Condensation Radiation of Water Vapor Drives Diurnal Temperature Range Patterns

ZHOU Shaoxiang

The global distribution and variation of the Diurnal Temperature Range (DTR) remain “an essential knowledge gap” in our understanding of climate dynamics in IPCC assessment reports. This study introduces the radiative mechanism of water vapor condensation as a novel physical driver of DTR dynamics. Contrary to classical heat conduction theory, which assumes latent heat transfers via temperature gradients, we demonstrate that condensation in near-thermal-equilibrium moist air cannot release heat conductively without violating the Second Law of Thermodynamics. Instead, latent heat is released directly via characteristic infrared radiation (Condensation Radiation). This radiative energy propagates over long distances and acts as a primary driver of the nighttime greenhouse effect. Quantitatively, the latent heat associated with global precipitation (~78 W/m²) exceeds anthropogenic heat emissions by three orders of magnitude. Using global datasets and case studies (e.g., precipitation events in Beijing), we show that higher condensation intensity - driven by evaporation, humidity, and geography - reduces DTR. This explains the observed spatial and temporal patterns: smaller DTR in coastal regions than inland, at low latitudes than high latitudes, and in summer than winter. Ultimately, the proposed mechanism offers a robust framework for reconciling long-standing inconsistencies in DTR trends under global warming.