A form of cellular convection, usually hexagonal as viewed from above (when the fluid has a free surface), formed in thin layers of a fluid initially at rest and heated from below. According to the usual dynamic stability analysis, the Rayleigh number must attain a critical minimum value before this type of circulation pattern develops. The vertical circulation may be upward in the core and downward on the edges, or it may be the reverse. It has been suggested that some cloud patterns give evidence of Bénard cells on a large scale.