Which specific microclimate and atmospheric phenomenon causes the formation of radiation fog over low-lying river valleys on clear, calm winter nights?

💡 Solution & Explanation

Radiation fog forms when intense terrestrial longwave radiation escapes into space on clear nights, rapidly chilling the ground and the thin layer of air immediately in contact with it below its dew point. Turbulent mixing destroys fog rather than creating it, while advection fog requires horizontal air movement over cold surfaces. Valley floors accumulate this cold, dense air through drainage, making them prone to dense morning fog.