ABSTRACT
The cooling power of a surface exposed to the sky derives from the difference between emitted thermal radiation and radiation received from the atmosphere. The radiative effect of clouds is then to partially or totally close the atmospheric window of transparency. The Planck’s law describes the electromagnetic radiation emitted by a black body in thermal equilibrium at a definite temperature. The Kirchhoff’s law of thermal radiation states that absorptivity and emissivity of radiation are equal when matter is at thermodynamic equilibrium at temperature. The presence of clouds modifies the sky emissivity by increasing atmospheric radiation. Any visually opaque cloud may be assumed to radiate as a black body at the temperature of the cloud base. The derivation of cloudy sky emissivity from clear sky emissivity is made by using radiometers to measure the cloudy sky downwelling longwave radiation power and comparing it with clear sky radiation power.
