Conventional wisdom suggests that warm air rises while cold air sinks. However, recent satellite observations show that, on average, rising air is colder than sinking air in the tropical free troposphere. This is due to the buoyancy effect of water vapor: the molar mass of water vapor is less than that of dry air, making humid air lighter than dry air at the same temperature and pressure. Unfortunately, this vapor buoyancy effect has been considered negligibly small and therefore overlooked in large-scale climate dynamics.
Here we use theory, reanalysis data, and a hierarchy of climate models to show that vapor buoyancy has a similar magnitude to thermal buoyancy in the tropical free troposphere. As a result, cold air rises in the tropical free troposphere.
We further show that vapor buoyancy enhances thermal radiation, increases subtropical stratiform low clouds, favors convective aggregation, and stabilizes Earth’s climate. However, some state-of-the-art climate models fail to represent vapor buoyancy properly. This flaw leads to inaccurate simulations of cloud distributions—the largest uncertainty in predicting climate change. Implications of our results on paleoclimate and planetary habitability will also be discussed.
Da Yang is an assistant professor in the Department of the Geophysical Sciences at the University of Chicago. His research interests have focused on understanding clouds, rainstorms, and climate change. In recent work, he asked why individual convective clouds tend to organize together, forming large-scale rainstorms, such as hurricanes, what environmental factors control rainstorms’ spatial scale, and what are essential elements in forecasting rainstorms. He has also discovered that cold air rises in the tropical atmosphere. This discovery led to explorations on the buoyancy effect of water vapor, which is less familiar than thermal buoyancy due to temperature contrasts.
Yang is a recipient of a Packard Fellowship for Science and Engineering, an NSF CAREER Award, and a Miller Research Fellowship. He also serves as an associate editor for the Journal of Climate. He received a bachelor’s degree from Peking University and a doctorate from the California Institute of Technology. Before joining the University of Chicago, Yang was on the faculty at the University of California, Davis, and the Lawrence Berkeley National Laboratory.