Vapor buoyancy increases clear-sky thermal emission

The molar mass of H2O (18 g mol−1) is smaller than that of dry air (29 g mol−1), which makes humid air lighter than dry air with the same temperature and pressure. This vapor buoyancy (VB) effect has been traditionally considered small in large-scale climate dynamics and even neglected in some leading climate models. Here, using theory and aquaplanet simulations with prescribed surface temperatures, we show that VB increases tropospheric air temperature, and that the warmer atmosphere emits more clear-sky thermal radiation by about 2–4 W m−2 in the dry subtropical areas, a significant radiative effect. We then analyze Coupled Model Intercomparison Project Phase 6 simulations with prescribed sea surface temperatures and realistic topography. The results show that VB can increase clear-sky thermal radiation by up to 5 W m−2 over the ocean and about 15 W m−2 over the subtropical arid land regions. The radiative effect over arid land is amplified by a substantial increase of surface temperature due to VB. Our results highlight the role of VB in regulating Earth’s energy balance both at the top of the atmosphere and at the land surface. This study points to new ways to improve climate models and their simulated energy balance.