Staying stably cool in the sunlight

Microporous ceramics passively cool buildings and reduce the need for air conditioners Terrestrial surfaces exposed to sunlight absorb solar heat and shed heat back to outer space as infrared radiation. If the radiated heat is greater than the solar energy absorbed, then daytime radiative cooling is achieved passively, without any energy input. However, this approach requires materials that strongly reflect sunlight and simultaneously emit long-wavelength infrared light-the wavelength needed to escape Earth's atmosphere and not be reflected back. Ceramics composed mainly of silica (SiO2) and alumina (Al2O3), and that permit long-wavelength infrared emission, can meet these requirements. Moreover, they are durable, thermostable, and water-resistant. On page 684 and 691 of this issue, Zhao et al. (1) and Lin et al. (2), respectively, describe microporous materials- a glass-based ceramic coating and a ceramic composite-that exhibit passive daytime radiative cooling and resistance to harsh environments. These advances may lead to more environmentally friendly ways for keeping buildings cool.