  Synthetic schlieren is an digital image processing optical method relying on
the variation of optical index to visualize the flow of a transparent fluid. In
this article, we present a step-by step, easy-to-implement and affordable
experimental realization of this technique. The method is applied to air
convection caused by a warm surface. We show that the velocity of rising
convection plumes can be linked to the temperature of the warm surface and
propose a simple physical argument to explain this dependence. Moreover, using
this method, one can reveal the tenuous convection plumes rising from ounce's
hand, a phenomenon invisible to the naked eye. This spectacular result may help
student realize the power of careful data acquisition combined with astute
image processing techniques.
