the wet-bulb temperature. Add a breeze to increase the evaporation rate, and in turn decrease the wet-bulb temperature.
–by Bill Streever