The molar mass of water vapour is less than that of dry air, making humid air lighter than dry air at the same temperature and pressure. This effect is known as vapour buoyancy and has been considered negligibly small in large-scale climate dynamics. Here, we use theory, reanalysis data and a hierarchy of climate models to show that vapour buoyancy has a similar magnitude to thermal buoyancy in the tropical free troposphere. We further show that vapour buoyancy makes cold air rise and increases subtropical stratiform low clouds by up to 70% of its climatological value. However, some widely used climate models fail to represent vapour buoyancy in the governing equations. This flaw leads to inaccurate simulations of cloud distributions-the largest uncertainty in predicting climate change. A combination of theory, reanalysis and model simulations suggests that tropospheric temperature and cloud cover are strongly influenced by vapour buoyancy, an effect currently neglected in some leading global climate models.
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