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.

水蒸气的摩尔质量小于干燥空气的摩尔质量，使得在相同温度和压力下，潮湿空气比干燥空气轻。这种效应被称为蒸汽浮力，在大尺度气候动力学中被认为是微不足道的。在这里，我们使用理论、再分析数据和气候模型的层次结构来表明蒸汽浮力与热带自由对流层中的热浮力具有相似的幅度。我们进一步表明，蒸汽浮力使冷空气上升，并使副热带层状低云层增加高达其气候值的 70%。然而，一些广泛使用的气候模型无法在控制方程中表示蒸汽浮力。这一缺陷导致对云分布的不准确模拟——这是预测气候变化的最大不确定性。