<P> The same radiative cooling mechanism can sometimes cause frost or black ice to form on surfaces exposed to the clear night sky, even when the ambient temperature does not fall below freezing . </P> <P> The term radiative cooling is generally used for local processes, though the same principles apply to cooling over geological time, which was first used by Kelvin to estimate the age of the Earth (although his estimate ignored the substantial heat released by radioisotope decay, not known at the time). </P> <P> Radiative cooling is one of the few ways an object in space can give off energy . In particular, white dwarf stars are no longer generating energy by fusion or gravitational contraction, and have no solar wind . So the only way their temperature changes is by radiative cooling . This makes their temperature as a function of age very predictable, so by observing the temperature, astronomers can deduce the age of the star . </P> <P> Cool roofs combine high optical reflectance with high infrared emissivity, thereby simultaneously reducing heat transfer from the sun and increasing heat removal through radiation . Radiative cooling thus offers immense potential for supplementary passive cooling to residential and commercial buildings . </P>

The earth loses energy to space only at night