Clear night sky radiative cooling is a natural process where surfaces lose heat by radiating infrared energy to the clear night sky. This phenomenon occurs because the sky, especially when clear and dry, acts as a cold sink with low thermal radiation, allowing surfaces to cool down below the ambient temperature.
Materials used for radiative cooling should have high emissivity in the infrared spectrum (8-13 micrometers) to effectively radiate heat. Additionally, they should have low absorptivity in the solar spectrum (0.3-2.5 micrometers) to minimize heat gain during the day. Materials like certain polymers, paints, and engineered coatings are often tailored to meet these criteria.
Atmospheric conditions significantly impact radiative cooling efficiency. Clear skies, low humidity, and low cloud cover enhance the cooling effect because they reduce the atmospheric infrared back-radiation. Conversely, cloudy or humid conditions diminish the cooling potential due to increased atmospheric thermal radiation.
The orientation and positioning of surfaces can influence radiative cooling efficiency. Horizontal surfaces are generally most effective as they have a direct line of sight to the sky. Ensuring that the surface is free from obstructions like buildings or trees can also enhance cooling by maximizing exposure to the clear sky.
Practical applications of radiative cooling include passive cooling of buildings, improving the efficiency of photovoltaic cells, and in certain agricultural practices to protect crops from frost. Additionally, it can be used in cooling water for industrial processes and in developing energy-efficient cooling systems for electronics and other devices.