waves can be visualized by rippled cloud patterns described as herringbone sky or mackerel sky. The outflow of cold air from a thunderstorm can launch large amplitude internal solitary waves at an atmospheric inversion. In northern Australia, these result in Morning Glory clouds, used by some daredevils to glide along like a surfer riding an ocean wave. Satellites over Australia and elsewhere reveal these waves can span many hundreds of kilometers. Undulations of the oceanic thermocline can be visualized by satellite because the waves increase the surface roughness where the horizontal flow converges, and this increases the scattering of sunlight (as in the image at the top of this page showing of waves generated by tidal flow through the Strait of Gibraltar). Buoyancy, reduced gravity and buoyancy frequency According to Archimedes principle, the weight of an immersed object is reduced by the weight of fluid it displaces. This holds for a fluid parcel of density surrounded by an ambient fluid of density . Its weight per unit volume is , in which is the acceleration of gravity. Dividing by a characteristic density, , gives the definition of the reduced gravity: If , is positive though generally much smaller than . Because water is much more dense than air, the displacement of water by air from a surface gravity wave feels nearly the full force of gravity (). The displacement of the thermocline of a lake, which separates warmer surface from cooler deep water, feels the buoyancy force expressed through the reduced gravity. For example, the density difference between ice water and room temperature water is 0.002 the characteristic density of water. So the reduced gravity is 0.2% that of gravity. It is for this reason that internal waves move in slow-motion relative to surface waves. Whereas the reduced gravity is the key variable describing buoyancy for interfacial internal waves, a different quantity is used to describe buoyancy in continuously stratified fluid whose density varies with height as . Suppose a water column is in hydrostatic equilibrium and a small parcel of fluid with density is displaced vertically by a small distance . The buoyant restoring force results in a vertical acceleration, given by This is the spring equation whose solution predicts oscillatory vertical displacement about in time about with frequency given by the buoyancy frequency: The above argument can be generalized to predict the frequency, , of a fluid parcel