  Radiative cooling, taking advantage of the coldness of the sky, has a
potential to be a sustainable alternative to meet cooling needs. The
performance of a radiative cooling device is fundamentally limited by the
emissivity of the sky, therefore depends heavily on the regional weather
conditions. Although the sky emissivity is known to increase with the dew point
temperature, this interdependence relates only to clear skies and does not
cover extreme weather conditions. Therefore, the feasibility of radiative
cooling remains elusive in the equatorial tropical climate. A case study of
Singapore is presented where the weather is humid, cloudy and constantly
changing. We point out that a high degree of thermal insulation of the
radiative cooling system can be effective under such extreme weather
conditions. A new method to characterise dynamic sky conditions is presented,
namely to measure the sky window emissivity in the zenith direction. We show
that a sub-ambient cooling up to 8 {\deg}C is possible during daytime and that
the cloud base is not a complete blackbody and can be used as a heat sink for
radiative cooling.
