As for obesity, it is complicated, Dr. Crandall said. Fat may insulate the interior from very hot external temperatures, but it also may compromise heat transfer from interior to skin. Carrying more weight generates more metabolic heat to get rid of. That means more sweat, but research suggests that large people cannot grow more sweat glands to cope with the extra heat load. Radiation of heat from skin to air may become especially important in their heat control.

Over all, though, these factors make small difference in sweat rate. The bigger differences come from activities that may fall short of exercise. Even brief spurts of walking or leg jiggling generate metabolic heat that turns into sweat, as do anger and frustration. The sweatiest person on the subway platform is probably the one who just ran for a train and missed it, Dr. Crandall said.

And as for clothing: less is not always better. In studies during World War II, researchers sat volunteers on wooden boxes in the California desert, some wearing standard olive drab military fatigues, some in light tan summer uniforms, and some “near naked.” The unclothed “soldiers” sweated about 30 percent more than the others — an indication of how much heat their unprotected skin was absorbing from the environment.

And so the average urban warrior might be forewarned that near-nudity on hot subway platforms may be counterproductive, as may be vigorous fanning, pacing and gesticulating if the train is late.

What will happen as the planet heats and more is asked of our sweat glands? No problem, experts say: the system can easily rev up into a high, efficient gear.

The process is called heat acclimation and is routinely seen in athletes training in hot weather. At first their internal temperatures climb, they sweat profusely, lose large quantities of salt in their sweat and feel miserable. But as the days pass they sweat even more, their salt loss diminishes, both skin and internal temperatures drop, and their endurance improves.

Image Mark Sanders wore a sweat suit for testing recently at a Dallas research laboratory. Credit... Mark Graham for The New York Times

At least in part, heat acclimation reflects bigger, juicier sweat glands: in monkeys exposed to continuous heat and humidity, individual sweat glands more than doubled in volume after only two months.