Boiling kettle, schlieren image. Steam and hot air rise from a kettle as it boils. This is due to the density of the hot gas in the kettle being less than that of the surrounding cool air, causing it to rise in turbulent thermal plumes. This is known as convection. Schlieren photography provides a way of visualising density changes in transparent fluids. Light passing through a density change is refracted (bent). The imaging technique alters the brightness and colour of this refracted light to highlight these areas.