Natural convection in water over an ice surface

A tank of water with an ice‐covered bottom and an upper surface maintained at about 25°C provides a simple way of studying the disturbance of a layer of stably‐stratified fluid by convective motion in an underlying layer of unstable fluid. Observations of temperature fluctuations and of the motion of dye streaks and suspended particles show that vigorous overturning is confined to a thin layer on the ice‐covered bottom, from which emerge rising columns of buoyant fluid. the columns penetrate a region of constant mean temperature (3·2°C) which occupies about two‐thirds of the tank volume but are deflected horizontally as they reach the region of stable stratification. the layer just beyond the farthest penetration of the columns is remarkable for large and continuous fluctuations of temperature, apparently caused by internal gravity waves excited by the impacts of columns on the stable fluid. Calculations show that the disturbance from an impulse at the interface spreads comparatively slowly into the stable fluid and the observed concentration of wave energy can be explained if the life‐time of each ripple is limited by energy loss either by ‘wave breaking’ or by viscous forces. Shallow layers with strong internal wave motion may occur in the atmosphere near the interface between an upper region of stable fluid and a lower region in convective motion.