Figure 2: (Top) Shadowgraph visualization of rising and falling plumes at $Ra=6.8$ $×$ $108$, $Pr=596$ (dipropylene glycol) in a $Γ=1$ cell (from Ref. [11]). (Bottom) Small thermochromic liquid-crystal spheres are seeded in the convecting fluid. Their Bragg-scattered light changes color from red to blue in a narrow temperature range. Streak pictures of the spheres with a long exposure time show the temperature and velocity fields simultaneously. Cooler regions appear brown and warmer regions appear green and blue. This image was taken near the top surface at $Ra=2.6$ $×$ $109$ and $Pr=5.4$ (water). The view shows an area of $6.5cm$ by $4cm$. Near the middle top one sees a brownish (cold) plume detaching from the boundary layer, extending down and to the left into the fluid, and forming a mushroom head consisting of two swirls (from Ref. [12]).