Human thermal plume, Schlieren image. The human body heats air to form a rising plume. This is revealed by Schlieren photography, a way of viewing density changes in transparent materials. These changes (here caused by heat and convection turbulence) cause light passing through the air to bend (refract). The imaging method alters the colour or brightness of this refracted light. The detection of chemicals in the human thermal plume may help detect terrorist explosives and diagnose diseases. Photographed by the world's only (as of 2003) full-scale Schlieren system, developed by Gary Settles, professor of mechanical engineering at Pennsylvania State University, USA.