  Vapor condensation plays a crucial role in solar water-purification
technologies. Conventional condensers in solar water-purification systems do
not provide sufficient cooling power for vapor condensation, limiting the water
production rate to $0.4 L m^{-2} hour^{-1}$. On the other hand, radiative dew
condensation, a technique used by existing radiative dew condensers, only works
at nighttime and is incompatible with solar water-purification technologies.
Here, we develop daytime radiative condensers that reflect almost all solar
radiation, and can thus create dew water even in direct sunlight. Compared to
state-of-art condensers, our daytime radiative condenser doubles the production
of purified water over a 24-hour period. The integration of our daytime
radiative condenser with solar water-purification systems can increase the
water production rate in sunlight from $0.4 L m^{-2} hour^{-1}$ to more than $1
L m^{-2} hour^{-1}$.
