Nocturnal cooling, also called night sky cooling, is the cooling of a building by radiation to the night sky. The roof of a building absorbs the greatest amount of solar radiation during summer. On a hot summer day, the surface temperature of the roof can rise as high 65°C. The roof also has the largest exposure to the sky and hence can be used effectively for nocturnal cooling.
Nocturnal cooling results from the loss of heat through long-wave radiation from a body towards another body of lower temperature, which plays the role of a heat sink. In the case of a building, the cooled body is the building and the heat sink is the sky, since the sky temperature is lower than the temperatures of most objects on the earth. Similarly, if water is exposed on a terrace, it cools because the temperature of the night sky is much less than the temperature of the water.