When a heat wave hits a city, the sidewalks, roads, and buildings end up making the air feel even hotter. Thanks to the urban heat island effect, all that infrastructure absorbs and then reemits the sun’s heat, raising temperatures even more. Getting cool, then, means protecting yourself not just from the sun’s radiation but also from all the radiation bouncing off the pavement and concrete. A new textile—made of plastic and silver nanowires—does just that, and can keep its wearers as much as 16...