Schlieren photograph of warmed air rising from a hot water filled goblet
Density variations in air caused by air being warmed by contact with the sides of a goblet filled with hot water cause the air to be warmed and to rise. This effect is hardly visible to the naked eye but the "Schlieren" technique allows us to readily visualize the effect. A subject without density gradients would look like a uniform color or shade of grey in a schlieren beam. Density variations, or "gradients', cause light rays to travel a different route than the same rays would have taken in a subject of uniform density and these lgive rise to brightness differences to appear which we then interpret as the effect. In this case warm air would be seen rising from the edges of the goblet much as we would expect this to be the case.