This image, taken looking upward from Mission Beach in San Diego, California, shows a particular cloud type called cirrocumulus stratiformis, with open cells on the left side of the image and a clear boundary showing closed cells on the right- Photo Credit: James D. Means PhD
Visible satellite images of stratocumulus and cumulus clouds will often exhibit hexagon-like structures that meteorologists call “open cell” and “closed cell” convection. The open cells are rings of cloud with rising air where the clouds are, and sinking air in the open middle of the cloud lattice. The closed cells are the reverse—rings of cloud-free sinking air with rising air and clouds in the middle. In both cases rising air causes clouds to form, while sinking air warms and dries the air and clears that region of clouds.
While such open and closed cell structures are easy to see from orbit, they are more rarely seen from the ground, but the image above is a good illustration of this. This image, taken looking upward from Mission Beach in San Diego, California, shows a particular cloud type called cirrocumulus stratiformis, with open cells on the left side of the image and a clear boundary showing closed cells on the right. If you look carefully you can pick out crude hexagons in the open cell region. The difference between what’s happening on the left and right may be no more than a thinning of the moist layer that generated the clouds, that can be enough to change them from open cells to closed cells.
Open and closed cell convection is a particular example of something that fluid dynamicists call convection, first discovered by Thomson when he was cleaning glasses in warm dishwater outside of a pub, then reproduced in the laboratory by Benárd and explained theoretically by Rayleigh. It’s found that cellular convection occurs in a wide variety of thin fluids that are heated from below. The hexagonal cells that often occur are a result of nature packing the cells into a planar space as close to each other as possible.