Thermal convection. A demonstration of thermal convection in water. The square tube is filled with water and a drop of yellow dye is added at the top centre. A candle is lit under the right hand side of the apparatus. This heats the water immediately above it, causing it to rise by convection. This forces the creation of a current flowing in the anti-clockwise direction as seen here. This is evident as the dye only descends along the left hand side of the apparatus. Eventually turbulent mixing will even out the distribution of the dye in the water.