A planetary-scale hydraulic jump driving Venus’ cloud front

Atmospheric motions generate clouds and influence planetary climate systems. Venus, permanently shrouded by sulfuric acid clouds, provides a striking example. Unlike the photochemically produced upper clouds, the lower cloud layer is thought to form through condensation driven by poorly understood atmospheric dynamics. Observations by the Akatsuki spacecraft revealed a persistent, planetary-scale massive cloud cover in the lower cloud region, moving westward with a sharply defined leading edge about 6000 km long. This feature, unexpected from existing atmospheric models, raised fundamental questions about Venusian meteorology. Here, we show that the cloud front results from the largest hydraulic jump (bore) in the solar system. A planetary-scale Kelvin wave becomes unstable due to a background static stability structure, generating an updraft along the front that triggers sulfuric acid condensation. Numerical simulations reproduce the observed morphology, including fine undulations. This process is likely the origin of the lower cloud. The westward momentum carried by the Kelvin wave is transferred to the mean flow through the hydraulic jump, thereby contributing to the maintenance of the planet's fast atmospheric superrotation. The resulting clouds modify the static stability, further facilitating a hydraulic jump. This previously unrecognized coupling between clouds and atmospheric dynamics represents a fundamental process likely to operate across planetary atmospheres. (Imamura et al., JGR-Planets 131, e2026JE009672, 2026)