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Rayleigh-Benard Convection Cells
This type of convection pattern occurs in a relatively shallow layer - this could mean a layer of fluid 1 millimeter thick in a petri dish, or the first 2 kilometers of the Earth's atmosphere.
Perfect Conditions, Perfect Pattern (Almost!)
Under the right conditions, convection cells will take the shape of hexagons. Why don't we see hexagon-shaped clouds in the sky? Take a look at the picture to the right, and notice the small glitch in the pattern. It was later discovered that there was a tiny dent in the copper plate under the fluid. This tells us that the pattern is very sensitive to the bottom surface. Think about our earth - it's surface has millions of dents and bumps in the form of mountains, valleys, canyons, and more. All of these surface features affect the convection patterns in the atmosphere.

Fluid in Motion

This picture shows a time lapse view of Rayleigh-Benard cells. The picture was taken over ten seconds, so the aluminum flakes in the fluid look like long trails instead of small particles. This helps to visulaize how the fluid is moving: up through the center of the cell, then spreading out and sinking at the edges of the cell.

Images used with permission of The Parabolic Press. Van Dyke, M., 1982: An Album of Fluid Motion.